Black''''s veterinary dictionary 21st edition - T ppsx

The bont tick, bont-legged blue tick, yellow dog tick, and the brown tick – all common in East Africa – transmit this disease.. Tick-Borne Fever of Cattle Tick-borne fever of cattle is c

LYMPHOCYTES from the thymus gland

con-cerned with cell-mediated immunity (See

IMMUNE RESPONSE.)

T2Toxin

This fungal toxin may poison cattle or poultry

eating stored corn containing the fungus

Fusarium tricinctum In cattle, the toxin may

cause multiple haemorrhages and sometimes

death; in poultry, there may be mouth lesions

Tachycardia

Tachycardia is a disturbance of the heart’s

action which produces great acceleration of the

pulse

Tachypnoea

An increase in the rate of breathing due to some

pathological condition (See BREATHLESSNESS;

PARAQUAT POISONING.)

Taenia

(see TAPEWORMS)

Tail, Amputation of

Amputation of the tail (docking) is, or has

been, undertaken for a variety of reasons In

the UK the Royal College of Veterinary

Surgeons has ruled that docking a puppy’s tail

is an unethical procedure except when it is

done for prophylactic or therapeutic reasons

Docking by lay persons is illegal In cattle,

amputation of the tail is illegal except following

injury and must, except in an emergency, be

undertaken by a veterinary surgeon Pigs’ tails

are often docked to prevent tail-biting Lambs

tails are docked to prevent faecal soiling and

fly strike (see under DOCKING; LAW; WELFARE

CODES)

Tail-Biting

In pigs this ‘vice’ can be of great economic

importance There are various reasons why it

occurs: boredom, absence of bedding, and

overcrowding (floor space of less than 1.5 m2(5

square feet) per pig), are regarded as

conducive to tail-biting High temperature

and humidity are possible causes Bitten tails

require amputation or dressing if pyaemia is to

be prevented

Tail sores in pigs These may follow tail-biting by 1 or 2 pigs out of a large batch,and if untreated can lead to pyaemia

In 6 months, out of 135 pig carcases demned in an Oslo abattoir, 56 were affectedwith pyaemia – and of these, 43 had tail sores

con-Talfan Disease (Teschen Disease; Porcine Viral Encephalomyelitis)

This disease of pigs was first recognised in theCzech Republic and occurs throughout Europe

In the UK, it was made a NOTIFIABLE DISEASE

in 1974 Its cause is an enterovirus.Experimentally, the incubation period is stated

to be 12 days Piglets 3 weeks old and upwardsare affected; adult pigs may be infected butshow no clinical signs By no means all piglets

in a litter or on a farm become ill, and the mortality is usually low The main symptom isweakness or paralysis of the hind-legs There

is little or no fever or loss of appetite Recoveryoccurs in a proportion of animals which arehand-fed The disease is present in Britain to asmall extent, and apparently may be associatedwith abortion

Tannin (Tannic Acid)

Tannin (tannic acid) is a non-crystallisablewhite or pale-yellowish powder, which is soluble in water and glycerine It is preparedfrom oak-galls, and is found in strong tea orcoffee When brought into contact with amucous surface, tannin causes constriction ofthe blood vessels When brought into contactwith many poisonous alkaloids it renders them temporarily inert by forming the insoluble tannate, and so is a valuable antidote

Uses Tannic acid has been used in diarrhoeaand dysentery in young animals, usually as catechu or kino – 2 vegetable drugs which contain a large amount of tannin It is oftenadministered, in the form of strong tea, as the first step in the antidotal treatment of poisoning by ALKALOIDS

Tannic-acid jelly is a valuable burn dressing

It lessens the absorption of breakdown productsfrom the burned area and hence diminishes the

T

secondary effects of a serious burn It is not

suitable for extensive areas owing to the danger

of liver damage if large quantities are absorbed

Tapetum

(see EYE)

Tapeworms

An intestinal parasite commonly found in

vertebrates Their life-cycle requires 2 hosts,

sometimes 3 The presence of the adult worm

may give rise to few if any symptoms or, on the

other hand, to anaemia, indigestion, and

nervous symptoms – or even to blockage of

the intestine The cystic stage of tapeworms

may involve the brain Tapeworms are of

considerable public-health importance

A typical tapeworm has a head or scolex,

provided with suckers and, in some species,

with hooks also

Behind the scolex follows a neck, and behind

that are the segments, each being called a

proglot-tis The segments nearest to the head are the

smallest, and are immature Next follow mature

segments, and lastly the gravid segments ing eggs These older segments fall off and arepassed out of the host’s body in the faeces

contain-Taenia This is the common genus of worms

found in dogs and cats, and includes:

T pisiformis (T serrata) is one of the monest Its cystic stage, Cysticercus pisiformis, is

com-found in rabbits and hares

T hydatigena (T marginata) is the largest

form, with mature segments wider than long

It may reach a length of over 5 metres (16 feet)

Its cystic stage, C tenuicollis, occurs in the

viscera of various animals, especially sheep,

cattle and pigs T ovis is frequently mistaken

for the last form, from which it can be distinguished only by microscopical examina-

tion Its cysticercus, C ovis, is found in the

muscles and organs of sheep and goats It is asmall form, easily overlooked

T multiceps (T coernurus) is a more delicate

form than the others, semi-translucent Theintermediate stage is a coenurus, found in thenervous system of sheep and other ruminantsand man

T serialis is a more robust form, its coenurus

being found in rabbits and hares Only 1

species is common in the cat, T taeniaeformis (T crassicollis) The cystic stage C fasciolaris is

found in the liver of rats and mice

Tapeworms 687

T

A typical tapeworm Each segment is called a

proglottis (From H T B Hall, Diseases and

Parasites of Livestock in the Tropics, Longman.)

Taenia Head, mature and gravid segments.

T saginata is a tapeworm of man which

produces cysticercosis infection in the muscles

of cattle; this is C bovis, known as measly

beef T solum is another tapeworm of man, the

intermediate stage of which (metacestode) is

found in the skeletal and heart muscles of pigs,

producing measly pork

Diphyllobothrium D latum is the broad

tapeworm of man, the dog, and the cat It is

rare in Britain, but has a wide distribution

Several species are found, but this is the

commonest The life-history is interesting

The ciliated larva liberated from the egg is

swallowed by a crustacean, Cyclops strenuus

or Diaptonius spp., in which it becomes an

elongated form with a terminal sphere

contain-ing three pairs of hooklets, called a ‘procercoid

larva’ The crustaceans are swallowed by a fish,

when the larva, migrating to the muscles,

becomes an elongated infective larva called a

‘plerocercoid’ The fish is eaten by a suitable

host, and the adults develop In man, the

tapeworm may attain a length of 18 metres

(60 feet), and it may cause a grave form of

anaemia (bothriocephalus anaemia) associated

with gastric and nervous symptoms

D mansoni is also widely distributed and has

a similar life-history, but the infective stage is

found in many hosts, including man, pig, and

carnivores It is common in frogs in Japan The

adult worm is found in carnivores

Treatment of dogs infested with

tape-worms is very important, because some of the

species in their intermediate stages are

danger-ous to food animals Farm dogs should never

be allowed to harbour tapeworms Routine use

of anthelmintics is essential: a wide range is

available, many based on praziquantel or

dichlorophen All material passed should be

destroyed

Dipylidium caninum infests cats also; and

may be transmitted by swallowing a flea

In pigs, cattle, and sheep cysts of the

tape-worm Taenia hydatigena (which infests the dog

and may occasionally attain a length of 5 metres(16 ft)) may be so numerous in the liver thatthe latter ruptures, causing death

Tapeworms in horsesThree species occur

in horses, all belonging to the genus

Anoplocephala A perfoliata and A mammillana are not uncommon in Britain, while A magna

is also sometimes encountered

A perfoliata, a stoutish worm with large head

and no hooks, is a cause not only of ness but occasionally also of ileal and caecalobstruction, and/or intussusception, where

unthrifti-numerous A perfoliata are present The

infection may therefore be more serious than

is generally supposed The intermediate host

The segments of Moniezia worms are much

broader than they are long The worms mayattain a length of several metres/yards, with aminute head little larger than a pin-head Morethan 1000 worms have been recorded from asingle host Numerous species have been

recorded H giardi is found in Europe,

Australia, and Africa and is from 1 to 2 metres(3 to 6 ft) long

A closely related form, Thysanosoma actinoides, is found in North America It is

about 30 cm (1 ft) long, and is found in the liver The sheep show general symptoms ofmalnutrition

Bovine cystercercosis in Denmark

Studies were conducted on 14 farms with a history of this disease On 6 of the farms thesource of infection was sludge from septic tanksapplied to pasture or crops In 2 herds the cattle grazed pasture near a sewage plant; while

on 3 farms people defecating on pasture was apossible source

Tapeworms in poultry A number of

tape-worms have been found in poultry, of which

the commonest are Davainea proglottina,

which has a larval stage in slugs and snails and is widely distributed, and several species

of Raillietina, with the larvae in house-flies,

dung beetles and ants The following are

also common in many countries: Amoebotoenia,

688 Tapeworms

T

with larvae in earthworms; and Hymenolepis of

various species, some of which may be very

numerous in individual birds

‘Measles’ in beefdue to the presence of the

cyst stage (Cysticercus bovis) of the tapeworm

Taenia saginata, which is a parasite of man.

Cattle swallow the eggs of the adult tapeworm,

and these hatch in the intestines, liberating

young embryos, which burrow until they settle

in muscle fibre or connective tissues Here

they appear as small oval cysts, containing fluid,

and each possessing the head of a potential

tapeworm

‘Measles’ in pork is due to the presence

of the cyst stage (Cysticercus cellulosae) of the

tapeworm of man, Taenia solium It is

extremely common among pigs in eastern

lands, which have access to garbage and human

faeces, from whence they pick up the eggs

passed through the human intestines The

eggs undergo a development similar to those of

the beef-measles tapeworm Man may also

himself harbour the cystic stage

Cysticercosis in man Very high sporadic

infection rates have been found in Africa with

Taenia saginata and T solium, the 2 tapeworms

of major importance in man Where T solium

is present, serious human infections with the

cysticercus stage may be observed, as well as

mild infections with the adult tapeworm

When it occurs in beef cattle, the cysticercus of

T saginata is a major economic problem and a

serious obstacle to the export of meat

A single human carrier of T saginata led to

an outbreak of cysticercosis among cattle on a

large farm in the USA

Coenuriasis (gid or sturdy) in sheep

This disease is caused by the pressure of cysts of

the tapeworm Taenia multicepts on cells of the

brain (or spinal cord)

Sheep become infested by swallowing the

unhatched eggs, excreted in a dog’s faeces, while

grazing In the digestive tract the eggs hatch,

and pass via the bloodstream to various parts

of the body; only those reaching the central

nervous system develop Here they form small

cysts, each containing 1 tapeworm head: this

larval stage is known as Coenurus cerebralis.

Over a period of months, each cyst increases

in size, and more heads are budded from the

lining membrane of the translucent cyst wall

Eventually a single coenurus may contain 50

or 100 or more tapeworm heads (scolices)

a proportion of cases

DiagnosisWhere there is no softening of theskull, a guide to the location of the cyst may begiven by interpretation of the neurological signs

as indicated by the sheep’s behaviour An dermal test has been used: 0.1 ml of cyst fluid is injected into a shaved area of skin.Thickening of the skin within 24 hours indicates the presence of a cyst in the animal

intra-TreatmentPhysical removal of the cyst may

be attempted The sheep is anaesthetised and,

in the absence of any skull softening, a trephineused to remove a disc of bone 1.5 cm (0.6 in) indiameter Draining the fluid from the cystbefore its removal obviates the need to enlargethe hole The cyst is then removed completely.(If this is not done, the remaining cyst wall isapparently capable of replacing the fluid.)

Hydatid disease is caused by the cystic

larval stage of the tapeworm Echinococcus granulosus, of which the dog and fox are the

usual hosts Eggs released from tapeworm segments passed in the faeces by these animalsare later swallowed by grazing cattle, sheep andhorses, which may become infested alsothrough drinking water contaminated by wind-blown eggs

People become infested through swallowingeggs attached to inadequately washed vegeta-bles, and possibly eggs may be inhaled in dust

or carried by flies to uncovered food The handling of infested dogs is an importantsource In Beirut, the risk is put at 21 timesgreater for dog-owners than others, by theWorld Health Organisation, which states alsothat in California nomadic sheep-rearers are

1000 times more likely to have hydatid diseasethan other inhabitants of the state (WHOTechnical Report 637)

There have been successful campaigns tocontrol human hydatid disease in both Cyprusand Iceland, by compulsory treatment and/orbanning of dogs

Swallowed eggs hatch in the intestines andare carried via the portal vein to the liver Some

Tapeworms 689

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remain there, developing into hydatid

cysts; others may form cysts in the lungs or

occasionally elsewhere, e.g spleen, kidney,

bone-marrow cavity, or brain Inside the cysts,

brood capsules, containing the infective stage of

the tapeworm, develop, and after 5 or 6 months

can infest dog or fox

In Wales, where the incidence of hydatid

disease is relatively high, farm dogs and

foxhounds are important in its spread

Only some 7 people are known to die from

this disease in England and Wales each year – a

figure which would probably be higher were

diagnosis less difficult Condemnation of sheep

and cattle offal from this cause runs into

hundreds of thousands of pounds annually

Routine worming of dogs is essential for

control

E granulosus is far from being a typical

tapeworm, as it has only 3 or 4 segments and

a total length of a mere 3 to 9 mm (0.12 to

0.35 in), so that the dog-owner will not notice

the voided segments

A problem of diagnosis also arises, in that

this worm’s eggs are indistinguishable from

those of Taenia tapeworms Examination of a

dog’s faeces following dosing with arecoline

would reveal the intact tapeworm However,

this drug has now been replaced by more

modern drugs which destroy the tapeworm

but leave it unrecognisable

Dichlorophen, praziquantel, nitroscanate,

and benzimidazoles are used for treatment

Equine hydatidosisin Britain is caused by

a strain of Echinococcus granulosus which has

become specifically adapted to the horse as its

intermediate host, and is often referred to now

as E granulosus equinus This apparently is of

low pathenogenicity for man

In a survey covering 1388 horses and ponies

examined at 2 abattoirs in the north of

England, 8.7 per cent were infected Prevalence

of infection was closely related to age, rising

from zero in animals up to 2 years old to over

20 per cent of those over 8 years old

Sixty-six per cent of the infected animals had

viable cysts

Treatment of human patientsHydatid

disease is one of the rare parasitic conditions

that can be treated by surgery However, the

result is often incomplete, with frequent

local recurrences or accidents of secondary

dissemination Repeated interventions are

often mutilating and do not guarantee a

definite cure Mebendazole is reported to have

been used successfully in patients

or bite the part The tar must be removed with

a bland fat or oil Crude tar should never

be used on an animal’s skin (See also PITCH POISONING.)

Tarantulas

These include the Chilean rose spider

(Grammostola spatulatus) If found lying on

its back, this creature should not be assumed to

be dead, but merely moulting In the UK,tarantulas are being kept as pets; in Australiawild tarantulas (‘red-back’ spiders) bite a few hundred people each year An antivenin

is available (See also PET ANIMALS ACT

1971;PETS.)

Tarsorraphy

An operation for producing union of upper andlower eyelids It is performed as a permanentmeasure after enucleation of an eyeball; andsometimes as a temporary expedient to giveprotection to an ulcerated or perforated cornea(but see LENSES,CONTACT)

It is important that accumulated tartar beremoved from time to time, for if it is allowed

to collect for an indefinite period the gumsshrink before the advancing deposit, the rootbecomes exposed and ultimately affected, andthe tooth loosens and falls out In addition

to this, there are generally signs of systemic disturbance, such as a bad smell from thebreath, indigestion from inability to feed properly, and in bad cases, great irritability and loss of condition ( , )

690 Tapping

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Tasmanian Grey

An Australian breed of beef cattle, similar to the

Murray Grey but developed from Aberdeen

Angus and White Shorthorns

Taste

This special sense is dependent upon the taste

buds, located in the crevices of the papillae The

taste buds have minute projections – the

endings of nerve fibres It is necessary for

the purpose of taste that the substance should

be dissolved in a fluid, and it seems that this is

one of the functions of the saliva The sense

of taste is closely associated with the sense of

smell (See TONGUE; SMELL; JACOBSON’S

ORGAN.)

Tattooing

Identifying marks or numbers may be applied

to animals by tattooing On black skins,

tattoo-ing is not an effective method, and the use of

nose prints has been tried for cattle The

tattooing of dogs is widely practised in France

(where it is compulsory for the Kennel Club’s

register of pedigree dogs), and in Canada and

the USA

Tattooing, usually in the ear, is used to

identify cattle, pigs, sheep and goats It is not

entirely free from the risk of introducing

infection, e.g blackquarter, tetanus

FREEZE-BRANDING and MICROCHIPPING are

alter-native methods (See also DANGEROUS DOGS

ACT 1991.)

Taurine

An amino acid essential to maintain the health

of cats, and which must be provided in the

food In the USA, feeding of cats on canned

dog foods is reported to have led to a taurine

deficiency, resulting in degeneration of the cat’s

retina

However, a level of taurine in the cat’s diet

sufficient to prevent degeneration of the retina

may be insufficient to prevent the heart disease,

dilated cardiomyopathy (DCM) Most modern

proprietary cat diets contain adequate amounts

of taurine

Taxis

Taxis is the method of pushing back into

the abdominal cavity a loop of bowel which

has passed through the wall as the result of a

rupture or hernia

Tear-Staining

Tear-staining of the face in the dog may be

due to atopic disease or to blockage of a

lacrimal duct

Tears

(see EYE; for ‘soapy’ tears, see ALGAE POISONING;

see also NAPHTHALENE POISONING)

‘Teart’ Pastures, Soils

(see under MOLYBDENUM)

of reducing coliform mastitis Results in the

UK are reported as variable

The liquid chiefly used for the purpose is

an iodophor, but good results can be obtainedwith hypochlorite teat dips containing 1 percent available chlorine (See under MASTITIS

IN COWS.)

Teat Necrosis

This is seen in piglets under intensive tions of rearing, and is sometimes accom-panied by skin necrosis affecting the limbs.Inadequate bedding and abrasive concrete may

Teeth 691

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of the mandible; they are situated in the front

of the mouth, and used for grasping and

cutting They are absent from the upper jaw

of cattle, sheep, and goats, as well as other

ruminating animals

The canines are situated behind the incisors,

and are used mainly for fighting purposes,

being most developed in carnivores and

omni-vores They are useless to the domesticated

herbivorous animals, and in them are usually of

small size They are not present in the upper

jaws of ruminants, and in the lower jaws have

the shape and function of incisors

The molars are the remaining teeth, situated

further back in the mouth They are used

mainly for chewing, and are specially adapted

for this purpose by having broad strong

irregu-lar tables or grinding surfaces The term ‘cheek

teeth’ is often applied to these teeth, since,

strictly speaking, they are composed of

‘pre-molars’, which are represented in the

milk dentition, and ‘molars’, which are not so

represented (See DENTITION.)

Each tooth has a portion covered with

enamel, the ‘crown’; a portion covered with

cement, the ‘root’; and a line of union between

these 2 parts known as the ‘neck’ A

constric-tion occurs at the neck in the temporary

incisors of the horse, in the incisors of the

ruminants, and in incisors and molars of the

dog and cat; in the remaining teeth there is no

such constriction

StructureTeeth consist of 4 tissues In the

middle of the tooth is the ‘pulp’, occupying

the ‘pulp cavity’ It is soft and gelatinous, well

supplied with blood vessels and nerves, and is

large in the young tooth It nourishes the

remaining tissues, and forms dentine for as long

as the pulp cavity is open In later life it is small

or absent, the pulp cavity having filled withdentine formed from the pulp The ‘dentine’forms the greater part of the tooth It is hard,yellowish, or yellowish-white in colour, and issurrounded in the crown by enamel, and in theroot by cement The ‘enamel’ consists of a comparatively thin layer of a brilliant whitecolour and extremely dense and brittle, whichforms a cap to the dentine, or is arranged in layers through it The ‘cement’ is always theoutermost layer of a tooth, being formed on the outside of the dentine in the root, and filling up the irregular spaces and hollows of thecrown The implanted part of a tooth is fixedinto the socket by a layer of vascular fibrous tissue, which serves as the periosteum both ofthe tooth root and of the lining of the alveolus

It is known as the ‘alveolar periosteum’.Enamel is the hardest tissue in the body, andconsists mainly of phosphate of lime It is com-posed of prisms placed side by side, with oneend resting on the dentine and the other endtowards the free surface in a simple tooth, such

as the canine of a dog Cement is practically ofthe same structure as bone, without possessingHaversian canals

Arrangement and form For times of cutting of the various teeth, see DENTITION

Teeth, Diseases of

Most diseases or disorders affecting the teeth areassociated with pain or discomfort, whichresults in absence of appetite, capriciousness infeeding, or other disturbances

Irregularities In certain cases, the incisor

or molar teeth develop out of their normal positions in the jaw, with the result that perfectapposition between the upper and lower teeth

is not possible, and the rate of toothwear is not uniform In other instances, extra or

‘supernumerary’ teeth are formed; in the incisor region these are usually placed behindthe arch of normal teeth, while extra molarsmay be found as projections from the gums

on the inside or the outside of the line of normal teeth

When the temporary teeth are shed, it sometimes happens that the permanent teetherupt irregularly to one side or behind the temporaries, and are distorted accordingly Thisfrequently happens in puppies, and to a lesserextent in the herbivora In the former, trouble

is likely to be experienced between 31⁄2 and 5

or 6 months, and in young horses at 21⁄2 and

31⁄2 years of age In such cases it is necessary

to extract any temporaries which persist, so that

692 Teeth, Diseases of

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Tooth structure (From de Coursey, The Human

Organism, McGraw-Hill.)

the permanent teeth can arrive in their proper

places in the mouth

In dogs frequently, in sheep sometimes, and

in other animals less commonly, there may be a

discrepancy in length between the upper and

lower jaws When the upper jaw is too long, the

condition is known as an ‘overshot jaw’, and

when the lower jaw projects too far forward,

it is popularly spoken of as an ‘undershot jaw’

In bulldogs, pugs, and other breeds of dogs

with very short upper jaws the undershot

condition is practically normal, while in certain

breeds with extremely long upper jaws, such as

the greyhound and show collie, overshot jaws

are very common

Abnormal wear, which is due to

malforma-tions of the jaws, to excessive softness of

the teeth, or to the direction of the teeth, is

another mechanical cause of tooth disorder

(See SOIL-CONTAMINATED HERBAGE with

reference to sheep.)

Abnormal wear varies in different cases, and

is productive of some well-known conditions,

as follows: (1) shear mouth, in which the molar

teeth of the upper and lower jaws wear so that

in time they appear like the blades of a pair of

sheep-shears, the upper row being worn away

on its inner border, and the lower one along its

outer border; (2) step mouth, where the cheek

teeth, instead of being all at the same level,

are arranged with some higher than others,

somewhat like steps – a high tooth in the lower

jaw being opposite a short one in the

corre-sponding upper jaw; (3) overhanging upper

jaw, which is where the first upper cheek tooth

on either side is placed too far forward in

the mouth, and does not come into accurate

apposition with the tooth immediately below

it, causing the formation of a hook – at the

same time the last lower cheek tooth is situated

too far back and also forms a hook; and (4)

curved tables, where the line of cheek teeth in

the upper jaw shows a convexity in its centre,

and a corresponding concavity exists in the

lower row

Signs In most of these instances the animal

affected (almost always a member of the horse

tribe), instead of chewing its food and

swallow-ing it in the usual way, rolls it round and round

in the mouth until it collects into a sodden

mass, often about the size of a couple of fingers,

and puts it out of the mouth instead of

swallowing it (See QUIDDING.) Pain may be

shown when the hand is passed along the

outside of the cheek, especially when pressure

is put upon the line of teeth

Treatment Rasping the teeth by means of

a special tooth-rasp will reduce smaller irregularities, and bring the teeth back intotheir proper function

Caries is not synonymous with tooth decay,

although the term – borrowed from humandentistry – is often used in veterinary practice

to include all tooth decay

Caries is the destruction of the tooth enameland invasion of the dentine by bacteria, resulting in the formation of a cavity Truecaries has been confirmed in dogs but is comparatively rare in farm animals

Neck lesions in cats’ teeth A painfulcondition affecting middle-aged to elderly cats,characterised by cavitation of the necks of teeth.This makes extraction difficult because of therisk of breakage of crowns

Inflammations of the periosteum

lining the root cavity of a tooth are common.They may be due to small particles of food getting forced down into the socket of thetooth, to fractures or fissures of the teeth, tocaries, tumour formation, depositions of tartar,and to certain specific diseases, such as actinomycosis, etc

SignsThese vary from a slight redness of thegum around the root of the tooth, which ispainful when pressed by the finger, to a largesuppurating tract running alongside the root ofthe tooth down into its socket, and perhapsthrough the skin to the outside or into one orother of the sinuses Abscess formation in thetooth socket may take place, and the abscessmay burst into the mouth, to the outsidethrough the skin, or up into a sinus In manycases there is a distinct bulge of the surfaceabove the diseased tooth, which may give to theface a one-sided appearance

TreatmentThe affected tooth or teeth must

be extracted, and the areas of suppurationcleansed and curetted if necessary The cavityusually has to be packed with antiseptic gauzeafterwards for a few days until it begins to fill

by healthy granulation tissue

Periodontal diseaseis a name for chronicinfection of the periodontal membrane It isone form of inflammation of the periosteum, oralveolar periostitis It causes loosening andshedding of the teeth, pain, failure to masticate,and loss of weight

Teeth, Diseases of 693

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Odontomata are tumours formed in

connection with the root of one tooth, or they

may be found in the jaw, sinuses, or even

involving part of the nasal passage, and be

composite or compound, when multitudes of

small rudimentary teeth are present They

cause swelling and bulging of the surface of the

face, and can only be treated surgically

Porphyria gives rise to a pink or brown

discoloration of teeth (See under BONE,

DISEASES OF.)

Toothache is most spectacular in the dog,

which rubs its mouth along the ground, paws at

its nose or mouth, works its jaws, salivates, and

may whine or moan

A veterinary surgeon will offer a diagnosis

and initiate the necessary treatment

‘Broken mouth’is important in hill sheep

(See undermain dictionary heading.)

Fracturesof the canine teeth in dogs are not

uncommon If the pulp is exposed, subsequent

infection can lead to a painful abscess

Extraction of the remainder of the tooth

obviates this but, for show dogs or guard dogs,

is undesirable Metal crowns have been applied

to dogs’ teeth, but are liable to be dislodged

Tooth transplantation has been used in

veterinary practice but the results are seldom

lasting, due to root resorption and bone

replacement Fracture of the transplanted tooth

is likely after a couple of years or so

Teeth, Ewes’, ‘Trimming’

It has been estimated that between 60 and 70

per cent of culling of ewes is on account of their

teeth A small percentage will involve loss of

molars or incisor wear, but the vast majority

will be incisor loss

Ewes have been treated for ‘bite correction’

by means of an electric grinder, a practice that

originated in Australia The procedure has been

strongly condemned on welfare grounds

(See also ‘BROKEN MOUTH’.)

Teeth Scaling

The use of ultrasonic dental scalers is widely

accepted in veterinary dentistry During the

scaling, an aerosol of water droplets is formed,

with a variable amount of periodontal debris

spattered from the patient’s mouth In the

debris there are likely to be viruses and/or

bacteria – a danger for operator, assistant, or

subsequent patient unless precautions are

taken An aerosol of mouth flora can remainairborne for up to 30 minutes following scaling

It is recommended that: (1) the working areashould be well ventilated – preferably withforced air extraction; (2) masks should be worn

at all times by anyone in the working area; and(3) a 0.2 per cent chlorhexidine solution should

be used as the coolant supplied to the scalingequipment

Temperament, Change in

This may follow a brain tumour or infection, asoccurs in BOVINE SPONGIFORM ENCEPHALOPA- THY and RABIES, for example A horse maybecome bad-tempered as the result of EQUINE VERMINOUS ARTERITIS Poisoning may causefrenzy or aggressiveness, e.g BENZOIC ACID

poisoning in the cat (See also BRAIN DISEASES;

3 of which have a bearing on body temperature.Heat is produced by the muscles and by thedigestive organs, and during very cold weather

or exercise, heat from the former increases,while that from the liver and other digestiveorgans decreases The animal may also absorbheat from the sun’s rays

Heat is lost by evaporation of water, and bysensible heat loss (see under HEAT LOSS) Waterloss is achieved via the lungs and the skin, e.g

by panting and sweating (The dog is, for allpractical purposes, a non-sweating animal apartfrom the pads of its feet, and has to rely mainly

usually at its lowest, and at its highest in the late

afternoon

For ordinary practical purposes the usual

average temperatures of animals are given as

Temperature-takingThe most satisfactory

place is within the rectum In females the

ther-mometer may also be inserted into the external

part of the genital canal; as a rule, the vaginal

temperature is about half a degree higher than

the rectal temperature, so that when a series of

temperatures is to be taken, one site or the

other should be selected

With dogs and cats, one person should hold

the animal, preferably on a table, while another

inserts and holds the thermometer In each

animal, after the bulb of the thermometer has

been lubricated with a little soap or Vaseline,

etc., the tail is raised vertically by the left hand,

and the thermometer is inserted through the

anal ring and into the rectum, by a screwing

movement if any resistance is encountered It is

held in position for 30 seconds, or 1 minute,

according to the make of the thermometer, and

then withdrawn With a piece of cotton-wool

any adherent faeces are wiped away, and the

temperature is read off Subsequently, the

ther-mometer should be washed in cold water, and a

cold solution of disinfectant used to disinfect it

For purposes of temperature stress research,

American scientists use a special ear

thermome-ter in cattle As in similar medical research, this

tympanic thermometer is more reliable than the

rectal thermometer, and can sense changes as

small as 0.05°C (1⁄50°F)

Temperature in diseaseA high

tempera-ture is one of the classic symptoms of fever, and

in greater or less measure accompanies

practi-cally all acute cases of disease A comparatively

steady rise in temperature is as a rule succeeded

by a correspondingly steady fall, and is to be

looked upon as a more favourable sign of the

natural course of a disease than when the

tem-perature rises and falls with greater suddenness

The reduction of temperature in simple fevers is

in almost all cases much slower than the rise Awavering temperature, which shows little tendency to come down to normal, generallyindicates that there is some active focus of disease, such as an abscess, which the body cannot overcome Sudden rise in temperature

in an animal which has shown a steady fall previously is an indication of a relapse or recurrence of the disease (See also FEVER;

HYPERTHERMIA;HEAT-STROKE;TROPICS.)Fall of temperature may be occasioned bygreat loss of blood, starvation, collapse, orcoma; it is characteristic of certain forms of kidney disease Certain chronic diseases inwhich emaciation is marked are also associatedwith a subnormal temperature (See also HYPOTHERMIA.)

Temperature, near calving time Ahealthy cow – even though showing the famil-iar signs – is unlikely to calve during the next

12 hours if her temperature is 39°C (102°F).This is a useful guide to herdsmen (See also under FEVER;HOUSING OF ANIMALS, etc.)

Temperature Control in Animal Housing

(see CONTROLLED-ENVIRONMENT HOUSING)

Temperature-Sensitive (TS) Viruses

of bundles of fibrous tissue, white in colour,and arranged in a very dense manner, so as to becapable of withstanding great strains Some are rounded; some are flattened into ribbons;others are arranged in the form of sheets; whilethose of a 4th variety are very short, the musclefibres being attached almost directly on to the bone or cartilage which they actuate Mosttendons are surrounded by sheaths lined withmembrane similar to that found in joint cavities, i.e synovial membrane In this sheaththe tendon glides smoothly over surroundingparts The fibres of a tendon pass into the fibres

of the periosteum covering a bone, and blendwith them One of the largest tendons in theanimal body is the Achilles tendon, which runs

Tendon 695

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from the large muscles at the back of the stifle

down to the point of the hock; it is often called

the ‘hamstring’, and is the structure that is

injured in the condition known as ‘hamstrung’

Tendons, Diseases and

Injuries of

(see also under MUSCLES;SPRAINED TENDONS) In

most cases the injuries to which tendons are

liable are in the nature of minute lesions in

which fibres have been torn across through

over-extension of the tendon as a whole

Accompanying these there are often slight

haemorrhages or extravasations of blood into

the substance of the tendon, and the tendon

itself is thickened at the injured part or, when

severe, practically over the whole of its length

At the same time, a certain amount of damage

has usually been sustained by the tendon

sheath, or by its lining, and an unusually large

amount of the lubricating synovial fluid is

thrown out, which fills the tendon sheath to the

point of dilatation, causing it to stand out on

the surface of the limb

When recovery occurs, the swelling subsides,

fluid is absorbed, and the broken ends of the

fibres become attached by strands of fibrous

tissue to other intact fibres nearby Pain

disap-pears, and the animal becomes sound

Sometimes, however, permanent thickening

results (See also KNUCKLING.)

Certain of the tendons of the horse’s limb are

liable to become ruptured when subjected to

great or sudden strains Suture of the ruptured

ends of the tendon has given good results when

performed early, and when a sufficient amount

of support can be provided by splints or other

means (See CARBON FIBRE.)

Severing of tendons in dogs’ legs has been

successfully treated (See also TENOSYNOVITIS.)

Tenesmus

Straining to pass urine or faeces with little or no

result

Tenosynovitis

Tenosynovitis is inflamation of the tendon and

its sheath It affects the legs of broiler chickens

and is usually caused by a virus Tendons

may enlarge and cease to function (See also

SYNOVITIS.)

Tenotomy

The surgical severing of a tendon

TEPP

Tetra-ethyl pyrophosphate, used in agriculture

as a pesticide, is a potential danger to livestock

A Texas rancher diluted 1 volume of TEPP with water to make 120 volumes, and sprayed

20 head of cattle All were dead within three-quarters of an hour Symptoms of poison-ing in a puppy comprised drowsiness, muscularincoordination, and vomiting The antidote isatropine sulphate

Teratogenic

Teratogenic agents, called teratogens, are thoseknown to cause congenital defects when thepregnant mother is exposed to them The mostnotorious is thalidomide but there are manyothers, not all of them drugs: alkaloids found insome plants, e.g hemlock, viruses and radiationcan all be teratogenic

Teratoma

Teratoma is a developmental irregularity inwhich the embryo, instead of growing normally in the uterus, develops structuraldefects or, in extreme cases, develops into a seriously deformed fetus The latter are com-paratively common in cattle, and give rise todifficulty at parturition ‘Teratology’ is thestudy of congenital deformities (See also under TUMOURS.)

A small aquatic turtle, of which the

diamond-backed terrapin (Malaclemys terrapin) is typical.

Males are smaller than females, reaching about

14 cm (51⁄2in) to the female’s 32 cm (9 in) Theyare popular domestic aquarium pets; however,they should be handled with care as cases of sal-monella poisoning in members of households

in which they are kept have occurred

696 Tendons, Diseases and Injuries of

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the testicle, guided by the fibrous cord known

as the gubernaculum, moves down from a

position close to the kidney to a ‘cooler climate’

in the scrotum Into this it is pulled by the

gubernaculum, which either fails to lengthen or

actually shortens

In some animals, e.g foals, one or both

testicles may go up again through the inguinal

canal This occurs occasionally in pigs, in which

a returning testicle has been known to become

a mere vestige by the age of 6 months

In certain of the wild animals, such as the rat,

and in many tropical animals, e.g the elephant,

the testes are found in the abdominal cavity,

either permanently or temporarily between

periods of sexual activity In the foal the testes

appear in the scrotum usually very soon after

birth, but they are subsequently drawn up into

the abdomen, and do not reappear until

between 5 or 6 months and 10 to 12 months

In a certain proportion of cases the testes are

retained in the abdomen until 2 years of

age, and then descend into the scrotum; in a

number of cases they do not descend at all The

name ‘rig’, or ‘cryptorchid’, is applied to

such animals, and the condition is known as

‘cryptorchidism’ (See CRYPTORCHID.)

The testes consist of a dense fibrous coat, the

‘tunica albuginea’ Blood vessels run

through-out the fibrous tissue, and nourish microscopic

tubules, lined by layers of specialised cells

which form the spermatozoa The tubules,

known as ‘seminiferous tubules’, are connected

with each other near the centre of the testes,

and communicate with the coiled tubes of the

epididymis, from which springs the vas deferens

connecting with the urethra at the opposite

end In the epididymis the sperms mature The

‘spermatic cord’, which consists of the vas

deferens, spermatic artery, veins, and nerves,

enclosed in the layer of serous membrane

(tunica vaginalis), passes upwards through the

inguinal canal and enters the abdomen, whence

it runs back to the region of the neck of

the urinary bladder, opening finally into the

urethra Along the course of the urethra are

the openings of the ducts from the secondary

sexual glands – seminal vesicles, prostate, and

bulbo-urethral glands – which pour out a

secre-tion which mixes with, nourishes, and protects

the masses of spermatozoa coming from the

testes

Externally, the testicle is covered by a layer

of serous membrane, lying immediately outside

the tunica albuginea, and known as the tunica

vaginalis propria, which also covers the

epi-didymis On the outside of this tunic is the

tunica vaginalis communis, or the parietal layer

Outside this is a fairly thick layer of scrotal fascia, in which is deposited the ‘cod-fat’ of thebullock and wedder A strong reddish, fibro-elastic tunica dartos forms the next outermostlayer, and provides the septum between theright and left pouches of the scrotum Finally,

on the outside, there is the practically hairless,thin, elastic, oily-feeling skin of the scrotum

FunctionsThe essential function of the testis

is to produce sperms (See SPERMATOZOA.)Between 60 and 80 million sperms are dis-charged at each copulatory act by the stallion atthe beginning of the breeding season Since astallion may serve more than 100 mares duringthe season, many of them upon 2 separate occa-sions, it will readily be understood that thetestes are extremely active organs, and make aconsiderable demand upon the vitality of thebody generally The necessity for a recuperativeperiod in breeding males will also be obvious.The other function of the testis is that associated with elaboration of the male sex-hor-mones, resulting in the production of the secondary sexual characteristics, such as thearched neck and great body size of the stallion,the broad forehead, massive development ofhorns, and deep voice of the bull, the horns ofthe ram, and the tusks of the boar, etc., as well

as the instinctive desire for sexual intercourse.The chief hormone is testosterone

(See also REPRODUCTION; ENDOCRINE GLANDS;ARTIFICIAL INSEMINATION.)

Testicle, Diseases of

During service, an irritable mare may kick astallion and rupture one of the testes, or seri-ously injure it Damage may also be occasioned

to these organs by the bites of dogs when ing, by gores from cattle, or by injuries from the tusks of boars, gunshots, etc However,infection is probably most common

fight-Orchitis, or inflammation of the testis, may be the result of infection (e.g by

Actinobacillus seminis, Brucella abortus, B suis, Corynebacterium pseudotuberculosis, tuber-

culosis) or of trauma which – if the skin is broken – may itself lead to infection A viralinfection of bulls – infectious orchitis – wasreported in the former Czechoslovakia.Necrotic orchitis in the bull has been caused inBritain by actinobacillosis The testis, beingenclosed in a fibrous, comparatively non-elasticcapsule, is not able to swell to a great extent,although the loose tissues of the scrotum often

do The scrotum becomes reddened in animalswhich have unpigmented skin in the inguinal

Testicle, Diseases of 697

T

region, and the whole area is very painful to

the touch

TreatmentAntibiotics or other therapy may

be needed to deal with an infection

Epididimytis (see under main dictionary

heading,and under RAM)

Hydroceleis a local oedema affecting usually

one tunica vaginalis, and distending that side of

the scrotum with fluid It is most frequently

encountered in the dog, although it may affect

other animals

Hypoplasia(see under INFERTILITY)

Tumoursaffecting the testicle and/or scrotum

include CARCINOMA,SARCOMA,FIBROMA,

PAPIL-LOMA,SEMINOMA, and SERTOLI-CELL TUMOUR

Torsion(see under SPERMATIC CORD,TORSION

OF)

Testosterone

The hormone, secreted by the testicle, which

controls development of the secondary sex

organs, sex characteristics and libido (See

ENDOCRINE GLANDS;HORMONES.)

Tests

(see LABORATORY TESTS)

Tetanus (Lockjaw)

Tetanus (lockjaw) is a specific disease of the

domesticated animals and man, caused by

Clostridium tetani, which obtains access to the

tissues through a wound Horses are most

com-monly affected The organism is present in

most cultivated soils, especially such as receive

heavy dressings of farmyard manure

In certain districts, tetanus is so common

that it is usual to take precautions by

inoculat-ing horses with antitoxin whenever they receive

even comparatively slight wounds, and always

before castration or major operations Lambs

are lost each year after docking and castration,

or before the umbilicus (navel) has closed after

birth, from tetanus

Cl tetani is an anaerobe, i.e it thrives only in

an absence of oxygen Its serious effects are

pro-duced by a toxin, which is absorbed into the

general circulation and exerts its effects upon

the nervous system of the brain and spinal cord

This toxin is one of the most powerful known

Deeply punctured wounds, from which

oxy-gen is excluded, are much more serious than

even large superficial wounds, the surfaces of

which are exposed to the action of sunlight andfresh air Picked-up nail wounds, cracked heels,injuries from the prongs of stable-forks, etc., areexamples of wounds which often become cont-

aminated with Cl tetani Tetanus may occur in

an animal which has had a slight wound whichappeared to heal without any complication Itmay follow tattooing Cases are met with where

no wound can be found on the surface of thebody, nor is there any history of an accident;such cases are probably the sequel to injuriesinflicted by worms in the intestinal wall, or toslight scratches from unusually hard or roughherbage

Intramuscular injections are a potential route

of infection when sterile precautions areneglected

Signs

Horsesbecome stiff and disinclined to move.There is difficulty in turning the head round tothe side, and the fore-legs are splayed outwards

as though to enable the unfortunate animal better to retain its balance

The ears may be turned in towards eachother

If the head is lifted sharply up, by placing thehand under the chin, the haw or 3rd eyelid(nictitating membrane) is seen to flicker acrossthe eye to an extent much greater than usual.Fixity of the jaws, or trismus, which has beenresponsible for the popular name given totetanus (i.e lockjaw), is not always in evidence

in the early stages of an attack

The tail may be held out quivering, and

OPISTHOTONOSmay be evident

During the course of an attack, faeces andurine are usually withheld, and digestive distur-bances may occur, sometimes resulting in fatalcollections of gas in the large intestines.(See HYPERAESTHESIA– another sign.) Cattle Early signs include a raising of the tail-head and, in some cases, bloat The gaitbecomes stiff and the animal may have diffi-culty in feeding because it cannot easily lowerits head because of stiffness in the neck Trismus(lockjaw) is a late sign Tetanus in cattle is not,however, at all common; occasionally out-breaks occur, possibly due to rough, abrasivefeed which allows entry of the infectionthrough the gut

SheepThe signs are similar to those in cattle

As the disease progresses, standing is difficult;the affected animals lie on their sides, rapidlybecome tympanitic, and die after a very short

698 Testosterone

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illness In lambs after castration or docking, the

disease is very rapid in its effects, and several are

affected at the same time

PigsTetanus is not common

Dogs The owner may notice something

peculiar about the eyes and mouth, and either

stiffness or recent lameness Later, the limbs are

usually stretched out as far from each other as

possible, in a sawhorse position Squinting and

grinning are common, but closure of the jaws is

not always in evidence When it is present it is

complete, and death practically always follows

Hyperaesthesia is also very marked The ears

may be bent inwards (as in the horse)

TreatmentFarm animals should be placed in

a darkened loose-box, away from noise, and

with food and water placed at a new level which

they can reach despite their stiffness

If nursed at home, a dog should be in a

room where there are no bright lights, noise,

television, or family activity

Tetanus antiserum, penicillin, and muscle

relaxants (such as acepromazine, which can

obviate exhaustion and save life) are all needed

Treatment must also include glucose saline

injec-tions, e.g in a dog which cannot drink or eat;

and large animals similarly (See DEHYDRATION,

NORMAL SALINE.)

Prevention Vaccination is effective, and on

land where tetanus is rife, the most susceptible

animals should be immunised

Lambsare given antitoxin on the day of

dock-ing or castration Vaccine can be injected at a

different site on the same occasion

HorsesThe usual practice is to give 2

injec-tions at an interval of 4 to 6 weeks, with a

booster dose 6 or 12 months later Further

booster doses may be required It is practicable

to vaccinate pregnant mares so that later their newborn foals will be protected againsttetanus infection via the navel (See also under IMMUNITY.)

PrognosisIn the absence of first-class nursingand intensive care, not many animals (otherthan cattle) recover from tetanus

If an animal regains the ability to drink, thatcan be regarded as a favourable sign

Tetany

Tetany is a condition in which localised modic contraction of muscles takes place Theremay be twitching or convulsions Tetany occurswhen the level of blood calcium falls below normal (See also under PARATHYROID;HYPOMAG- NESAEMIA;TRANSIT;MILK FEVER;RABIES.)

spas-Tethering

The Cruel Tethering Act 1988 makes it illegal

to tether a horse, ass or mule in such a way as

to cause suffering The animal must haveenough to eat and be supplied with fresh waterregularly The tether must not be able to causeinjury, e.g by being too tight or too short

Tetracyclines

Tetracyclines are bacteriostatic antibiotics with

a wide range of activity which includes Gram-positive and Gram-negative bacteria, certain protozoa, rickettsia, and mycoplasma.Tetracyclines are absorbed from the gut, butoral administration may upset the gut flora.They are irritant when injected

Tetracyclines cause fluorescence in bone andteeth In late pregnancy or in young growing ani-mals, high dosage can result in teeth discolorationand can interfere with the formation of enamel.Horses treated with tetracyclines while suf-fering from stress may become affected withdiarrhoea and die (See DIARRHOEAin horses.)

In cats, tetracyclines occasionally cause severeloss of hair

Tetraiodophenolphthalein

Tetraiodophenolphthalein is used in phy of the gall-bladder and bile-ducts for diagnostic purposes

tempera-Diagram of attitude assumed by a dog affected

with tetanus The hind-limbs are kept well out

behind the body, the tail is held rigidly or

quiver-ing, and the muscles of the face are drawn into a

sardonic grin – the ‘risus sardonicus’ of ancient

authors.

Texas Fever 699

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post-mortem examination the blood is bright

red and abnormally fluid, while the tissues

are paler The spleen is enlarged from 2 to

4 times its normal size and is reddish-brown

(‘anthrax spleen’) The liver is swollen and

pale and the gall-bladder is distended with

thick, viscid, dark-coloured bile The muscles

are normal

The chronic form is similar but milder, and

occurs in late autumn Recovery is frequent,

but convalescence is long (although it is stated

to be very short in Argentine cattle)

Treatmentis fairly effective Imidocarb is one

of several proprietary preparations that have

replaced the trypan blue formerly used

Transmissionis by the following ticks:

Boophilus (Margaropus) annulatus (North

America)

B microplus (South America)

B australis (many countries)

B argentinus (South America)

B calcaratus (Asia)

B decoloratus (South Africa)

Rhipicephalus appendiculatus (South Africa)

R evertsi (South Africa)

R bursa (North Africa)

Haemaphysalis punctata (Europe)

Texel

A Dutch breed of sheep, and the most common

breed in Europe Noted for its milk production,

it has good growth rate and meat potential

TGE

(see TRANSMISSIBLE GASTROENTERITIS OF PIGS)

Thalamus

A part of the brain consisting of 2 large ovoid

structures at the base of the cerebrum (see under

BRAIN)

Thallium

Thallium sulphate is used in poison baits to

destroy rats, ants, and other pests, and

acciden-tal poisoning in domestic animals may occur

Thallium poisoning in dogs gives rise to

gas-troenteritis, profuse vomiting, and severe pain

If death does not immediately follow, there may

be a brick-red discoloration of lips, skin of

groin or axilla Hair begins to fall out In

human medicine, thallium poisoning has been

successfully treated with prussian-blue

There are several species in cattle and insheep, including:

T parva (EAST COAST FEVER in tropicalAfrica)

T mutans (Benign bovine theileriosis).

T lawrencei, causing CORRIDOR DISEASE

T annulata, causing MEDITERRANEAN FEVER.(See also TZANEEN DISEASE.)

Thermography

The mapping of temperature over surfaces.Infra-red thermography, using a camera, hasbeen tested in the diagnosis of orthopaediclesions in horses

sec-700 Texel

T

Signs of this deficiency include loss of

appetite, a staggering gait, and muscular

spasms

Thin Sow Syndrome

Groups of sows or gilts lose weight, usually in

the middle or later stages of pregnancy, and

remain emaciated for perhaps 6 months or

more Prolonged under-feeding may eventually

result in some sows being unable to cope with

adverse conditions encountered at times of

stress, e.g weaning It has also been suggested

that infestation with the stomach worm

Hyostrongylus or with the nodular worm

Oesophagostomum may be a cause The use of

sow stalls, in which animals cannot move away

to escape draughts, is another possible cause

Thiopental

A widely used anaesthetic for horses, dogs and

cats It is administered intravenously, as an

aqueous solution of the sodium salt; other

routes cause necrosis of the tissues

Thiouracil

An antithyroid agent which lowers the rate of

metabolism It has been used as a growth

promoter; such use is banned in the EU

Thiourea

This is naphythyl antu, a rat poison which

caus-es oedema of the lungs It is dangerous to

domestic animals and birds

Thirst

(see WATER; DIABETES; SALT POISONING;

COMPULSIVE POLYDIPSIA)

Thogoto Virus

Thogoto virus is a cause of abortion in ewes

in Africa It was first isolated from a tick,

Rhipicephalus appendiculatus, near Thogoto in

Kenya In one flock of some 600 Dorper

ewes, more than 200 aborted over a 2-month

period

Thoracic Duct

The thoracic duct is the large lymph vessel

which collects the contents of the lymphatics

proceeding from the abdomen, hind-limbs,

part of the thorax, etc., and which discharges

its contents into the left innominate vein (See

aspiration under PARACENTESIS.)

Thoracocentesis

Draining off from the thorax of the fluid found

in certain diseases of the chest (See aspiration under

Thread-Worm

Thread-worm is a popular term for oxyuris

worms (See ROUNDWORMS.)

see PHARYNGITIS

Thrombasthenia

This is a rare, congenital disorder of the blood,occurring in man and dogs, in both sexes (com-pare HAEMOPHILIA) It arises from a defect ofthe platelets, and gives rise to prolonged bleed-ing resulting in anaemia It has been described

in foxhounds, otterhounds, etc

Thrombocytopenia

A condition of the blood in which the number

of platelets is below normal Causes include viralinfections, poisoning, auto-immune disease.The signs may include petechial haemorrhagesand fever

Thrombosis

The blocking of a blood vessel by a blood clot

It may follow atheroma, or some injury to thevessel In cats, thrombosis of the femoral arter-ies is by no means rare, and causes paralysis ofthe hind-legs and often pain There is completeabsence of pulse in the arteries In dogs, throm-bosis of the iliac and femoral arteries occurs

Thrombosis 701

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occasionally Euthanasia is nearly always

necessary (See also PARAPLEGIA.)

Aortic-iliac thrombosis is seen in the horse;

the worm Strongylus vulgaris may be a cause.

Thrombosis of a blood vessel in the brain is

a cause of apoplexy (in human medicine a

stroke) (For thrombosis of the vena cava

in cattle, see under VENA CAVA.) (See also under

ANTICOAGULANTS.)

Thrombus

A blood clot in a blood vessel or the heart

Thymus Gland

Situated in the anterior part of the chest cavity,

this gland attains its largest size during early life

and thereafter gradually dwindles The thymus

has a role in immunity, as it removes young

T-cells that happen to reccognise the body’s

own components as foreign Failure of this

function can result in auto-immune disease (See

T-CELLS, which are thymus derived; also

LEUKAEMIA.)

Thyroid Cartilage

The thyroid cartilage is the largest cartilage of

the larynx, and forms a well-marked

promi-nence at the upper end of the trachea It gives

attachment to one end of each of the vocal

folds, which are concerned in the production of

voice (See LARYNX.)

Thyroid Gland

This is a very highly vascular ENDOCRINE

GLAND, situated near the thyroid cartilage of the

larynx The gland usually consists of 2 lobes,

one on either side of the larynx, joined by an

isthmus in some species and individuals

Located within or near the thyroid gland are

the PARATHYROID GLANDS

Minute structureEach lobe is enveloped

in a thin capsule of fibrous tissue, strands

from which pass into the organ, dividing it into

lobules

Function The most important hormone

secreted by the thyroid gland is an

iodine-containing compound called thyroxin This

increases the rate of metabolism, and is released

when an animal is exposed to cold, for example

In hot weather, thyroid activity is reduced

Thyroxin is essential for growth and

reproduc-tion, and influences lactation

Secretion of the hormone is controlled

wholly or in part by a hormone from the

ante-rior lobe of the pituitary gland (See also

PARATHYROID GLANDS.)

Thyroid Gland, Diseases of

Enlargement of the thyroid gland is known as

GOITRE Goitre may occur when there is eithertoo little or too much of the thyroid hormone,thyroxin, produced

Dwarfism in young animals (cretinism) canresult from failure of the gland to produce sufficient thyroxin

Hypothyroidism An insufficiency of roxin is known as hypothyroidism, and may beassociated with insufficiency of iodine in thediet (see GOITRE) The rate of metabolism isslowed, while there is an increase in bodyweight, loss of hair, and lethargy

thy-One form of hypothyroidism, MYXOEDEMA,affects the skin, causing its deterioration

Treatmentincludes the use of thyroid extract;and iodides if appropriate

Hyperthyroidism, or excess thyroxin in theblood, is characterised by loss of weight, some-times an increase in appetite, polyuria, thirst,increased rate of metabolism and heartbeat.Enlargement of the gland may be detected

on palpation The animal may become less or irritable Protrusion of the eyeballs (exophthalmic goitre) may occur

rest-Hyperthyroidism is seen in elderly cats Theyare mostly thin, and it is this loss of weightwhich causes the owner to seek veterinary advice

in many instances In addition to the symptomsmentioned above, diarrhoea may occur

Treatmentis surgical: removal of one gland,

or ligation of the anterior arteries; alternatively,the use of drugs such as sodium fluoride ormethylthiouracil

Tumours of the gland include ADENOMA;sarcoma and carcinoma (see under CANCER); and EPITHELIOMA

Thyroxine

The active principle of the THYROID GLAND It

is used in pharmaceutical preparations to rect hypothyroidism, a common endocrinedeficiency in dogs

cor-Tiamulin

A macrolide antibiotic active against Treponema hyodysenteriae (swine dysentery), various Gram- positive organisms, and Mycoplasma hyosinoviae

(a cause of arthritis in pigs) It must not be used

at the same time as MONENSINor SALINOMYCIN

which possess fewer than 5 digits in their

hind-limbs, the tibia has become modified so that it

sustains the greater part of the weight borne by

the limb – the fibula, its complementary bone,

having become reduced in size and importance

The tibia lies just below the skin on the inside

of the limb, in such a position that it is liable to

be injured by kicks, blows, etc., and in this

connection is of more importance than those

bones that are surrounded by massive muscles

which afford some protection It is not

uncom-mon for the tibia to become fractured, but the

parts remain held together by the very dense

periosteum that covers the bone In the smaller

animals, the setting of the fractured bone is a

routine (See BONES;FRACTURES.)

Tibial dyschondroplasia A crippling

deformity occurring in certain strains of

chick-ens, ducks, and turkeys selected for high growth

rates It is due to a cartilage abnormality

Tick-Bite Fever of Man in Africa

CauseA RICKETTSIA Local reactions, swelling

of lymph nodes, occur in some individuals So

far as is known, tick-bite fever is not fatal

The bont tick, bont-legged blue tick, yellow

dog tick, and the brown tick – all common in

East Africa – transmit this disease It can be

transmitted to the guinea-pig by inoculation of

blood

Tick-Borne Encephalitis (TBE)

A meningioencephalitis following infection by a

flavivirus transmitted by the sheep/cattle tick

Ixodes ricinus The ticks become infected by field

mice, voles, shrews, and occasionally moles It

occurs throughout continental Europe, being

especially prevalent in mountainous regions

with coniferous forests It is more common

in humans than in animals, but the infection in

dogs has been confused with rabies

The human illness resembles influenza in

its symptoms, with a high fever This may

be followed by meningitis Mortality is about

1 per cent

In differential diagnosis, the flavivirus causing

TBE has to be distinguished from louping-ill

virus

Tick-Borne Fever of Cattle

Tick-borne fever of cattle is caused by Cytoectes

(Erlichia) phagocytophila, transmitted by the

common sheep tick, Ixodes ricinus Symptoms

of this infection are high but transient fever,

and a considerable reduction in milk yield

Abortion may also occur Oxytetracycline is

used in treatment (Red-water, caused by

Babesia divergens, often occurs simultaneously.)

Tick-Borne Fever of Sheep

Tick-borne fever of sheep is a disease caused by

Cytoectes phagocytophilia transmitted by the tick Ixodes ricinus.

Tick-borne fever is a mild febrile disease ofsheep in which the essential symptom is a rise

in temperature occurring after an incubationperiod of 4 to 8 days, and lasting about 10 days,when it subsides During this period (whichmay be prolonged) there is dullness and listless-ness, and a considerable loss of weight mayoccur Death occurs in only a small percentage

of cases; most sheep recover unless some othercomplicating condition such as louping-illsupervenes Abortion is an important result

of infection in many instances, and may affect

50 per cent of breeding stock introduced fromtick-free areas

Rickettsiae can be demonstrated in the polymorphonuclear white cells of the blood.The importance of tick-borne fever is that it

is capable of rendering the vasculo-meningealbarrier of the central nervous system vulnerable

to the virus of louping-ill Without its presence,though the louping-ill virus may be introducedinto the bloodstream (by the bite of a tick), itcannot pass this barrier to attack the nerve cellsand so produce the typical nervous symptoms

It has been shown that both infective agents –that of tick-borne fever and of louping-ill – frequently exist together in ticks found on animals on farms where louping-ill is common,and it is probable that under natural conditionsthe great majority of adult sheep on such farmshave been infected with tick-borne fever infection and have recovered

Tick-borne fever increases the susceptibility

of lambs to tick pyaemia, often caused by

infec-tion with Staphylococcus aureus following

tick bites Abscesses occur in the joints and elsewhere, causing lameness, unthriftiness, anddeath

various species of Ixodes (especially the dog tick)

in South Africa and Australia, and Dermacentor

in America In East Africa, the bont-legged tick

(Hyalomma spp.) and possibly the Red tick (Rhipicephalus evertsi) cause paralysis.

The paralysis is caused by toxin(s) present inthe saliva of ticks

Tick Paralysis 703

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In human beings, 3 or 4 days after the ticks

attach themselves, paralysis of the legs occurs,

then paralysis of the arms takes place, later the

chest and neck become involved, and

ultimate-ly the heart and respiratory centres are attacked

In the sheep, the parts are affected in the same

general sequence

This form of paralysis is peculiar in that

symptoms disappear within 2 to 6 days after the

ticks are removed, and recovery takes place

subsequently Individual lambs, for example,

can be reinfected and recover more than once,

if the ticks are removed by hand They are

usu-ally not easily seen unless a deliberate search is

made in the wool over the vertebral column

from the base of the skull back to the tail

In the dog, they may cause QUADRIPLEGIA

Tick Pyaemia

(see TICKS;TICK-BORNE FEVER OF SHEEP)

Ticks

These are among the most serious parasites of

domestic animals In the tropics they transmit

bacterial, protozoal and viral diseases; in the

UK, tick-borne fever, red-water fever and

louping-ill Tick pyaemia is caused in sheep

by the transfer of staphylococcal infection

Lyme disease is considered to be partly tick

spread

Some cause illness by means of a toxin, while

all feed on the host’s blood – which can result

in a serious anaemia Large numbers of ticks

also worry the host, and cause unthriftiness

Suppurating wounds may also result In the

British Isles, Ixodes ricinus is the main tick

found, although Haemophysalis punctatis is

present in some southern coastal areas

Life-cyclesOn this basis, ticks can be divided

into 3 groups:

1-host ticks, such as Boophilus, which spend

all 3 stages of their life-cycle on the same

ani-mal Larvae having attached themselves to the

host, they feed on it, moult, feed again on it as

nymphs, moult, and the adult ticks also feed on

it – the females subsequently dropping to the

ground to lay their eggs

2-host ticks: these, such as some Hyalomma

species, feed as both larvae and nymphs on

the same host, but then moult on the

ground; emerging adults find and feed on a

2nd host

3-host ticks: larva, nymph, and adult each

feeds on a different host, with moulting taking

place on the ground between each stage in the

life-cycle Ixodes and Dermacentor species are

included in this group

Family Ixodidae (hard ticks) In this family the dorsum of the body is more or lessprotected by a hard shield of chitin, and insome species the male has ventral plates also.The principal species attacking the domesticanimals are dealt with below (See also DOG TICKS

for those occurring in Britain.)

Ixodes.There are over 50 species in this genus,including the following:

(a) I ricinus attacks all the domesticated

ani-mals and is found in most parts of the world It

is known locally as the castor-bean tick, orEuropean sheep tick A 3-host tick, it leaves itshost before each moult, and then seeks a newhost In this way 3 animals are attacked by thesame tick: one as a larva, one as a nymph, andone as an adult The animals attacked need not

be of the same species This tick transmits borne fever in sheep, louping-ill, and causes tickparalysis in sheep and cattle It can also transmitBabesia, the cause of red-water

tick-(b) I hexagonus attacks especially the dog,

but is found on other hosts, notably sheep It

704 Tick Pyaemia

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Ixodes (Ventral and dorsal views of male × 12.)

Ixodes (Dorsal and ventral views of a small

female × 8.) In this and subsequent drawings of ticks only the fore parts of the legs are shown in diagrams of the ventral surface.

occurs in Europe, North Africa, and America;

it is common on hunting dogs in France In

addition it is a transmitter of babesiosis

(c) I canisuga is the common species found

on the dog in Britain It occurs also in Western

Europe and North America Like the last

species, only females are found on the host It is

known popularly as the British dog tick

(d) I pilosus attacks all the domestic

mam-mals in South Africa It is a reddish-brown tick,

with the body larger behind than in front It is

known locally as the russet tick, and is a causal

agent of tick paralysis

(e) I rubicundus, another South African tick,

which is found only on sheep, also causes tick

paralysis

(f ) I holocyclus, in Australia and India, is

found on ruminants, dogs, and pigs It is the

cause of Australian tick paralysis, symptoms of

which may appear within an hour of attachment

It transmits Q fever

Haemaphysalis. The following species are

important:

(a) H punctata (H cinnabarina var

puncta-ta) is a common tick in Europe, North

America, and North Africa on all the domesticanimals The life-history is identical with that

of H leachii It transmits Babesia bovis in

Britain

(b) H leachii is a 3-host African species

which has been found in Western Asia andAustralia It attacks carnivores, but is sometimesfound on ruminants In East Africa it is calledthe yellow dog tick; it is also known as theSouth African dog tick It transmits caninebabesiosis, Q fever, and tick-bite fever

Dermacentor. The following species areimportant:

(a) D reticulatus is common in Europe, but

also occurs in North Asia It attacks ruminants,and also the dog and the horse It is occasional-

ly found in Western England It transmitsequine and canine babesiosis

(b) D variabilis (D electus) is found on dogs

in North America It also occurs on cattle andhorses It is known as the American dog tick

(c) D occidentalis occurs in western North

America on various domestic mammals It is

considered by some authorities to be D latus.

reticu-Life-cycle of a 3-host tick, Ixodes ricinus (Reproduced with permission from H T B Hall, Diseases and

Tick 705

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(d) D venestus is found in the Rocky

Mountain District of North America and is

called the Pacific Coast tick Adults are found

on various mammals, including man It is the

transmitter of Rocky Mountain spotted fever in

man, and of canine babesiosis It is the cause of

American tick paralysis It is a 3-host tick

Rhipicephalus. The following species are

important:

(a) R sanguineus is found in all parts of the

world on dogs and ruminants It is brown in

colour It is known as the European brown tick

and also as the European dog tick – a name

shared with Ixodes hexagonus.

(b) R appendiculatus is found in Africa,

where it attacks cattle, sheep, goats It is called

the brown tick, and is a 3-host tick This species

transmits East Coast fever, corridor disease,

mild gall sickness, red-water, Nairobi sheep

disease

(c) R bursa is found in North Africa and

Southern Europe on all animals It is a 2-host

tick It transmits ovine babesiosis in Europe

(d) R capensis is found in South Africa on

cattle, horses and dogs It is called the Capebrown tick The life-cycle is similar to the

2nd species It can transmit Theileria parva (e) R simus is found in Africa on dogs and her-

bivores It is called the dark pitted tick Its cycle is similar to the 2nd species It can transmit

life-Theileria parva, Anaplasma marginale, T mutans (f ) R evertsi in Africa may be found on

all the domestic mammals except pigs It hasorange-red legs with round convex distinct eyes.The scutum is black and densely pitted Theunderside of the male is red; the females arebrown or reddish brown It is called the red tick

or the red-legged tick This 2-host species

trans-mits Nuttallia equi and T parva, causing East

Coast fever, babesiosis, spirochaetosis

Boophilus

(a) B decoloratus is found on cattle and other

animals in Africa It is a 1-host tick, called theblue tick This tick, which may be a variety of

B annulatus, transmits Babesia bigemina, Anaplasma marginale, and Spirochaeta theileri.

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COMMON TICKS IN EAST AFRICA

Tick species of hosts attachment affected Parasite transmitted Brown-ear tick 3 Ears, base of horns, Cattle Theileria parva East Coast fever

(Rhipicephalus around eyes, tail Cattle Theileria lawrencei Corridor disease

appendiculatus) brush, and heels Cattle Theileria mutans Mild gall-sickness

Cattle Babesia bigemina Red-water Sheep and goats Virus Nairobi sheep

disease Sheep, cattle, and Louping-ill goats

Man Rickettsia Tick-bite fever Red-legged tick 2 Larvae and Cattle Babesia bigemina Red-water

(Rhipicephalus nymphae in ears Cattle Theileria parva East Coast fever

evertsi) Adults perineal Cattle Theileria mutans Mild gall-sickness

region Horses Babesia nuttali

Babesia caballi } Biliary fever Cattle, horses

sheep, and goats Spirochaeta theileri Spirochaetosis Lambs ?Tick toxin ?Paralysis Yellow dog tick 3 Whole body Dogs Babesia canis Biliary fever

(Haemaphysalis Man and animals Virus Q fever

leach) Man Rickettsia Tick-bite fever Blue tick 1 Face, neck, dewlap, Cattle Babesia bigemina Red-water

(Boophilus and sides of the Cattle Anaplasma Gall-sickness

decoloratus) body marginale

Man Rickettsia Tick-bite fever Horses, cattle, Spirochaeta theileri

Goats and sheep Bont tick 3 Larvae and Cattle/Sheep/ Ricketsia Heartwater

(Amblyomma nymphae on head Goats ruminantium

adults on perineum, Man and animals Virus Q fever udder, scrotum, Man Rickettsia Tick-bite fever and tail

Bont-legged tick 2 or 3 Adults on perineum, Cattle, sheep, Tick toxin Sweating sickness

(Hyalomma udder, scrotum, and goats, and pigs

Man Tick toxin Tick paralysis Man Rickettsia Tick-bite fever

(b) B australis is found in Australia, India,

Africa, and tropical America It is called the

Australian blue tick It also is probably a variety

(c) B annulatus is the Texas fever tick, and is

found in southern North America

The tick remains on the host for 3 to

9 weeks It transmits B bigemina.

Hyalomma. This genus has an oval body

with longish pedipalps and distinct eyes

(a) H aegyptium is found on all the domestic

animals in Africa, Southern Europe, and Asia

It has a brown scutum Only adults are found

on the domestic animals, the younger stages

being found on small mammals It is called the

striped-leg tick, or the bont-leg tick The tick

produces ulcerating sores in cattle, and is

fre-quently the cause of lameness in sheep and

goats owing to its attachment between the

claws It is believed to transmit both species of

Theileria, and equine and bovine babesiosis.

(b) H truncatum, the African bont-legged

tick, is usually a 2-host, occasionally a 3-host

parasite Cattle and goats are the main hosts It

transmits sweating sickness and Q fever A

toxin is thought to be produced by this species

capable of causing necrosis of skin and mucous

membrane at the site of bites, as well as some

degree of paralysis The necrosis may be

exten-sive In one case, in a terrier bitch, it extended

from vulva to umbilicus, with exposure of the

urethra and much sloughing

Amblyomma. In this genus the body is

broadly oval

(a) A hebraeum is an African tick attacking

all the domestic mammals It has a

conspicu-ously marked scutum, yellowish with a red and

blue tinge, and brown or black markings The

eyes are flat and flush with the body It is called

the bont tick This species causes ulcerating

sores at the points for attachment, and is a

fre-quent cause of sore teats It conveys heart-water

to ruminants

(b) A variegatum is an African species

attack-ing herbivores It has distinct convex eyes

The scutum is reddish yellow bordered with

green with black markings It is called the

var-iegated tick Its life-history is as above It also

transmits heart-water, Nairobi sheep disease,

and Q fever

(c) A lepidum, an African 3-host bont tick,

apparently transmits no diseases but gives rise

to unpleasant sores

(d) A gemma, an African 3-host bont tick

which infests cattle, camels, and other domestic

animals It can transmit both heart-water and

Nairobi sheep disease

(e) A cayannense in South and Central

America attacks all the domestic mammals It

is a most vicious biter, and transmits equinenuttalliosis

(f ) A americanum is similar to the last

species, but the scutum has a silvery white spot, giving it its popular name of the lone startick

An American species of Amblyomma transmits Anaplasma argentinum.

Family Argasidae (soft ticks)This

fami-ly is distinguished from the hard ticks by theabsence of a scutum and by the fact that themales and females are almost indistinguishable.Looked at from above, the capitulum is invisi-ble in the adult, whereas in the Ixodidae thehead is always visible

Only 2 genera exist in this family, Argas and Ornithodorus The adults do not permanently

attach themselves to 1 host, like the hard ticks,but resemble the bed-bug in habits The femalealso generally lays more than one batch of eggs.Some ticks in this family are carriers ofspirochaetal diseases to man and birds

Argas.

(a) A persicus (A miniatus) is the well-known

fowl tick, or blue bug, or tampan It is practicallycosmopolitan in its distribution

It is essentially a bird tick, but will bite manand other mammals (horses and cattle) on occa-sion It particularly attacks chickens A largenumber on a fowl will suck so much blood thatthe bird will die from anaemia It is the carrier

of fowl spirochaetosis, and fowl piroplasmosis.The tick normally feeds at night, spendingthe day in crevices, and accordingly is seldomseen – as is the case with bed-bugs, which alsoattack chickens It is easily distinguished fromthis pest by the presence of 8 legs – the bed-bugbeing an insect, and in consequence having

Argas × 4.

Tick 707

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only 6 legs The larval tick (seed tick) remains

several days on the host, and is more frequently

seen The adults can live for 2 years without

food

(b) A reflexus, a closely related species, is

found mainly on pigeons, but also attacks

poul-try and man It is found in Europe, Africa, and

America

Ornithodorus.

(a) O savignyi, the sand tampan, is a soft tick

of great economic importance in Africa, Asia,

and the Near East The tick lives below the

sur-face of the sand, emerging to feed on the blood

of cattle, other domesticated stock, wild

ani-mals, and man The tick’s saliva contains a

potent toxin and this, together with massive

blood loss, readily kills young or debilitated

animals Amitraz, cypermethrin and ivermectin

are among effective controlling agents

(b) O megninii is the spinose ear tick of

America and South Africa The larvae creep

into the ear of some mammalian host, and in

a few days moult The nymphs, which are

covered with minute spines, may live for 1 to

7 months in the ear, increasing in size from 3

to 17 mm (1⁄8to 2⁄3in) They finally drop to the

ground, moult, mate, lay their eggs, and die

The adult is not parasitic The eggs hatch in

about 10 days As many as 80 ticks have been

found in a single ear The irritation is

consider-able and heavy losses may result A modern

treatment is IVERMECTIN

(c) O coriaceus (pajaroello) is a venomous

species (found in North America) which causes

a very painful bite

Transmission of diseaseWhen an

infect-ed tick feinfect-eds upon a calf, it transmits the

para-sites – or causal organisms – of the tick-borne

disease in question The calf soon becomes ill,

and either dies or recovers As a rule, recovery is

associated with immunity However, relapses

may occur in animals thought to be immune to

red-water, for example

With the exception of ticks of the Boophilus

species, larvae hatching from a tick’s eggs willnot immediately be infective because these lar-vae have not yet fed on any host; but as soon asthey start feeding they may ingest the causalorganisms of a tick-borne disease When theymoult and become nymphs, they may then becapable of transmitting disease Similarly, whenthe nymph, on moulting, becomes an adulttick, it will be infective if there were already parasites in its blood

Not all tick vectors will transmit all causalorganisms; and, of course, not all species of hostare susceptible to the same causal organisms

An infective 3-host tick feeding on a susceptible host ‘cleans’ itself of infection andwill not transmit disease in the next stage of itslife-cycle This fact provides a useful controlmeasure

non-The specific parasites transmitted by theticks are not passed on mechanically, but mustundergo a special development in the tick This

is easily understood when it is realised that anyone stage in the life-history of a hard tick bitesonly one animal Accordingly, a tick infected inone stage must be capable of producing the dis-ease in some succeeding stage, which depends

on the tick

Control of ticksIn many tropical countries,energetic measures for the regular and frequentdipping of cattle and sheep are necessary Inorder to achieve adequate control of the tick-borne diseases, it is important that hand-dressing of certain parts of the body should becarried out in addition to the dipping or spray-ing This applies to inside the ears, around thebase of the horns, around the eyes, anus, etc.The acaricides, or tick-killing chemicals,have comprised: (1) arsenical compounds; (2) chlorinated hydrocarbon compounds; (3) the organo-phosphorus compounds; and(4) avermectin compounds

Dipwashes containing arsenic are unsuitablefor spraying because of the danger of pasturecontamination Although cheap, stable, and sol-uble, arsenic compounds are very poisonous:another disadvantage is that some species ofticks acquire a resistance to arsenic preparations.Accordingly, a change to chlorinated hydrocar-bons followed Of these, BHC and toxaphenehave been widely used Unfortunately, ticks canbecome resistant to these too (See also BHC.)Organophosphorus compounds tend to beexpensive, and are used mostly against ticksresistant to other acaricides

Coumaphos; cypermethrin, a syntheticpyrethroid; diazinon, an organophosphorus

708 Tick

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Ornithodorus × 3.