Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 2) docx

Introduction to Infectious Diseases: Host–Pathogen Interactions Part 2 Host Factors in Infection For any infectious process to occur, the pathogen and the host must first encounter ea

Chapter 113 Introduction to Infectious Diseases:

Host–Pathogen Interactions

(Part 2)

Host Factors in Infection

For any infectious process to occur, the pathogen and the host must first encounter each other Factors such as geography, environment, and behavior thus influence the likelihood of infection Although the initial encounter between a susceptible host and a virulent organism frequently results in disease, some organisms can be harbored in the host for years before disease becomes clinically evident For a complete view, individual patients must be considered in the context

of the population to which they belong Infectious diseases do not often occur in isolation; rather, they spread through a group exposed from a point source (e.g., a contaminated water supply) or from one individual to another (e.g., via respiratory droplets) Thus, the clinician must be alert to infections prevalent in the community as a whole A detailed history, including information on travel, behavioral factors, exposures to animals or potentially contaminated environments, and living and occupational conditions, must be elicited For

example, the likelihood of infection by Plasmodium falciparum can be

significantly affected by altitude, climate, terrain, season, and even time of day

Antibiotic-resistant strains of P falciparum are localized to specific geographic

regions, and a seemingly minor alteration in a travel itinerary can dramatically influence the likelihood of acquiring chloroquine-resistant malaria If such important details in the history are overlooked, inappropriate treatment may result

in the death of the patient Likewise, the chance of acquiring a sexually transmitted disease can be greatly affected by a relatively minor variation in sexual practices, such as the method used for contraception Knowledge of the relationship between specific risk factors and disease allows the physician to influence a patient's health even before the development of infection by modification of these risk factors and—when a vaccine is available—by immunization

Many specific host factors influence the likelihood of acquiring an infectious disease Age, immunization history, prior illnesses, level of nutrition, pregnancy, coexisting illness, and perhaps emotional state all have some impact on the risk of infection after exposure to a potential pathogen The importance of individual host defense mechanisms, either specific or nonspecific, becomes apparent in their absence, and our understanding of these immune mechanisms is enhanced by studies of clinical syndromes developing in immunodeficient patients (Table 113-1) For example, the higher attack rate of meningococcal disease

among people with deficiencies in specific complement proteins of the so-called membrane attack complex (see "Adaptive Immunity," below) than in the general population underscores the importance of an intact complement system in the prevention of meningococcal infection

Table 113-1 Infections Associated with Selected Defects in Immunity

Host Defect Disease or Therapy

Associated with Defect

Common Etiologic Agent of Infection

NONSPECIFIC IMMUNITY

Impaired cough Rib fracture,

neuromuscular dysfunction

Bacteria causing pneumonia, aerobic and anaerobic oral flora

Loss of gastric

acidity

Achlorhydria, histamine blockade

Salmonella spp., enteric pathogens

Loss of Penetrating trauma, Staphylococcus

cutaneous integrity athlete's foot spp., Streptococcus spp

aeruginosa

Intravenous catheter Staphylococcus

spp., Streptococcus spp.,

gram-negative rods, coagulase-negative

staphylococci

Implantable

device

Heart valve Streptococcus spp.,

coagulase-negative staphylococci,

Staphylococcus aureus

spp., Streptococcus spp.,

gram-negative rods

Loss of normal Antibiotic use Clostridium

bacterial flora difficile, Candida spp

Impaired

clearance

Poor drainage Urinary tract infection Escherichia coli

Abnormal

secretions

Cystic fibrosis Chronic pulmonary

infection with P aeruginosa

INFLAMMATORY RESPONSE

Neutropenia Hematologic

malignancy, cytotoxic chemotherapy, aplastic anemia, HIV infection

Gram-negative enteric bacilli,

Pseudomonas spp.,

Staphylococcus spp.,

Candida spp

Chemotaxis Chédiak-Higashi

syndrome, Job's syndrome,

S aureus, Streptococcus pyogenes,

protein-calorie malnutrition Haemophilus influenzae,

gram-negative bacilli

defects 1 and 2

Bacteria causing skin and systemic infections, gingivitis

Phagocytosis

(cellular)

Systemic lupus erythematosus (SLE), chronic myelogenous leukemia, megaloblastic anemia

Streptococcus pneumoniae, H influenzae

pneumoniae, other streptococci,

Capnocytophaga spp.,

Babesia microti, Salmonella spp

Microbicidal

defect

Chronic granulomatous disease

Catalase-positive bacteria and fungi:

staphylococci, E coli, Klebsiella spp., P aeruginosa, Aspergillus

spp., Nocardia spp

syndrome

S aureus, S pyogenes

defect, interleukin 12 deficiency, interleukin 12 receptor defect

Mycobacterium

spp., Salmonella spp

INNATE IMMUNITY

Complement system

C3 Congenital liver disease,

SLE, nephrotic syndrome

S aureus, S pneumoniae,

Pseudomonas spp.,

Proteus spp

C5 Congenital Neisseria spp.,

gram-negative rods

C6, C7, C8 Congenital, SLE Neisseria

meningitidis, N gonorrhoeae

Alternative

pathway

Sickle cell disease S pneumoniae,

Salmonella spp

Toll-like

receptor 4

Congenital Gram-negative

bacilli

Interleukin 1

receptor–associated

kinase (IRAK) 4

Congenital S pneumoniae, S

aureus, other bacteria

Mannan-binding

lectin

Congenital N meningitidis,

other bacteria

ADAPTIVE IMMUNITY

T lymphocyte

deficiency/dysfunction

Thymic aplasia, thymic hypoplasia, Hodgkin's disease, sarcoidosis, lepromatous leprosy

Listeria monocytogenes, Mycobacterium spp.,

Candida spp., Aspergillus

spp., Cryptococcus neoformans, herpes simplex virus, varicella-zoster virus

cytomegalovirus, herpes simplex virus,

Mycobacterium avium-intracellulare, C neoformans, Candida

spp

candidiasis

phosphorylase deficiency

Fungi, viruses

deficiency/dysfunction

Bruton's X-linked agammaglobulinemia

S pneumoniae,

other streptococci

chronic lymphocytic leukemia, multiple myeloma, dysglobulinemia

H influenzae, N meningitidis, S aureus, Klebsiella pneumoniae, E coli, Giardia lamblia, Pneumocystis,

enteroviruses

deficiency

S pneumoniae, H influenzae, E coli

Selective IgA deficiency G lamblia,

hepatitis virus, S pneumoniae, H

influenzae

Mixed T and B

cell

deficiency/dysfunction

Common variable hypogammaglobulinemia

Pneumocystis,

cytomegalovirus, S pneumoniae, H influenzae, various other

bacteria

Ataxia-telangiectasia S pneumoniae, H

influenzae, S aureus,

rubella virus, G lamblia

immunodeficiency

S aureus, S pneumoniae, H influenzae, Candida albicans, Pneumocystis,

varicella-zoster virus, rubella virus, cytomegalovirus

syndrome infections associated with

T and B cell abnormalities

syndrome

Pneumocystis,

cytomegalovirus,

Cryptosporidium parvum

Medical care itself increases the patient's risk of acquiring an infection in several ways: (1) through contact with pathogens during hospitalization, (2) through breaching of the skin (with intravenous devices or surgical incisions) or mucosal surfaces (with endotracheal tubes or bladder catheters), (3) through introduction of foreign bodies, (4) through alteration of the natural flora with antibiotics, and (5) through treatment with immunosuppressive drugs

Infection involves complicated interactions of microbe and host and inevitably affects both In most cases, a pathogenic process consisting of several steps is required for the development of infections Since the competent host has a complex series of barricades in place to prevent infection, the successful pathogen must use specific strategies at each of these steps The specific strategies used by bacteria, viruses, and parasites (Chap 114) have some remarkable conceptual

similarities, but the strategic details are unique not only for each class of microorganism but also for individual species within a class