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

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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...

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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 Common Associated with Defect Etiologic Agent of Infection NONSPECIFIC IMMUNITY Impaired cough Rib fracture, Bacteria causing neuromuscular dysfunction pneumonia, aerobic and anaerobic oral flora Loss of gastric Achlorhydria, histamine Salmonella spp., acidity blockade enteric pathogens Loss of Penetrating trauma, Staphylococcus
cutaneous integrity athlete's foot spp., Streptococcus spp. Burn Pseudomonas aeruginosa Intravenous catheter Staphylococcus spp., Streptococcus spp., gram-negative rods, coagulase-negative staphylococci Implantable Heart valve Streptococcus spp., device coagulase-negative staphylococci, Staphylococcus aureus Artificial joint Staphylococcus 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 Cystic fibrosis Chronic pulmonary secretions infection with P. aeruginosa INFLAMMATORY RESPONSE Neutropenia Hematologic Gram-negative malignancy, cytotoxic enteric bacilli, chemotherapy, aplastic anemia, Pseudomonas spp., HIV infection Staphylococcus spp., Candida spp. Chemotaxis Chédiak-Higashi S. aureus, syndrome, Job's syndrome, Streptococcus pyogenes,
protein-calorie malnutrition Haemophilus influenzae, gram-negative bacilli Leukocyte adhesion Bacteria causing defects 1 and 2 skin and systemic infections, gingivitis Phagocytosis Systemic lupus Streptococcus (cellular) erythematosus (SLE), chronic pneumoniae, H. myelogenous leukemia, influenzae megaloblastic anemia Splenectomy — H. influenzae, S. pneumoniae, other streptococci, Capnocytophaga spp., Babesia microti, Salmonella spp. Microbicidal Chronic granulomatous Catalase-positive defect disease bacteria and fungi:
staphylococci, E. coli, Klebsiella spp., P. aeruginosa, Aspergillus spp., Nocardia spp. Chédiak-Higashi S. aureus, S. syndrome pyogenes Interferon γreceptor Mycobacterium defect, interleukin 12 spp., Salmonella spp. deficiency, interleukin 12 receptor defect INNATE IMMUNITY Complement system C3 Congenital liver disease, S. aureus, S. SLE, nephrotic syndrome pneumoniae, Pseudomonas spp., Proteus spp.
C5 Congenital Neisseria spp., gram-negative rods C6, C7, C8 Congenital, SLE Neisseria meningitidis, N. gonorrhoeae Alternative Sickle cell disease S. pneumoniae, pathway Salmonella spp. Toll-like Congenital Gram-negative receptor 4 bacilli Interleukin 1 Congenital S. pneumoniae, S. receptor–associated aureus, other bacteria kinase (IRAK) 4 Mannan-binding Congenital N. meningitidis, lectin other bacteria
ADAPTIVE IMMUNITY T lymphocyte Thymic aplasia, thymic Listeria deficiency/dysfunction hypoplasia, Hodgkin's disease, monocytogenes, sarcoidosis, lepromatous Mycobacterium spp., leprosy Candida spp., Aspergillus spp., Cryptococcus neoformans, herpes simplex virus, varicella- zoster virus AIDS Pneumocystis, cytomegalovirus, herpes simplex virus, Mycobacterium avium- intracellulare, C. neoformans, Candida spp. Mucocutaneous Candida spp.
candidiasis Purine nucleoside Fungi, viruses phosphorylase deficiency B cell Bruton's X-linked S. pneumoniae, deficiency/dysfunction agammaglobulinemia other streptococci Agammaglobulinemia, H. influenzae, N. chronic lymphocytic leukemia, meningitidis, S. aureus, multiple myeloma, Klebsiella pneumoniae, E. dysglobulinemia coli, Giardia lamblia, Pneumocystis, enteroviruses Selective IgM S. pneumoniae, H. deficiency influenzae, E. coli Selective IgA deficiency G. lamblia, hepatitis virus, S. pneumoniae, H.
influenzae Mixed T and B Common variable Pneumocystis, cell hypogammaglobulinemia cytomegalovirus, S. deficiency/dysfunction pneumoniae, H. influenzae, various other bacteria Ataxia-telangiectasia S. pneumoniae, H. influenzae, S. aureus, rubella virus, G. lamblia Severe combined S. aureus, S. immunodeficiency pneumoniae, H. influenzae, Candida albicans, Pneumocystis, varicella-zoster virus, rubella virus, cytomegalovirus Wiskott-Aldrich Agents of
syndrome infections associated with T and B cell abnormalities X-linked hyper-IgM Pneumocystis, syndrome 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.