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

Host Factors in InfectionFor 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) 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 firstencounter each other. Factors such as geography, environment, and behavior thusinfluence the likelihood of infection. Although the initial encounter between asusceptible host and a virulent organism frequently results in disease, someorganisms can be harbored in the host for years before disease becomes clinicallyevident. For a complete view, individual patients must be considered in the contextof the population to which they belong. Infectious diseases do not often occur inisolation; rather, they spread through a group exposed from a point source (e.g., acontaminated water supply) or from one individual to another (e.g., via respiratorydroplets). Thus, the clinician must be alert to infections prevalent in thecommunity as a whole. A detailed history, including information on travel,behavioral factors, exposures to animals or potentially contaminatedenvironments, and living and occupational conditions, must be elicited. Forexample, the likelihood of infection by Plasmodium falciparum can besignificantly affected by altitude, climate, terrain, season, and even time of day.Antibiotic-resistant strains of P. falciparum are localized to specific geographicregions, and a seemingly minor alteration in a travel itinerary can dramaticallyinfluence the likelihood of acquiring chloroquine-resistant malaria. If suchimportant details in the history are overlooked, inappropriate treatment may resultin the death of the patient. Likewise, the chance of acquiring a sexuallytransmitted disease can be greatly affected by a relatively minor variation in sexualpractices, such as the method used for contraception. Knowledge of therelationship between specific risk factors and disease allows the physician toinfluence a patients health even before the development of infection bymodification of these risk factors and—when a vaccine is available—byimmunization. Many specific host factors influence the likelihood of acquiring aninfectious disease. Age, immunization history, prior illnesses, level of nutrition,pregnancy, coexisting illness, and perhaps emotional state all have some impact onthe risk of infection after exposure to a potential pathogen. The importance ofindividual host defense mechanisms, either specific or nonspecific, becomesapparent in their absence, and our understanding of these immune mechanisms isenhanced by studies of clinical syndromes developing in immunodeficient patients(Table 113-1). For example, the higher attack rate of meningococcal diseaseamong people with deficiencies in specific complement proteins of the so-calledmembrane attack complex (see Adaptive Immunity, below) than in the generalpopulation underscores the importance of an intact complement system in theprevention 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, Staphylococcuscutaneous integrity athletes 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 Clostridiumbacterial flora difficile, Candida spp. Impairedclearance Poor drainage Urinary tract infection Escherichia coli Abnormal Cystic fibrosis Chronic pulmonarysecretions infection with P. aeruginosa INFLAMMATORY RESPONSE Neutropenia Hematologic Gram-negative malignancy, c ...