Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 6)

The microbiology laboratory must be an ally in the diagnostic endeavor. Astute laboratory personnel will suggest optimal culture and transport conditions or alternative tests to facilitate diagnosis. If informed about specific potential pathogens, an alert laboratory staff will allow sufficient time for these organisms to become evident in culture, even when the organisms are present in small numbers or are slow-growing. The parasitology technician who is attuned to the specific diagnostic considerations relevant to a particular case may be able to detect the rare, otherwise-elusive egg or cyst in a stool specimen. ...
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Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 6) Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 6) The microbiology laboratory must be an ally in the diagnostic endeavor.Astute laboratory personnel will suggest optimal culture and transport conditionsor alternative tests to facilitate diagnosis. If informed about specific potentialpathogens, an alert laboratory staff will allow sufficient time for these organismsto become evident in culture, even when the organisms are present in smallnumbers or are slow-growing. The parasitology technician who is attuned to thespecific diagnostic considerations relevant to a particular case may be able todetect the rare, otherwise-elusive egg or cyst in a stool specimen. In cases where adiagnosis appears difficult, serum should be stored during the early acute phase ofthe illness so that a diagnostic rise in titer of antibody to a specific pathogen can bedetected later. Bacterial and fungal antigens can sometimes be detected in bodyfluids, even when cultures are negative or are rendered sterile by antibiotictherapy. Techniques such as the polymerase chain reaction allow the amplificationof specific DNA sequences so that minute quantities of foreign nucleic acids canbe recognized in host specimens. Infectious Diseases: Treatment Optimal therapy for infectious diseases requires a broad knowledge ofmedicine and careful clinical judgment. Life-threatening infections such asbacterial meningitis or sepsis, viral encephalitis, or falciparum malaria must betreated immediately, often before a specific causative organism is identified.Antimicrobial agents must be chosen empirically and must be active against therange of potential infectious agents consistent with the clinical scenario. Incontrast, good clinical judgment sometimes dictates withholding of antimicrobialdrugs in a self-limited process or until a specific diagnosis is made. The dictumprimum non nocere should be adhered to, and it should be remembered that allantimicrobial agents carry a risk (and a cost) to the patient. Direct toxicity may beencountered—e.g., ototoxicity due to aminoglycosides, lipodystrophy due toantiretroviral agents, and hepatotoxicity due to antituberculous agents such asisoniazid and rifampin. Allergic reactions are common and can be serious. Sincesuperinfection sometimes follows the eradication of the normal flora andcolonization by a resistant organism, one invariant principle is that infectiousdisease therapy should be directed toward as narrow a spectrum of infectiousagents as possible. Treatment specific for the pathogen should result in as littleperturbation as possible of the hosts microflora. Indeed, future therapeutic agentsmay act not by killing a microbe, but by interfering with one or more of itsvirulence factors. With few exceptions, abscesses require surgical or percutaneous drainagefor cure. Foreign bodies, including medical devices, must generally be removed inorder to eliminate an infection of the device or of the adjacent tissue. Otherinfections, such as necrotizing fasciitis, peritonitis due to a perforated organ, gasgangrene, and chronic osteomyelitis, require surgery as the primary means of cure;in these conditions, antibiotics play only an adjunctive role. The role of immunomodulators in the management of infectious diseaseshas received increasing attention. Glucocorticoids have been shown to be ofbenefit in the adjunctive treatment of bacterial meningitis and in therapy forPneumocystis pneumonia in patients with AIDS. The use of these agents in otherinfectious processes remains less clear and in some cases (in cerebral malaria, forexample) is detrimental. Activated protein C (drotrecogin alfa, activated) is thefirst immunomodulatory agent widely available for the treatment of severe sepsis.Its usefulness demonstrates the interrelatedness of the clotting cascade andsystemic immunity. Other agents that modulate the immune response includeprostaglandin inhibitors, specific lymphokines, and tumor necrosis factorinhibitors. Specific antibody therapy plays a role in the treatment and preventionof many diseases. Specific immunoglobulins have long been known to prevent thedevelopment of symptomatic rabies and tetanus. More recently, CMV immuneglobulin has been recognized as important not only in preventing the transmissionof the virus during organ transplantation but also in treating CMV pneumonia inbone marrow transplant recipients. There is a strong need for well-designedclinical trials to evaluate each new interventional modality. Perspective The genetic simplicity of many infectious agents allows them to undergorapid evolution and to develop selective advantages that result in constantvariation in the clinical manifestations of infection. Moreover, changes in theenvironment and the host can predispose new populations to a particular infection.The dramatic march of West Nile virus from a single focus in New York City in1999 to locations throughout the North American continent by the summer of2002 caused widespread alarm, illustrating the fear that new plagues induce in thehuman psyche. The intentional release of deadly spores of Bacillus anthracis viathe U.S. Postal Service awakened many from a sense of complacency regardingbiologic weapons. The terror of the unknown is seldom better displayed than by the responseof a population to the appearance of an epidemic, particularly when the epidemicstrikes without apparent cause. Edward H. Kass made this statement in 1977 inreference to the newly discovered Legionnaires disease, but it could apply equallyto SARS, H5N1 (avian) influ ...