Chapter 113. Introduction to Infectious Diseases: Host–Pathogen Interactions (Part 5) doc

Chapter 113. Introduction to Infectious Diseases:
Host–Pathogen Interactions
(Part 5)

The complement system (Chap. 308) consists of a group of serum proteins
functioning as a cooperative, self-regulating cascade of enzymes that adhere to—
and in some cases disrupt—the surface of invading organisms. Some of these
surface-adherent proteins (e.g., C3b) can then act as opsonins for destruction of
microbes by phagocytes. The later, "terminal" components (C7, C8, and C9) can
directly kill some bacterial invaders (notably, many of the neisseriae) by forming a
membrane attack complex and disrupting the integrity of the bacterial membrane,
thus causing bacteriolysis. Other complement components, such as C5a, act as
chemoattractants for PMNs (see below). Complement activation and deposition
occur by either or both of two pathways: the classic pathway is activated primarily
by immune complexes (i.e., antibody bound to antigen), and the alternative
pathway is activated by microbial components, frequently in the absence of
antibody. PMNs have receptors for both antibody and C3b, and antibody and
complement function together to aid in the clearance of infectious agents.
PMNs, short-lived white blood cells that engulf and kill invading microbes,
are first attracted to inflammatory sites by chemoattractants such as C5a, which is
a product of complement activation at the site of infection. PMNs localize to the
site of infection by adhering to cellular adhesion molecules expressed by
endothelial cells. Endothelial cells express these receptors, called selectins (CD-
62, ELAM-1), in response to inflammatory cytokines such as tumor necrosis
factor α and interleukin 1. The binding of these selectin molecules to specific
receptors on PMNs results in the adherence of the PMNs to the endothelium.
Cytokine-mediated upregulation and expression of intercellular adhesion molecule
1 (ICAM 1) on endothelial cells then take place, and this latter receptor binds to β
2

integrins on PMNs, thereby facilitating diapedesis into the extravascular

compartment. Once the PMNs are in the extravascular compartment, various
molecules (e.g., arachidonic acids) further enhance the inflammatory process.
Approach to the Patient: Infectious Diseases
The clinical manifestations of infectious diseases at presentation are
myriad, varying from fulminant life-threatening processes to brief and self-limited
conditions to indolent chronic maladies. A careful history is essential and must
include details on underlying chronic diseases, medications, occupation, and
travel. Risk factors for exposure to certain types of pathogens may give important
clues to diagnosis. A sexual history may reveal risks for exposure to HIV and
other sexually transmitted pathogens. A history of contact with animals may
suggest numerous diagnoses, including rabies, Q fever, bartonellosis, Escherichia
coli O157 infection, or cryptococcosis. Blood transfusions have been linked to
diseases ranging from viral hepatitis to malaria to prion disease. A history of
exposure to insect vectors (coupled with information about the season and
geographic site of exposure) may lead to consideration of such diseases as Rocky
Mountain spotted fever, other rickettsial diseases, tularemia, Lyme disease,
babesiosis, malaria, trypanosomiasis, and numerous arboviral infections. Ingestion
of contaminated liquids or foods may lead to enteric infection with Salmonella,
Listeria, Campylobacter, amebas, cryptosporidia, or helminths. Since infectious
diseases may involve many organ systems, a careful review of systems may elicit
important clues as to the disease process.
The physical examination must be thorough, and attention must be paid to
seemingly minor details, such as a soft heart murmur that might indicate bacterial
endocarditis or a retinal lesion that suggests disseminated candidiasis or
cytomegalovirus (CMV) infection. Rashes are extremely important clues to
infectious diagnoses and may be the only sign pointing to a specific etiology
(Chap. 18; Chap. e5). Certain rashes are so specific as to be pathognomonic—e.g.,
the childhood exanthems (measles, rubella, varicella), the target lesion of
erythema migrans (Lyme disease), ecthyma gangrenosum (Pseudomonas
aeruginosa), and eschars (rickettsial diseases). Other rashes, although less

specific, may be exceedingly important diagnostic indicators. The prompt
recognition of the early scarlatiniform and later petechial rashes of meningococcal
infection or of the subtle embolic lesions of disseminated fungal infections in
immunosuppressed patients can hasten life-saving therapy. Fever (Chaps. 17, 18,
and 19) is a common manifestation of infection and may be its sole apparent
indication. Sometimes the pattern of fever or its temporally associated findings
may help refine the differential diagnosis. For example, fever occurring every 48–
72 h is suggestive of malaria (Chap. 203). The elevation in body temperature in
fever (through resetting of the hypothalamic setpoint mediated by cytokines) must
be distinguished from elevations in body temperature from other causes such as
drug toxicity (Chap. 19) or heat stroke (Chap. 17).
Laboratory Investigations
Laboratory studies must be carefully considered and directed toward
establishing an etiologic diagnosis in the shortest possible time, at the lowest
possible cost, and with the least possible discomfort to the patient. Since mucosal
surfaces and the skin are colonized with many harmless or beneficial
microorganisms, cultures must be performed in a manner that minimizes the
likelihood of contamination with this normal flora while maximizing the yield of
pathogens. A sputum sample is far more likely to be valuable when elicited with
careful coaching by the clinician than when collected in a container simply left at
the bedside with cursory instructions. Gram's stains of specimens should be
interpreted carefully and the quality of the specimen assessed. The findings on
Gram's staining should correspond to the results of culture; a discrepancy may
suggest diagnostic possibilities such as infection due to fastidious or anaerobic
bacteria.