Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (473 page)

Pasteurella multocida
, a fastidious aerobic GNR, is a common part of the endogenous oral flora of domesticated cats and dogs, as well as other domesticated and wild animals.

   Who Should Be Suspected?

Infection is usually manifested as cellulitis or wound infections associated with cat bites or scratches. Close contact with animals and underlying medical conditions, especially hepatic disease and malignancy, predispose to infection. Infections at the site of inoculation are painful with marked erythema and swelling. Because of the nature of cat bites (deep penetrating wounds), deep soft tissue infection, septic arthritis, and osteomyelitis are common complications. Localized infection may progress to bacteremia with hematogenous spread to other organ systems, including endocarditis and CNS infections. Colonization of the upper respiratory tract predisposes to pneumonia and pararespiratory abscesses, like sinusitis or empyema.

   Laboratory Findings

Gram staining
: Possibly small, faintly staining gram-negative coccobacilli.

Cultures
: Isolates grow well on SBA or chocolate agar incubated in increased CO
₂
.

PSEUDOMONAS AERUGINOSA
INFECTION

   Definition

Pseudomonas aeruginosa
is a nonfastidious, glucose nonfermenting GNB that is intrinsically virulent for humans; it is capable of producing a wide range of localized and systemic infections. This organism can metabolize a variety of substrates and can be isolated from many environmental reservoirs, including water sources (e.g., sink traps), aqueous solutions, disinfectant solutions, and condensates in respirators, contributing to its role in nosocomial infections.
Pseudomonas aeruginosa
exhibits intrinsic and acquired resistance to commonly used antibiotics.

   Who Should Be Suspected?

Pseudomonas aeruginosa
may cause such infections as bacteremia/endocarditis and systemic infection in neutropenic and ICU patients, burn wound infection with sepsis, chronic pneumonia in patients with CF, keratoconjunctivitis due to contaminated contact lens solutions and other eye infections, nosocomial pneumonia, osteomyelitis due to nail puncture injuries or hematogenous spread (especially in IV drug abusers), otitis externa (swimmer’s ear and malignant otitis externa), and/or UTI.

   Laboratory Findings

Culture
:
Pseudomonas aeruginosa
grows well on routine laboratory media after overnight incubation. Special selective media are recommended to improve isolation of
P. aeruginosa
from lower respiratory specimens submitted from patients with CF.

Susceptibility
: Susceptibility testing should be performed on all significant isolates. Isolates may develop resistance during prolonged therapy with any antibiotic; testing of repeat isolates may be indicated. Reported susceptibility to beta-lactam and beta-lactam/beta-lactamase combinations implies the need for high-dose therapy for serious infections; combination therapy is often recommended.

Q FEVER (
COXIELLA BURNETII
)

   Definition

Q fever describes zoonotic infections caused by
Coxiella burnetii
, a small, obligately intracellular gram-negative bacterium. Cattle, sheep, and goats are the primary reservoir for organisms, which are very stable in the environment. Human infection is usually acquired by inhalation of organisms from environments contaminated with urine, feces, products of gestation, or other materials from infected animals. Infection may also be acquired by ingestion of unpasteurized dairy products.

   Who Should Be Suspected?

Coxiella
infection may cause acute or chronic infection, but many infections remain asymptomatic. Acute infection is usually manifested by flu-like illness, hepatitis, and/or pneumonitis. Endocarditis may develop, usually in patients with preexisting valve disease. Chronic disease is defined as infection lasting >6 months and is usually manifested by endocarditis, aneurism, or infection of prosthetic material.

   Laboratory Findings

Histology
: “Doughnut” granulomas in liver biopsy or bone marrow are highly suggestive but not pathognomonic.

Culture
:
C. burnetii
may be isolated by special eukaryotic cell culture, but this testing is not widely available.

Serology
: The basis of definitive diagnosis. IFA testing is more sensitive (approximately 91%) than CF testing (78%). Serum (1:50 dilution) is screened for antiphase II anti-immunoglobulin. Positive specimens are tested for anti-phase I and anti-phase II IgG, IgM, and IgA, with titer. Single phase IgG titer ≥1:800 by immunofluorescence is diagnostic and strongly suggests
C. burnetii
endocarditis; any positive IgM titer is diagnostically significant. High specific IgM titer suggests hepatitis. High specific IgA titer is common in chronic Q fever and suggests culture-negative endocarditis. ELISA testing is sensitive (approximately 94%) in early convalescence.

Molecular diagnostics
: PCR techniques have been described, but there is no FDA-approved kit for NAA.

ROCKY MOUNTAIN SPOTTED FEVER

   Definition