Chapter 94: Infections Caused by Miscellaneous Gram-Negative Bacilli

Chapter 94: Infections Caused by Miscellaneous Gram-Negative Bacilli is a topic covered in the Harrison's Manual of Medicine.

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BRUCELLOSIS

MICROBIOLOGY

Brucellae are small, gram-negative, unencapsulated, nonsporulating, nonmotile rods or coccobacilli that can persist intracellularly. The genus Brucella includes four major clinically relevant species: B. melitensis (acquired by humans most commonly from sheep, goats, and camels), B. suis (from swine), B. abortus (from cattle or buffalo), and B. canis (from dogs).

EPIDEMIOLOGY

Brucellosis is transmitted via ingestion, inhalation, or mucosal or percutaneous exposure; the disease in humans is usually associated with exposure to infected animals or their products in either occupational settings (e.g., slaughterhouse work, farming) or domestic settings (e.g., consumption of contaminated foods, especially dairy products). The global prevalence of brucellosis is unknown because of difficulties in diagnosis and inadequacies in reporting systems.

CLINICAL MANIFESTATIONS

Regardless of the specific infecting species, brucellosis often presents with one of three patterns: a febrile illness similar to but less severe than typhoid fever; fever and acute monoarthritis, typically of the hip or knee, in a young child (septic arthritis); or long-lasting fever, misery, and low-back or hip pain in an older man (vertebral osteomyelitis).

  • An incubation period of 1 week to several months is followed by the development of undulating fever; sweats; increasing apathy and fatigue; and nonspecific symptoms such as anorexia, headache, myalgias, and chills.
  • Brucella infection can cause lymphadenopathy, hepatosplenomegaly, epididymoorchitis, neurologic involvement, and focal abscess.
  • Given the persistent fever and similar symptoms, tuberculosis is the most important differential diagnosis (Table 94-1).
TABLE 94-1: Radiology of the Spine: Differentiation of Brucellosis from Tuberculosis
 BRUCELLOSISTUBERCULOSIS
SiteLumbar and othersDorsolumbar
VertebraeMultiple or contiguousContiguous
DiskitisLateEarly
BodyIntact until lateMorphology lost early
Canal compressionRareCommon
EpiphysitisAnterosuperior (Pom’s sign)General: upper and lower disk regions, central, subperiosteal
OsteophyteAnterolateral (parrot beak)Unusual
DeformityWedging uncommonAnterior wedge, gibbus
RecoverySclerosis, whole-bodyVariable
Paravertebral abscessSmall, well-localizedCommon and discrete loss, transverse process
Psoas abscessRareMore likely

DIAGNOSIS

Laboratory personnel must be alerted to the potential diagnosis to ensure that they take precautions to prevent occupational exposure.

  • The organism is successfully cultured in 50–70% of cases. Cultures using the BACTEC system usually become positive in 7–10 days and can be deemed negative at 3 weeks.
  • PCR analysis of blood or tissue samples is more sensitive, faster, and safer than culture.
  • Agglutination assays for IgM are positive early in infection. Single titers of ≥1:160 and ≥1:320 are diagnostic in nonendemic and endemic areas, respectively.

Treatment: Brucellosis

  • The recommended regimen is streptomycin at a dosage of 0.75–1 g/d IM (or gentamicin at 5–6 mg/kg qd) for 14–21 days plus doxycycline at 100 mg bid for 6 weeks.
    • Rifampin (600–900 mg/d) plus doxycycline (100 mg bid) for 6 weeks constitute an alternative regimen (the current World Health Organization [WHO] recommendation).
    • Significant neurologic disease requires at least 3–6 months of treatment, with ceftriaxone supplementation of a standard regimen.
    • Endocarditis requires a four-drug regimen (an aminoglycoside, rifampin, a tetracycline, and ceftriaxone or a fluoroquinolone) for at least 6 weeks.
    • Relapse rates range from 5% to >20% and depend on the specific antibiotic regimen used; pts should be monitored for at least 2 years.

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