Brucellosis¶
Chapter 174 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Brucellosis is a zoonotic bacterial infection caused by Brucella species, transmitted via contact with infected animals or their products, presenting as undulant fever with variable clinical manifestations.
- Key diagnostic features include undulating fever, musculoskeletal symptoms, and radiographic findings differentiating it from tuberculosis (e.g., late diskitis, noncaseating granulomas).
- Treatment requires prolonged antibiotic therapy (6–12 weeks) with combinations of streptomycin/doxycycline, rifampin/doxycycline, or gentamicin/doxycycline, with strict adherence to prevent relapse.
- Complications include endocarditis, osteomyelitis, neurological involvement, and septic arthritis, necessitating early differentiation from tuberculosis and other infections.
- Serologic tests (agglutination, PCR) and imaging (isotope scans) are critical for diagnosis, with culture confirmation being definitive but challenging due to Brucella's intracellular survival.
1. DEFINITION & OVERVIEW¶
Brucellosis is a bacterial zoonosis transmitted directly or indirectly from infected animals (primarily ruminants and swine) to humans. It is characterized by undulant fever due to its remittent fever pattern. The disease presents variably, ranging from acute febrile illness to chronic, systemic infection. Clinical diagnosis requires support from bacteriologic/serologic tests due to nonspecific symptoms.
Table 174-1 Radiology of the Spine: Differentiation of Brucellosis from Tuberculosis¶
| Feature | Brucellosis | Tuberculosis |
|---|---|---|
| Site | Lumbar and others | Dorsolumbar |
| Diskitis | Late | Early |
| Vertebrae | Multiple or contiguous | Contiguous |
| Body Integrity | Intact until late | Morphology lost early |
| Canal Compression | Rare | Common |
| Osteophyte | Anterolateral (parrot beak) | Unusual |
| Deformity | Wedging uncommon | Anterior wedge, gibbus |
| Recovery | Sclerosis, whole-body | Variable |
| Psoas Abscess | Rare | More likely |
1.1 Etiologic Agents¶
Brucellosis is caused by Brucella species, with B. melitensis (sheep/goats/camels), B. abortus (cattle/buffalo), B. suis (swine), and B. canis (dogs) being the most common human pathogens. Other species like B. neotomae, B. ceti, and B. pinnipedialis are less frequently implicated.
1.2 Transmission¶
Transmission occurs via inhalation, ingestion of unpasteurized dairy products, or direct contact with infected animal tissues. Vaccination with live Brucella strains (e.g., S19, RB51) can cause disease in humans.
2. EPIDEMIOLOGY¶
Brucellosis is a global zoonosis with high prevalence in rural areas and regions with poor veterinary control. Annual cases exceed 2 million worldwide. Risk factors include occupational exposure (farmers, veterinarians), consumption of unpasteurized dairy, and travel to endemic regions. Incidence inversely correlates with GDP, with higher rates in low-resource settings.
2.1 Demographics¶
Common in Mediterranean, Middle Eastern, and South/Central American regions. Endemic in countries like India, Pakistan, China, and parts of Africa. Mortality is 50% in severe cases due to sepsis.
2.2 Transmission¶
Primary routes include ingestion of contaminated food (soft cheeses, unpasteurized milk), inhalation, and direct contact with infected animal tissues. V. vulnificus infections are associated with seawater exposure and open wounds.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Brucella species evade host immunity through intracellular survival, immune suppression, and antigenic variation. Key mechanisms include: (1) inhibition of phagosome-lysosome fusion, (2) suppression of TNF- α and IFN- γ responses, (3) resistance to reactive oxygen/nitrogen intermediates, and (4) type IV secretion system-mediated intracellular replication.
3.1 Immune Evasion¶
Brucella LPS (smooth variant) has low endotoxic activity, preventing innate immune activation. Virulent strains suppress macrophage activation and resist apoptosis. Type IV secretion system (Vir) facilitates intracellular survival and trafficking.
3.2 Pathogenesis¶
Infection begins with phagocytosis by neutrophils/macrophages, followed by intracellular replication in endoplasmic reticulum. Chronic infection leads to granuloma formation, abscesses, and systemic dissemination to bones, joints, and organs.
4. CLINICAL FEATURES¶
Classic presentation includes undulating fever, musculoskeletal pain, and fatigue. Complications include septic arthritis, osteomyelitis, endocarditis, and neurological involvement. Variants include acute febrile illness, monoarthritis, and chronic spinal disease.
4.1 Systemic Symptoms¶
Fever (often with night sweats), fatigue, weight loss, and nonspecific myalgia. Leukopenia, thrombocytopenia, and elevated liver enzymes are common.
4.2 Musculoskeletal Involvement¶
Arthritis (knee, hip, sacroiliac), osteomyelitis (lumbar spine), and psoas abscesses. Spinal involvement mimics tuberculosis with late diskitis and vertebral sclerosis.
4.3 Neurological Complications¶
Depression, meningitis, encephalitis, and mycotic aneurysms. Neurological symptoms may mimic tuberculosis or other infections.
5. DIFFERENTIAL DIAGNOSIS¶
Differentiate from tuberculosis (radiographic patterns, response to antibiotics), typhoid fever, malaria, and other intracellular infections. Consider brucellosis in patients with undulant fever, musculoskeletal pain, and history of animal exposure.
5.1 Spinal Infections¶
Brucellosis vs. tuberculosis: Brucellosis shows late diskitis, noncaseating granulomas, and minimal canal compression. Tuberculosis presents with early diskitis and caseating granulomas.
5.2 Other Infections¶
Distinguish from leptospirosis, viral infections (e.g., HIV, CMV), and fungal infections. Brucellosis is often misdiagnosed as glandular fever or malaria in endemic regions.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis relies on clinical suspicion, serology, PCR, and imaging. Blood cultures and tissue biopsies are definitive but challenging. Radiology and isotope scans aid in spinal and bone involvement.
6.1 Laboratory Tests¶
Serology (agglutination, complement fixation, ELISA), PCR for brucella DNA, and blood cultures. Isolation from blood, CSF, or bone marrow confirms infection.
6.2 Imaging¶
X-ray and isotope scans detect late bony changes. MRI/CT identify spinal involvement. Contrast-enhanced imaging may reveal abscesses or granulomas.
7. MANAGEMENT & TREATMENT¶
Combination antibiotic therapy (6–12 weeks) is essential. First-line regimens include streptomycin/doxycycline or rifampin/doxycycline. Alternative regimens (gentamicin/doxycycline) are used in specific populations. Surgical debridement is required for abscesses or osteomyelitis.
7.1 Antibiotic Regimens¶
Standard: Streptomycin (0.75–1 g/day) + doxycycline (100 mg BID) for 2–3 months. Alternatives: Rifampin (600–900 mg/day) + doxycycline. Gentamicin (5–6 mg/kg/day) + doxycycline for 1–2 weeks.
7.2 Special Populations¶
Avoid tetracyclines in children/pregnant women; use TMP-SMX or rifampin. Monitor for drug resistance and adjust therapy for immunocompromised patients.
8. PROGNOSIS & COMPLICATIONS¶
Mortality is 50% in severe sepsis. Complications include endocarditis, osteomyelitis, neurological deficits, and chronic fatigue. Poor adherence to therapy leads to relapse (10–20% in nontrial settings).
8.1 Long-Term Outcomes¶
Chronic fatigue, arthralgia, and persistent low-grade fever may occur. Relapse is common with inadequate or non-compliant treatment.
8.2 Public Health¶
Control programs in livestock reduce human incidence. Eradication efforts face challenges due to animal reservoirs and vaccine resistance.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Risk of fetal loss but no teratogenicity. Pediatrics: Infections often present as chronic arthritis or osteomyelitis. Immunocompromised patients require prolonged therapy and monitoring for drug resistance.
9.1 Occupational Exposure¶
Farmers, veterinarians, and slaughterhouse workers are at high risk. Use PPE and pasteurize dairy products to prevent transmission.
9.2 Laboratory Safety¶
Handle cultures under biosafety level 2 conditions. Brucella species are classified as BSL-2 pathogens due to their zoonotic potential.
10. KEY POINTS & CLINICAL PEARLS¶
- Brucellosis is a zoonotic infection with undulant fever and variable presentations.
- Diagnosis requires serology, PCR, and imaging to differentiate from tuberculosis.
- Treatment with combination antibiotics (6–12 weeks) is critical to prevent relapse.
- Spinal involvement mimics tuberculosis but responds better to antibiotics.
- Avoid tetracyclines in children/pregnant women; use rifampin or TMP-SMX as alternatives.