Brucellosis in Humans - A Complete Overview

Definition and Historical Names

• Goldman-Cecil Medicine, Chapter 286
• Harrison's Principles of Internal Medicine 22E (2025)

The Pathogen

• Goldman-Cecil Medicine

Epidemiology

• More than 500,000 cases reported yearly to the WHO from 100 countries; the true incidence is estimated at 10-25 times higher, with recent estimates suggesting potentially >2 million infections per year globally (Harrison's 2025).
• B. melitensis accounts for the most diagnosed cases; it infects free-range sheep and goats which are difficult to vaccinate.
• Endemic regions: Mediterranean basin, Latin America, Arabian Gulf, China, Indian subcontinent, parts of Africa.
• In the United States: Control programs reduced cases from >6,000/year in 1947 to ~80-120 cases annually, mostly in Texas, California, Arizona, and Florida. Most US cases involve meat processing (especially "kill areas"), contact with feral swine, or imported unpasteurized dairy products from Mexico.
• At-risk groups: slaughterhouse workers, farmers, dairy workers, veterinarians, laboratory workers, feral pig hunters, military personnel, travelers.
• Incidence in Europe inversely correlates with GDP - disease is linked to rural poverty and inadequate access to medical care.
• Childhood brucellosis (mostly school-aged) accounts for 3-10% of all cases worldwide.
• B. canis incidence is increasing in North America and Europe, often linked to imported dogs.

Transmission Routes

1. Ingestion - most common: unpasteurized dairy products (soft cheeses, milk, ice cream), undercooked meat, raw bone marrow.
2. Inhalation - occupational (slaughterhouse workers, laboratory workers); major route for bioterrorism scenarios.
3. Direct contact/percutaneous - conjunctival, skin inoculation from infected animal fluids/tissues or secretions.
4. Accidental vaccine inoculation - live attenuated strains S19, RB51 (B. abortus) and Rev 1 (B. melitensis).
5. Human-to-human - rare but documented: sexual transmission, breastfeeding, blood transfusion, bone marrow transplantation, nosocomial, perinatal vertical transmission.

Pathogenesis

1. Brucella crosses intestinal mucosa or respiratory epithelium and triggers a delayed inflammatory response (macrophages and dendritic cells dominant).
2. Granulomas (lymphocytes + infected macrophages + dendritic cells) form.
3. Phagocytosed organisms spread to regional lymph nodes.
4. If defenses are overwhelmed: bacteremia (incubation 2-4 weeks).
5. Free organisms are phagocytosed by macrophages primarily in the spleen, liver, and bone marrow, where small noncaseating granulomas persist as reservoirs.

Immune Evasion Mechanisms

• LPS differs from most gram-negatives: poor TLR4 activation and complement resistance.
• Deploys a protein that interferes with TLR signaling.
• ~10% of organisms in phagosomes survive acidification and lysosome fusion, forming a Brucella-containing vacuole (BCV).
• Intercepts ER-Golgi trafficking to replicate intracellularly.
• Outer membrane proteins Omp25/Omp31 mediate virulence.
• Key virulence factors: catalase, superoxide dismutase, alkyl hydroperoxide reductase, cytochrome oxidase, nitric oxide reductase, BvfA, Nramp1.
• Infected neutrophils may act as "Trojan horses" to spread infection.

Immunology

• Cell-mediated immunity (Th1) is predominant: IFN-γ, TNF-α, IL-1, IL-12 activate macrophages to kill Brucella.
• Antibody plays a peripheral role but is key for serology.
• Inflammation (not toxins or proteases) drives tissue damage - particularly in osteoarticular sites.
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Clinical Classification

Clinical Manifestations

Strain-Specific Differences

• B. abortus: usually mild to moderate sporadic disease.
• B. suis and B. melitensis: suppurative or disabling complications; prolonged course.
• B. canis: insidious onset, frequent relapses, chronic but relatively mild.

Acute Illness (most common presentation)

• Fever (characteristically undulant - waves separated by afebrile periods)
• Drenching night sweats with offensive odor
• Malaise, fatigue, weakness
• Arthralgia and myalgia
• Headache
• Anorexia and weight loss

Focal/Localized Complications

• Sacroiliitis - most frequent joint manifestation; often in younger patients
• Peripheral arthritis - large joints (hip, knee, shoulder)
• Spondylitis - especially lumbar spine; older patients; may cause cord compression (can mimic tuberculosis osteomyelitis)
• Osteomyelitis

• Meningitis (often chronic lymphocytic meningitis)
• Encephalitis, myelitis, radiculopathy, cranial nerve palsies
• Pseudotumor cerebri
• Brain abscesses

• Endocarditis - most common cause of death; B. melitensis most frequently implicated; often requires combined medical + surgical treatment

• Epididymo-orchitis - most common genitourinary complication in men; typically unilateral
• Prostatitis
• Glomerulonephritis (rare)
• In pregnancy: placental involvement, spontaneous abortion (especially B. melitensis)

• Hepatitis (very common, mild elevation of transaminases)
• Hepatic abscesses (rare, usually B. suis)
• Granulomatous hepatitis

• Pneumonia, pleuritis, lung abscess, hilar lymphadenopathy

• Pancytopenia (secondary to hypersplenism or bone marrow granulomas)
• Hemolytic anemia, thrombocytopenia

• Uveitis (granulomatous), optic neuritis, retinal detachment

Diagnosis

Laboratory Non-specific Findings

• Leukopenia (characteristic), lymphocytosis, anemia, thrombocytopenia
• Elevated ESR, CRP
• Elevated liver enzymes (mild)
• CSF in neurobrucellosis: lymphocytic pleocytosis, elevated protein, low glucose

Definitive Diagnosis: Culture

• Blood culture is gold standard (positive in 15-70% of acute cases).
• Slow-growing organism - may take 1-4 weeks; automated systems (BACTEC) improve detection.
• Cultures must be held for at least 4 weeks.
• Biosafety level 3 (BSL-3) precautions required - laboratory workers at significant risk.
• Bone marrow culture is more sensitive than blood culture (~90% vs ~70%) for disseminated disease.

Molecular Diagnosis: PCR

• Blood-based PCR is more sensitive and quicker than culture and avoids biohazard risk.
• Detects bacteremia; may predict relapse.
• Targets: spacer region between 16S and 23S rRNA genes (rrs-rrI), outer membrane protein-encoding genes, insertion sequence IS711, protein BCSP31.
• No single standardized procedure adopted globally.
• Limitation: prolonged PCR positivity after successful treatment is common and of unclear clinical significance.

Serology

• Often the only positive finding, especially in chronic disease.
• Standard Agglutination Test (SAT) (and microagglutination) - mainstay of diagnosis.
  • In endemic areas: titers ≥1:320-1:640 are diagnostic.
  • In non-endemic areas: titer ≥1:160 is significant.
  • In acute bacteremia in endemic settings: >90% of patients have SAT titer ≥1:320.
• Rose Bengal test - rapid screening but only partially validated for human diagnostic use.
• Additional tests: complement fixation, Coombs' antiglobulin test, ELISA.
• Antibody kinetics: IgM appears early; IgG and IgA follow. As disease progresses, IgM declines; IgG/IgA detectable by Coombs' or ELISA when agglutinin titers are undetectable.

Cross-Reactions (false positives)

• Yersinia enterocolitica O:9
• E. coli O157
• Francisella tularensis
• Salmonella group N
• Stenotrophomonas maltophilia
• Vibrio cholerae

Special Diagnostic Notes

• Rough strains (B. canis, B. ovis) give false-negative results in standard serologic tests.
• B. abortus vaccine strain RB51 does not elicit antibody in tests using smooth antigens - serology is unreliable after accidental RB51 exposure.
• Harrison's Principles of Internal Medicine 22E (2025)

Treatment

Principles

• Combination therapy required - monotherapy leads to high relapse rates.
• Prolonged courses needed due to intracellular localization.

First-Line Regimen (Adults - Non-focal Disease)

• The injectable regimen (doxycycline + streptomycin) has lower relapse rates than doxycycline + rifampin, based on clinical trials and observational studies.
• Gentamicin can substitute for streptomycin when streptomycin is unavailable.

Special Circumstances

Other Antibiotics

• Ciprofloxacin and TMP-SMX are used in combination regimens.
• Beta-lactams: may be active in vitro but poor clinical response (likely due to intracellular failure of penetration).
• Fluoroquinolone monotherapy - inadequate.

Monitoring and Relapse

• Relapse occurs in 5-15% of cases, usually within the first year.
• Relapse is typically due to inadequate treatment duration or poor compliance rather than antibiotic resistance.
• PCR positivity post-treatment is common but of uncertain significance.

Recent Evidence (PubMed 2024-2026)

1. Huang S et al. (2024) - Network meta-analysis of RCTs on treatment of human brucellosis [PMID: 39172763] - PLoS Neglected Tropical Diseases
2. Maduranga S et al. (2024) - Systematic review and meta-analysis of comparative antibiotic studies [PMID: 39152180] - Scientific Reports
3. Lanza Galvão E et al. (2024) - Systematic review on treatment of childhood brucellosis [PMID: 38754009] - Pediatric Infectious Diseases Journal

Bioterrorism Considerations

• High infectivity via aerosol (very low infectious dose, ~10-100 organisms)
• Long-term environmental viability
• Incapacitating disease with long incubation period
• Historically weaponized by several countries

Prevention

• Pasteurization of dairy products is the single most effective public health intervention.
• Animal vaccination programs (B. abortus S19/RB51 in cattle; Rev 1 in sheep/goats).
• Occupational precautions: PPE for veterinarians, slaughterhouse workers, laboratory staff.
• No approved human vaccine currently available (a major unmet need).
• Post-exposure prophylaxis (after significant occupational or lab exposure): doxycycline + rifampin for 3-6 weeks; expert consultation recommended.
• Laboratory handling requires BSL-3 containment.

Prognosis

• Mortality is low (<1%) with appropriate treatment.
• Endocarditis carries higher mortality and often requires valve surgery.
• Untreated or inadequately treated disease can persist for months to years.
• Chronic disease (symptoms >1 year) is controversial and often represents psychosomatic sequelae, reinfection, or another diagnosis rather than active infection.
• Recovery may take weeks to several months depending on severity and timing of treatment.
• No evidence for increased severity in HIV-infected or otherwise immunocompromised individuals.

• Goldman-Cecil Medicine (International Edition), Chapter 286 - Brucellosis
• Harrison's Principles of Internal Medicine 22E (2025, McGraw-Hill), Chapter on Brucellosis
• Fitzpatrick's Dermatology (9e) - Brucellosis section
• Sherris & Ryan's Medical Microbiology (8e)
• Adams and Victor's Principles of Neurology (12e)
• PubMed 2024: PMIDs 39172763, 39152180, 38754009 (systematic reviews on treatment)
• CDC Clinical Overview of Brucellosis (cdc.gov)