Brucellosis is caused by gram-negative facultative intracellular coccobacilli of the genus Brucella, with the most clinically significant species being B. melitensis (sheep, goats; most virulent for humans), B. abortus (cattle), B. suis (pigs), and B. canis (dogs). The bacteria infect macrophages and persist intracellularly within the reticuloendothelial system, explaining the chronic and relapsing nature of the disease. Transmission occurs through ingestion of unpasteurized dairy products from infected animals (cheese, milk, yogurt), direct contact with infected animal tissues or fluids during occupational exposure, inhalation of aerosolized organisms, and accidental laboratory exposure. Person-to-person transmission is rare but has been documented through breast milk, sexual contact, and blood transfusion.
Clinical presentation is protean. Acute brucellosis presents with insidious or sudden onset of high undulant fever (classic feature), profound sweating with characteristic odor, fatigue, weight loss, anorexia, headache, arthralgias, myalgias, hepatosplenomegaly, and lymphadenopathy. Focal complications develop in 20-30% of untreated cases and include osteoarticular disease (sacroiliitis, spondylitis with vertebral collapse, peripheral arthritis—most common at 50-80%), neurobrucellosis (meningitis, encephalitis, cranial neuropathy), endocarditis (most common cause of brucellosis-related death), genitourinary involvement (orchitis, epididymitis), and hepatic abscess or granulomatous hepatitis. Chronic brucellosis is defined as illness >12 months and may present with relapsing fever or focal disease.
Diagnosis combines clinical suspicion with epidemiologic risk factors, serology (Rose Bengal as rapid screening, standard agglutination test ≥1:160, ELISA for IgG/IgM, Coombs test for chronic cases), blood culture using automated systems with prolonged incubation up to 21 days (sensitivity 50-70%), bone marrow culture (higher yield in chronic cases), and PCR (increasingly available). Treatment requires combination antibiotic therapy due to intracellular persistence and high relapse rates with monotherapy. Standard regimen for adults is doxycycline 100 mg twice daily for 6 weeks plus rifampin 600-900 mg daily for 6 weeks (oral, well-tolerated, recommended) OR doxycycline 6 weeks plus streptomycin 1 g IM daily for 14-21 days (slightly more effective but parenteral). Spondylitis, neurobrucellosis, and endocarditis require triple therapy and prolonged duration (3-6 months minimum), with surgical intervention often needed for endocarditis. Prevention focuses on dairy product pasteurization, animal vaccination programs, and occupational protection.