Brucellosis remains one of the world’s most widespread bacterial zoonoses, silently affecting millions across the globe. Caused by various Brucella species, primarily B. melitensis (goats/sheep), B. abortus (cattle), B. suis (swine), and B. canis (dogs), this disease infiltrates human populations through direct contact with infected animals or consumption of contaminated products.
The bacteria demonstrate remarkable adaptability, preferentially infecting reproductive systems of livestock and causing late-term abortions, placental infections, and reduced milk production. In humans, initial symptoms mimic common influenza: fever, joint pain, fatigue, and headache, often appearing anywhere from five days to several months after exposure.
Without proper treatment, approximately 5-10% of cases progress to chronic infections lasting years, potentially triggering severe complications like endocarditis (heart lining inflammation), spondylitis (spinal arthritis), or neurological disorders.
Global Burden and Regional Hotspots
The World Health Organization identifies brucellosis as endemic in over 170 countries, with approximately 500,000 human cases annually. Its distribution reveals striking geographic patterns:
- Mediterranean/Middle East: B. melitensis dominates, with Oman, Lebanon, and Kuwait showing exceptionally high human seroprevalence rates (up to 44% in some studies) linked to raw dairy consumption.
- Sub-Saharan Africa: Tanzania’s Kagera region reports 7.7% human seroprevalence among pastoralists, with cattle herds showing 18.2% infection rates.
- South Sudan: Recent studies in Central Equatoria State found 21.7% of cattle, 11.8% of goats, and 4.8% of pastoralists seropositive.
- Central Asia/China: Xinjiang’s Aksu Prefecture identifies environmental factors like vegetation density (NDVI) and temperature as key drivers, with incidence peaks following specific weather patterns.
Economic impacts are severe in endemic regions, costing low- and middle-income countries an estimated $110 billion annually through healthcare burdens and livestock productivity losses.
Transmission Pathways and Risk Factors
Human infection primarily occurs through three routes:
- Foodborne: Consuming unpasteurized dairy products (accounting for >70% of global cases) or undercooked meat from infected animals. Egyptian studies confirm homemade soft cheese consumption doubles infection risk (OR=2.3).
- Occupational Exposure: Farmers, butchers, and veterinarians face risk through contact with placental tissues, aborted fetuses, or animal blood. Tanzanian data shows assisting births without gloves increases odds fivefold (OR=5.6).
- Environmental Inhalation: Slaughterhouse workers and laboratory staff may inhale aerosolized bacteria.
Unique cultural practices amplify risks in certain regions. In South Sudan, pastoralists traditionally blow air into cows’ vaginas to stimulate milk letdown, a practice significantly associated with human seropositivity (OR=1.4). Similarly, Georgia’s sheep owners face 19-fold higher infection odds due to husbandry practices.
Diagnosis Challenges and Prevention Strategies
Diagnosing brucellosis remains problematic due to nonspecific symptoms and unreliable serological tests. While the gold standard involves blood culture, PCR and fluorescence polarization assays (FPA) show higher sensitivity (up to 99%). Control requires integrated One Health approaches:
- Animal Vaccination: Essential in enzootic areas, though B. melitensis vaccines remain less effective in free-ranging goats/sheep.
- Food Safety: Pasteurization of dairy products is singularly effective, regions enforcing sales bans see dramatic case reductions.
- Occupational Hygiene: Protective gear for birth assistance and carcass handling reduces exposure.
- Environmental Management: In China’s Aksu Prefecture, predictive models using temperature/humidity data help target interventions before seasonal outbreaks.
Molecular genotyping reveals alarming transmission dynamics: identical B. melitensis strains circulate between Egyptian farmers and their cattle, confirming bidirectional transmission. Similarly, Georgian studies show environmental persistence enables reinfection despite animal culling. These insights underscore that sustainable control requires combining veterinary interventions with community health education, especially in pastoral communities where disease awareness itself proves protective (OR=0.1 in Tanzania). As climate change expands conducive environments, global coordination through programs like WHO’s GLEWS will be critical for early warning and containment.
