The agents are small, green, negative rod–shaped, non–motile, non–sporing and intracellular coccobacilli of the genus brucella. Four species infect humans viz. B Melitensis, B Abortus, B Duis and B Canis.
- B Melitensis is the most virulent and invasive species; it usually infects goats and occasionally, even sheep.
- Abortus is less virulent and is primarily a disease that afflicts cattle.
- B Duis is of intermediate virulence and chiefly infects pigs.
- B Canis is a parasite of dogs.
The main sources of human infection are cattle, sheep, goats, swine, buffaloes, horses and dogs. In animals, the disease can cause abortion, premature expulsion of the fetus or death. Cross–infections can often occur between animal species. The infected animals excrete Brucella in their urine, milk, placenta, uterine and vaginal discharges, particularly during birth or abortion. These animals may remain infected for life.
Human Brucellosis is predominantly a disease of adult males. Farmers, shepherds, butchers, and abattoir workers need to especially watch out. Veterinarians and laboratory workers are particularly at risk because of occupational exposure.
Brucellosis is most prevalent under conditions of advanced domestication of animals where there is an absence of correspondingly advanced standards of hygiene. Overcrowding of herds, high rainfall, lack of exposure to sunlight, unhygienic practices in milk and meat production, all favor the spread of Brucellosis. The infection can travel long distances in milk and dust. The environment of a cowshed may be heavily infected. The organism can survive for weeks or months in favorable conditions of water, urine, feces, damp soil and manure.