Brucellosis is a bacterial zoonosis transmitted directly or indirectly to humans from infected animals, predominantly domesticated ruminants and swine. The disease is known colloquially as undulant fever because of its remittent character. Its distribution is worldwide apart from the few countries where it has been eradicated from the animal reservoir. Although brucellosis commonly presents as an acute febrile illness, its clinical manifestations vary widely, and definitive signs indicative of the diagnosis may be lacking. Thus the clinical diagnosis usually must be supported by the results of bacteriologic and/or serologic tests.
Human brucellosis is caused by strains of Brucella, a bacterial genus that was previously suggested, on genetic grounds, to comprise a single species, B. melitensis, with a number of biologic variants exhibiting particular host preferences. This view was challenged on the basis of detailed differences in chromosomal structure and host preference. The traditional classification into nomen species is now favored both because of these differences and because this classification scheme closely reflects the epidemiologic patterns of the infection. The nomen system recognizes B. melitensis, which is the most common cause of symptomatic disease in humans and for which the main sources are sheep, goats, and camels; B. abortus, which is usually acquired from cattle or buffalo: B. suis, which generally is acquired from swine but has one variant enzootic in reindeer and caribou and another in rodents; and B. canis, which is acquired most often from dogs. B. ovis, which causes reproductive disease in sheep, and B. neotomae, which is specific for desert rodents, have not been clearly implicated in human disease. Other brucellae have been isolated from marine mammals, and two new nomen species, B. ceti sp. nov. and B. pinnipedialis sp. nov., have been proposed for these isolates; at least one case of laboratory-acquired human disease due to one of these proposed species has been described, and apparent cases of natural human infection have been reported. As infections in marine mammals seem widespread, more cases of zoonotic infection may be identified. Other newly proposed species include B. microti sp. nov. isolated from field voles and B. inopinata sp. nov. isolated from a patient with a breast implant. Moreover, it has become apparent that Brucella is closely related to the genus Ochrobactrum, which includes environmental bacteria sometimes associated with opportunistic infections.
All brucellae are small, gram-negative, unencapsulated, nonsporulating, nonmotile rods or coccobacilli. They grow aerobically on peptone-based medium incubated at 37°C; the growth of some types is improved by supplementary CO2. In vivo, brucellae behave as facultative intracellular parasites. The organisms are sensitive to sunlight, ionizing radiation, and moderate heat; they are killed by boiling and pasteurization but are resistant to freezing and drying. Their resistance to drying renders brucellae stable in aerosol form, facilitating airborne transmission. The organisms can survive for up to 2 months in soft cheeses made from goat's or sheep's milk; for at least 6 weeks in dry soil contaminated with infected urine, vaginal discharge, or placental or fetal tissues; and for at least 6 months in damp soil or liquid manure kept under cool dark conditions. Brucellae are easily killed by a wide range of common disinfectants used under optimal conditions but are likely to be much more resistant at low temperatures or in the presence of heavy organic contamination.
Brucellosis is a zoonosis whose occurrence is closely related to its prevalence in domesticated animals. The true global prevalence of human brucellosis is unknown because of the imprecision of diagnosis and the inadequacy of reporting and surveillance systems in many countries. Even in developed countries, the true incidence may be 10–20 times higher than the reported figures. Bovine brucellosis has been the target of control programs in many parts of the world and has been eradicated from the cattle populations of Australia, New Zealand, Bulgaria, Canada, Cyprus, Great Britain (including the Channel Islands), Japan, Luxembourg, Romania, the Scandinavian countries, Switzerland, and the Czech and Slovak Republics, among other nations. Its incidence has been reduced to a low level in the United States and most Western European countries, with a varied picture in other parts of the world. There is evidence of a resurgence in Eastern Europe following economic changes in recent years, and outbreaks have also occurred in Ireland. Efforts to eradicate B. melitensis infection from sheep and goat populations have been much less successful. These efforts have relied heavily on vaccination programs, which have tended to fluctuate with changing economic and political conditions. In some countries (e.g., Israel), B. melitensis has caused serious outbreaks in cattle. Infections with B. melitensis still pose a major public health problem in Mediterranean countries; in western, central, and southern Asia; and in parts of Africa and South and Central America.
Human brucellosis is usually associated with occupational or domestic exposure to infected animals or their products. Farmers, shepherds, goatherds, veterinarians, and employees in slaughterhouses and meat-processing plants in endemic areas are occupationally exposed to infection. Family members of individuals involved in animal husbandry may be at risk, although it is often difficult to differentiate food-borne infection from environmental contamination under these circumstances. Laboratory workers who handle cultures or infected samples are also at risk. Travelers and urban residents usually acquire the infection through consumption of contaminated foods. In countries that have eradicated the disease, new cases are most commonly acquired abroad. Dairy products, especially soft cheeses, unpasteurized milk, and ice cream, are the most frequently implicated sources of infection; raw meat and bone marrow may be sources under exceptional circumstances. Infections acquired through cosmetic treatments using materials of fetal origin have been reported. Person-to-person transmission is extremely rare, as is transfer of infection by blood or tissue donation. Although brucellosis is a chronic intracellular infection, there is no evidence for increased prevalence or severity among individuals with HIV ...