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DEFINITION

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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. 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.

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ETIOLOGIC AGENTS

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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. Two new species, B. ceti and B. pinnipedialis, have recently been identified in marine mammals, including seals and dolphins. At least one case of laboratory-acquired human disease due to one of these species has been described, and apparent cases of natural human infection have been reported. As infections in marine mammals appear to be widespread, more cases of zoonotic infection in humans may be identified. Other newly reported species include B. microti (isolated from field voles) and B. inopinata (isolated from a patient with a breast implant). Additional novel strains have been described from diverse species, including baboons, foxes, frogs, and various rodents, and the genus likely will expand further in forthcoming years. Moreover, it has become apparent that Brucella is closely related to the genus Ochrobactrum, which includes environmental bacteria sometimes associated with opportunistic infections. Genomics-based studies are beginning to elucidate the pathway of evolution from free-living soil bacteria to highly successful intracellular pathogens.

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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 ...

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