Bovine brucellosis, caused by the bacterium Brucella abortus, is an economically important cause of abortions in cattle. B. abortus also affects other species including bison, buffalo and elk; some species are maintenance hosts for this organism. Infections in wildlife can hinder eradication efforts in cattle. In addition, B. abortus is a human pathogen. In humans, brucellosis can be a serious, debilitating and sometimes chronic disease that may affect a variety of organs. Most cases are the result of occupational exposure to infected animals, but infections can also occur from ingesting contaminated dairy products. In addition, B. abortus could be used in a bioterrorist attack.
In cattle, bison and buffalo, brucellosis is mainly caused by Brucella abortus, a Gram-negative coccobacillus or short rod. This organism is a facultative intracellular pathogen. Up to nine B. abortus biovars (1-9) have been reported, but some of these biovars differ only slightly and their status is unresolved. Other Brucella species uncommonly associated with disease in cattle include Brucella melitensis and B. suis. (For information on B. suis or B. melitensis, see the factsheets titled “Porcine Brucellosis” and “Ovine and Caprine Brucellosis,” respectively.)
Genetic and immunological evidence suggests that all members of the genus Brucella are closely related, and some microbiologists have proposed that this genus be reclassified into a single species (B. melitensis), which contains many biovars. This proposal is controversial, and both taxonomic systems are currently in use. The multiple species nomenclature is used in this factsheet.
Most species of Brucella are primarily associated with certain hosts; however, infections can also occur in other species, particularly when they are kept in close contact. Maintenance hosts for Brucella abortus include cattle, bison (Bison spp.) water buffalo (Bubalus bubalus), African buffalo (Syncerus caffer), elk and camels. A feral pig population has recently been reported to maintain B. abortus. A variety of other species can become "spill-over" hosts where this organism is enzootic. B. abortus has been reported in horses, sheep, Rocky Mountain bighorn sheep, goats, chamois, pigs, raccoons, opossums, dogs, coyotes, foxes, wolves and other species. Moose and llamas can be infected experimentally.
B. abortus is found worldwide in cattle-raising regions, except in Japan, Canada, some European countries, Australia, New Zealand and Israel, where it has been eradicated. Eradication from domesticated herds is nearly complete in the U.S. B. abortus can be found in wildlife hosts in some regions, including the Greater Yellowstone Area of North America.
In animals, B. abortus is usually transmitted by contact with the placenta, fetus, fetal fluids and vaginal discharges from infected animals. Animals are infectious after either abortion or full-term parturition. B. abortus may also be found in the milk, urine, semen, feces and hygroma fluids. Shedding in milk can be prolonged or lifelong, and may be intermittent. Many infected cattle become chronic carriers.
Infection usually occurs by ingestion and through mucous membranes, but B. abortus can be transmitted through broken skin. Although the mammary gland is usually colonized during the course of an infection, it can also be infected by direct contact, with subsequent shedding of the organisms in the milk. In utero infections also occur. Venereal transmission seems to be uncommon. Transmission by artificial insemination is reported to occur when contaminated semen is deposited in the uterus but not in the midcervix. B. abortus can be spread on fomites including feed and water. In conditions of high humidity, low temperatures, and no sunlight, these organisms can remain viable for several months in water, aborted fetuses, manure, wool, hay, equipment and clothes. Brucella species can withstand drying, particularly when organic material is present and can survive in dust and soil. Survival islonger when the temperature is low, particularly when it is below freezing.
Other species can be infected with B. abortus after contact with infected cattle or other maintenance hosts. Carnivores do not seem to be a significant source of infection for other animals. Dogs and coyotes can be infected with B. abortus, shed bacteria in reproductive discharges, and can infect cattle if these species are kept in close confinement under experimental conditions. However, no confirmed cases of transmission from dogs to cattle have been reported under natural conditions, and there is no epidemiological evidence that carnivores act as a source of infection for ruminants in B. abortus eradication programs. Experimentally infected wolves excrete few organisms in the feces, and the number of organisms is much lower than the reported infective dose for cattle.
Humans usually become infected by ingesting organisms (including contaminated, unpasteurized dairy products) or by the contamination of mucous membranes and abraded skin.
In cattle, abortions and stillbirths usually occur two weeks to five months after infection. Reproductive losses typically occur during the second half of gestation; thus, the incubation period is longer when animals are infected early in gestation.
In cattle, B. abortus causes abortions and stillbirths; abortions usually occur during the second half of gestation. Some calves are born alive but weak, and may die soon after birth. The placenta may be retained and secondary metritis can occur. Lactation may be decreased. After the first abortion, subsequent pregnancies are generally normal; however, cows may shed the organism in milk and uterine discharges. Epididymitis, seminal vesiculitis, orchitis or testicular abscesses are sometimes seen in bulls. Infertility occurs occasionally in both sexes, due to metritis or orchitis/epididymitis. Hygromas, particularly on the leg joints, are a common symptom in some tropical countries. Arthritis can develop in some long-term infections. Systemic signs do not usually occur in uncomplicated infections, and deaths are rare except in the fetus or newborn. Infections in nonpregnant females are usually asymptomatic.
In camels, bison, water buffalo, bighorn sheep and other ruminants, the symptoms are similar to cattle. Abortions have also been reported in experimentally infected llamas. Other herbivores may develop more serious disease. Moose die rapidly in experimental infections. Two bighorn sheep rams with no apparent disease other than testicular lesions also died inexplicably, giving rise to speculation that B. abortus infections might sometimes be lethal in this species.
Symptomatic infections have been reported in some species of carnivores. Abortions, epididymitis, polyarthritis and other symptoms occur in some B. abortus-infected dogs. Experimentally infected wolves remained asymptomatic, although the organism could be recovered from lymphoreticular tissues for at least one year. Infected coyotes and foxes are also reported to be asymptomatic.
In horses, B. abortus can cause inflammation of the supraspinous or supra-atlantal bursa; these syndromes are known, respectively, as fistulous withers and poll evil. The bursal sac becomes distended by a clear, viscous, straw-colored exudate and develops a thickened wall. It can rupture, leading to secondary inflammation. In chronic cases, nearby ligaments and the dorsal vertebral spines may become necrotic. Brucella-associated abortions are rare in horses.