(Bang's disease, contagious abortion, Malta fever, undulant fever)
Brucellosis is a highly contagious disease of many animals and is caused by bacteria of the genus Brucella. Brucella abortus most commonly affects cattle; B. Suis is most common in swine; B. melitensis is most common in goats. Although infections may occur in other species, including man, such situations are rare. B. abortus is the species of most concern to wildlife workers.
Brucellosis has been reported throughout the world since its discovery in the latter part of the nineteenth century, and has long been considered an important disease in man, cattle, goats and swine. It has been found in bison, elk, moose, Dall sheep, caribou and several species of deer. It has been diagnosed in dogs, foxes, hares, mice, rats, ticks and fleas. The disease is relatively rare in deer in the U.S., slightly more significant in elk, bison and moose. Hares in Europe have been afflicted with the disease for many years, but it is not felt that they play an important part in maintaining brucellosis in domestic animals. In Michigan, brucellosis has never been identified in any wildlife species.
Transmission and Development
Brucella infections are usually transmitted by oral exposure, but susceptible animals can also be infected by contamination of the eyes, wounds and the genital tract. Males can transmit brucellosis during copulation, either by contaminated semen, or by genitals contaminated by an infected female.
Edible tissues of infected animals can infect humans and predators. Several species are known to spread Brucella organisms in their feces and urine. Milk from infected cattle and goats is a potential source of infection. Within animal populations, aborted fetuses, vaginal discharges and drainage from abscesses are all likely means of disseminating the disease. As bison constitute a potential threat to the bovine brucellosis eradication program in the U.S., they are subject to federal interstate regulations similar to those affecting cattle. In Alaska and Siberia, reindeer seem to be affected more disastrously by brucellosis than other wild species.
During bacteremic stages of infection, blood-sucking parasites may serve as vectors. The role of ticks, fleas and other parasites can only be conjectured; but the fact that they become contaminated establishes their role as potential vectors.
In many areas of the U.S., white-tailed deer (Odocoileus virginianus) are known to associate with cattle. Yet, of over 17,000 white-tailed and mule deer (O. hemionus) tested, only 20 white-tailed deer reacted positively to Brucella tests. No reactors were found in over 1,200 white-tailed deer in Michigan. Also, of 435 miscellaneous wild birds and mammals representing 23 species from southern Michigan, all were negative. It is generally felt that Brucella organisms are not readily transmissible from their preferential host to dissimilar hosts, and that no serious or threatening reservoir of infection exists presently in wild animals in the U.S.
Brucellosis affects many different organs in animals and consequently the signs of the disease will be influenced by the nature and extent of the infection and the species involved. Some infected animals may not show signs, yet will shed organisms in their feces and urine. In infected ruminants, brucellosis commonly induces abortion during the latter half of gestation. Calves are often born immature and weak. Inflammation of the uterus and excessive vaginal discharge are common signs which often result in reproductive failure or breeding difficulties in cattle and possibly in reindeer. Chronic infections of bones and joints occur in livestock and reindeer, resulting in lameness and abscesses. No significant signs of brucellosis have been reported in deer, moose or elk.
Brucella infection in ruminants usually localizes in the pregnant uterus and associated tissues. Abortions usually occur late in the gestation period and are followed by retained fetal membranes. Carpal bursitis is the most common condition associated with brucellosis in Siberian reindeer.
The common method for diagnosing brucellosis is the plate agglutination test. This is accomplished by mixing a drop of serum from the suspect animal with a drop of Brucella antigen. Clumping indicates infection. If the mixture remains clear, the result is negative.
Treatment and Control
Antibiotics have been used with some success to reduce the severity of brucellosis in man and animals. In cattle, a national eradication program in the U.S. has been quite successful. This, coupled with an extensive vaccination program, has practically eliminated the threat of serious epidemics of brucellosis in cattle.
Of wild animals, only reindeer and bison warrant serious consideration for treatment, the former, for reasons of economy in reindeer husbandry, the latter as a threat to the success of the bovine brucellosis eradication program in the U.S.
Before a nation-wide eradication program was instituted in the U.S. in the 1940's, brucellosis was of major economic significance to the entire dairy industry. Pasteurization of milk practically eliminated the disease (undulant fever) in man.
It is generally agreed that, rather than wildlife posing a threat to domestic species, the reverse is true: wildlife are more likely to acquire the disease from domestic species. Regardless, the incidence of brucellosis in wildlife is extremely low, and is of little significance in the population dynamics of any wild species.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.