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Bovine Tuberculosis

Tuberculosis is a contagious disease of both animals and humans and can be caused by 3 types of the Mycobacterium bacteria. Bovine TB, caused by Mycobacterium bovis, primarily affects cattle and other bovine-like animals (e.g., bison, deer, and goats) but it can be transmitted to humans and other animals.

Transmission between deer and cattle can occur via either direct or indirect means. Direct transmission would be something like nose-to-nose contact. However, studies have found that TB is more often transmitted from deer to cattle through contaminated feed (hay bales, feed troughs, and bait/feed piles) and other indi¬rect routes, rather than by direct contact.

White-tailed deer are infected when they inhale or ingest infectious organisms. Transmission is aided by high deer density and prolonged contact, as occurs at supplemental feeding sites. Disease organisms commonly invade the tonsils first, later spreading to other cranial lymph nodes. If the infection is contained, it spreads no further. However, in some animals the infection spreads and the bacteria can then be transmitted by aerosol or oral secretions, milk, and feces.

The significance of bovine TB is reflected in the efforts to eradicate it from the United States since 1917. By the mid-1990s, only a few known infected cattle herds remained, and it looked like the disease would be eradicated in the United States. However, between 1975 and 1998, TB was increasingly documented in Michigan white-tailed deer, and scientific evidence suggested that deer had transmit¬ted the disease to cattle (Schmitt et al. 2002). Consequently, Michigan lost its "Accredited-Free Status," which allows for unrestricted interstate move¬ment of cattle and large eco¬nomic costs were incurred by the state and the livestock industry. It has been estimated that Michi¬gan will incur losses of $22-74 million over a 5-year period.

Measures implemented in Michigan in an interagency attempt to slow the spread of the disease include more testing of cattle herds, depopula¬tion of infected cattle herds, liberal hunting seasons to reduce deer density, restrictions on artificial feeding of deer to avoid concentrating deer in small areas where disease transmission is more likely, and the depopulation of a large, private herd of captive cervids (mostly white¬-tailed deer).

Current research efforts are aimed at understanding the role of wildlife as reservoirs and vectors of the disease. Other studies are documenting the prevalence of the disease in other wildlife species i.e., raccoons, opossums, gray foxes, and coyotes. Coyotes have an apparent high prevalence of infection with TB, nearly 25%; whereas, the other species are less than 4%. Coyotes, therefore, may serve as a good indicator species for the disease because they apparently magnify the infection rate found in Michigan deer which average about 2%.

To help control the disease biologists are attempting to reduce the indirect contact between deer and cattle in the TB infected area in Michigan. Those studies involve the use of fencing, guard dogs, and various scare devises aimed at keeping deer away from cattle and their feed, such as haystacks and silage. Some of these experiments have shown success, including the use of fences around cattle feed sources and the use of dogs to keep deer away from the cattle farms.

Summary above taken from:

MCLEAN, R. G., L. CLARK, M. R. DUNBAR, K. C. VERCAUTEREN, AND T. A. CAMPBELL. 2005. Wildlife disease research at the APHIS National Wildlife Research Center. Pages 123-135 in Proceedings of the One Hundred and Eighth Annual Meeting of the United States Animal Health Association.

USDA/APHIS. 2002. Environmental assessment: an integrated wildlife damage management approach for the management of white-tailed deer damage in the state of Michigan as conducted by USDA-Wildlife Services. USDA/APHIS/Wildlife Services, Okemos, Michigan.

Literature Cited

SCHMITT, S.M., D.J. O'BRIEN, C.S. BRUNING-FANN, AND S.D. FITZGERALD. 2002. Bovine tuberculosis in Michigan wildlife and livestock. Annals of the New York Academy of Sciences. 969: 262-268.