Bovine tuberculosis was recently discovered in a herd in Hutchinson County, S.D.
"While tuberculosis in U.S. cattle is very uncommon now compared to when eradication efforts began in 1917, the disease still shows up periodically," says Russ Daly, SDSU Extension veterinarian. "State officials investigated tuberculosis in two South Dakota herds in early 2010. As will be done with the current case, the 2010 cases were extensively investigated by the South Dakota Animal Industry Board. Because of quick, effective action on the part of state and federal officials, South Dakota was allowed to keep its 'TB-Accredited-Free' status."
Bovine tuberculosis is a long-term, slowly developing disease of cattle. Its incubation time ranges from months to years. Most often, infected cattle will show little to no outward signs of infection.
"When signs are present, they will often be vague, such as weight loss, depression, and sluggishness," Daly says.
Cases of bovine tuberculosis are usually diagnosed on the basis of tell-tale abnormalities found on slaughter inspection. Tuberculosis in live cattle is diagnosed using the caudal fold test, where a small dose of tuberculin is injected into the skin around the tail and subsequent swelling around the injection site indicates infection. When infected animals are identified, state and federal officials trace them back to their herd of origin and work to identify other infected herds that may be associated with the infected animal.
Transmission of tuberculosis between animals occurs when susceptible animals are in close contact with respiratory secretions or aerosols from infected animals. Close contact is necessary for transmission; the infection is not considered to spread easily between cattle separated by any distance.
Tuberculosis also occurs in people, however the bacteria that usually infects humans is Mycobacterium tuberculosis, which is different than the bacteria causing bovine tuberculosis.
"Every year there are cases of human tuberculosis reported in South Dakota -- an average of 15 cases per year over the past five years. But these cases are due to the human bacteria, Mycobacterium tuberculosis, and have nothing to do with tuberculosis in animals," says Daly, who also serves as State Public Health Veterinarian.
Bovine tuberculosis can be transmitted to other species such as wildlife and humans, and as such, does present a slight public health concern. However, people would have to be in very close contact with infectious animals in order for transmission to occur.
"The general public is protected from bovine tuberculosis by the federal and state meat inspection system identifying tuberculosis in slaughtered animals. In addition, cooking and pasteurization will easily kill the bovine tuberculosis bacteria in meat and milk," he says.
Transmission of bovine tuberculosis to and among wildlife has been significant to the ecology of the disease in certain areas. For example, in Michigan, white-tailed deer are considered to be significant in the persistence of bovine tuberculosis in certain areas of that state. As with people and cattle, close contact is necessary for transmission.
Although surveillance for tuberculosis in wildlife in South Dakota has not shown the disease to be a problem in the state, hunters are always encouraged to look for abscesses or other suspicious lesions and report unusual findings to the South Dakota Game, Fish and Parks Department.
For more details about tuberculosis, refer to SDSU Veterinary Extension at www.sdstate.edu/vs/extension, or the South Dakota Animal Industry Board at http://aib.sd.gov.