Bovine Tuberculosis

This fact sheet has been updated on 1/30/20.

Cause

Tuberculosis (TB) is a zoonotic disease of the respiratory (or gastrointestinal tract, depending on the route of exposure) system. Of the three major types of TB (human, avian, and bovine), bovine TB has the greatest host range and is known to infect many species of mammals including humans. Bovine TB (BTB) is caused by the bacteria Mycobacterium bovis.

Significance

Humans can become infected with BTB by consuming contaminated cow milk or by handling contaminated tissues or materials from infected cattle. Pasteurization has significantly decreased the number of human cases of BTB in the United States, and the likelihood of humans contracting this disease in the US is currently remote (<2%), except in certain groups such as hunters, farmers, and slaughterhouse workers who are at greater risk.

In the past, bovine tuberculosis was the most prevalent disease of domestic cattle and swine in the United States and has caused major losses to the agricultural industry. A nationwide eradication program has nearly eliminated the disease from US domestic livestock and has also contributed to the large decline in the number of human cases.

Wild white-tailed deer are reservoir hosts for BTB. This means that white-tailed deer can carry and spread BTB to other individuals but also to other species, like cattle. Because white-tail deer move freely across the landscape they have the potential of contacting multiple cattle herds and pose a great threat to the establishment of BTB in cattle.

BTB in white-tail deer has been recorded in the state of New York in 1933, 1937 and 1961. One case of BTB in wild white-tailed deer was reported in Indiana in 2016. In Minnesota, BTB in wild deer was first detected in 2005 but after implementing an aggressive management program, BTB has not been detected in a wild deer since 2009. Currently, Michigan is the single focus of BTB in wildlife in the United States with the rate of occurrence at less than 2%.  

Species Affected

BTB infection has been reported in a wide variety of mammal species worldwide. Wildlife species susceptible to BTB include white-tailed deer, mule deer, elk, bison, black bears, bobcats, badgers, wild boar, coyotes, opossums, raccoons, and foxes. Many domestic animals can also be infected including cattle, swine, ferrets, and cats.

Humans can also be infected with BTB and according to the Centers for Disease Control and Prevention (CDC), BTB can be as dangerous as M. tuberculosis (the human type of TB). There have been 3 reported cases (all in Michigan) of hunters contracting BTB from wild deer while field dressing or processing their harvested deer.

Distribution

BTB has a worldwide distribution and is most prevalent in Africa and parts of Asia and the Americas. The disease was most likely introduced to North American wildlife when European cattle were brought to the continent. BTB was once common throughout United States cattle herds, but the Federal and State cooperative eradication program has nearly eliminated the disease from domestic animals. Tuberculosis occurs sporadically in free ranging and captive deer throughout the world. Currently there are management efforts to control BTB in wildlife in Africa, Europe, North America, and New Zealand, but it is likely that the disease is present, endemic and under-diagnosed in wildlife in other parts of the world. Canada, Great Britain, New Zealand, Sweden, and the United States have all reported BTB in farmed deer or elk. Until 1994 BTB was considered a rare disease of wild deer in the US, with only 8 known cases of the disease in white-tailed deer and mule deer in North America prior to that year. In 1994, Michigan began to routinely survey their wild deer population for BTB after it was detected in a hunter killed white-tailed deer. Michigan found a prevalence rate greater than 4% in free ranging white-tailed deer, which has since leveled to around 2%. In 2005, there were 18 cases of bovine tuberculosis in hunter killed free-ranging deer in Minnesota, though since 2009, no additional deer have been found to be positive for BTB in spite of regular surveillance.

Transmission

BTB can be transmitted by inhalation or ingestion. M. bovis is shed through nasal or oral secretions, draining abscesses, feces, urine and milk. Infected animals spread the bacteria by coughing or sneezing, so the disease is more likely to spread when animals are in close contact. BTB can also spread through feed or watering sites that are contaminated with saliva, urine and/or manure. BTB can be transmitted from domestic cattle to wildlife and vice versa so the probability of exposure is increased when wild and domestic animals commingle.  Predators and scavengers such as coyotes and foxes can acquire BTB by consuming infected deer carcasses. M. bovis can survive in the environment for at least 5 months in cold, damp conditions. The bacteria survive less than 4 weeks in dry conditions with direct sunlight.

Clinical Signs

BTB is difficult to diagnose based on clinical signs alone. It is a chronic disease and clinical signs may not appear until several years after initial infection. One of the earlier clinical signs in deer may be enlarged superficial lymph nodes due to abscess formation. These abscesses might drain through the skin. Infected animals usually become emaciated, weak and depressed. Infection often involves the lungs so severely affected deer may exhibit coughing, nasal discharge, and difficulty breathing, and yet some animals will die suddenly without showing clinical signs

Upon post-mortem examination, abscesses can be found in lymph nodes throughout the body. When cut open, these nodules contain yellow-green or tan pus. Tan or yellow lumps may be found throughout the chest cavity and in the lungs. The lungs may also develop dark and ­firm areas.

Diagnosis

If an animal’s post-mortem examination suggests tuberculosis, then further diagnostic tests such acid-fast staining and culture must be conducted to confirm the diagnosis. The BTB organism can be slow growing meaning that the diagnosis can be delayed, influencing management responses.

Live animals can also be tested for BTB. Any suspected animals should be reported to the appropriate state or federal animal health authority.

Treatment

There is no treatment for BTB in wildlife. While there are currently no effective vaccines for cattle, progress has been made on vaccines for wildlife. Currently however, no such vaccine exists in the United States.

Management

 In 1917 the United States began a bovine tuberculosis eradication program, and the disease has been nearly eliminated from the nation’s livestock. Tuberculosis can pass between domestic livestock and free ranging wildlife, so the presence of the disease in widely scattered individuals within free-ranging populations makes it difficult to completely eradicate. Management practices that congregate domestic livestock and wild deer pose a threat of reintroducing and maintaining BTB in domestic herds and wild populations. Several management techniques that can help keep the disease from spreading and enhance the probability of. These include raising hunter awareness of the disease and its prevention, surveying domestic animals for TB, preventing interactions between wild and domestic animal populations, decreasing deer population density through increased hunting, and preventing baiting or supplemental feeding. Should a vaccination protocol be developed, it may become a part of BTB control and eradication where the disease is established.

Hunters should wear rubber gloves when field dressing wild game because infection with BTB can occur through direct contact with a skin wound or by inhaling the bacteria in the air exhaled by an infected animal. Fully cooking the meat to an internal temperature of 165oF will destroy the bacteria, but animals with suspect lesions should be examined by state biologists.

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