Tuberculosis (TB) is a zoonotic disease of the respiratory 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.
Humans can become infected with bovine TB 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 bovine TB in the United States, and the likelihood of humans contracting this disease in the US is currently remote, 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 it 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.
Only two cases of M.bovis in white tail deer have ever been recorded in the state of New York. Currently, Michigan is the single focus of bovine TB in wildlife in the United States.
M. bovis infection has been reported in a wide variety of mammal species worldwide. Wildlife species susceptible to bovine TB 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 bovine TB and according to the Centers for Disease Control and Prevention (CDC), M. bovis can be as dangerous as M. tuberculosis (the human type of TB).
BTB has a worldwide distribution and is more 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. Bovine TB was once common throughout United States cattle herds, but the 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 bovine TB in wildlife in Africa, Europe, North America, and New Zealand, but it is likely that the disease is present, though undiagnosed, in wildlife in other parts of the world. Canada, Great Britain, New Zealand, Sweden, and the United States have all reported bovine TB in farmed deer or elk. Until 1994 bovine TB 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 bovine TB 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.
Bovine TB can be transmitted by inhalation or ingestion. M. bovis is shed through nasal or oral discharges, 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. Bovine TB can also spread through feed or watering sites that are contaminated with saliva, urine and/or manure. The disease can be transmitted from domestic cattle to wildlife and vice versa. Predators and scavengers such as coyotes and foxes can acquire bovine TB 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.
Bovine TB 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. Some animals will die suddenly without showing clinical signs. Hunters may not notice pathological lesions in live deer. Abscesses can be found in lymph nodes throughout the body as well as in the tonsils. 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.
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.
Live animals can also be tested for BTB. Any suspected animals should be reported to the appropriate state or federal animal health authority.
There is no treatment for Bovine TB 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.
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 potential threat of reintroducing and maintaining BTB in domestic herds and wild populations. Management techniques that can help keep the disease from spreading and hope for eradication include raising hunter awareness, 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. Vaccination is a potential alternative, though no practical or effective system has been established.
Hunters should wear rubber gloves when field dressing wild game because infection with bovine TB can occur through direct contact with a 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.