Bovine Tuberculosis Investigation
Updates and resources for the October 2018 investigation of a bovine TB-infected herd.
Bovine tuberculosis (TB) is a chronic disease of animals caused by a bacteria called Mycobacterium bovis (M. bovis), which is closely related to the bacteria that cause human tuberculosis (M. tuberculosis) and avian tuberculosis (M. avium). Although cattle are considered to be the true hosts of M. bovis, the disease has been reported in many other domesticated and non-domesticated animals. This disease causes a general state of illness, coughing and eventual death. M. bovis is a zoonotic disease, meaning people may become infected following exposure to infected animals or animal products. In addition, infected people can be a source of infection to animals.
M. bovis infection can be contracted by direct contact with infected animals or exposure to products from infected animals. The bacteria can be transmitted through aerosolized droplets generated by breathing or coughing, consumption of unpasteurized milk or milk products, or exposure to contaminated feed/food. Transmission is affected by many variables such as environment, shedding animals, susceptible animals, stocking density, and type of facility. Cool, moist climates, close contact, and commingling of animals from multiple sources increase the risk of introduction and transmission of bovine tuberculosis.
Clinical Signs of Illness
BTB has a prolonged course and symptoms take months or years to appear. Typical clinical signs include weakness, loss of appetite, weight-loss, fluctuating fever, intermittent hacking cough, diarrhea, and large prominent lymph nodes. However, the bacteria can also lie dormant in the host without causing an obvious disease. Bovine TB lesions are primarily in the respiratory tract, but can be found in other areas depending on how the infection was introduced and stage of infection.
In cattle, the standard method for detection of bovine TB in live animals is the tuberculin skin test, performed by a licensed, accredited, and TB certified veterinarian. Secondary skin testing is conducted by regulatory veterinarians on animals that respond to initial skin tests. Animals that test positive on secondary skin testing require further testing and may need to be slaughtered to determine if they are infected with bovine TB.
The main way we monitor cattle for bovine tuberculosis is through slaughter inspections. Post mortem meat inspection of animals looks for the tubercles in the lungs and lymph nodes. Detecting these infected animals prevents unsafe meat from entering the food chain and allows veterinary services to trace-back to the herd of origin of the infected animal, which can then be tested and eliminated if needed.
The bovine tuberculosis eradication program, which was initiated more than 100 years ago, focuses on identifying infected herds through post mortem meat inspection. Pasteurization of milk from infected animals to a temperature sufficient to kill the bacteria has been very successful in preventing the spread of this disease and many others to humans. Treatment is not an option available for food producing animals.
Although vaccination is sometimes practiced in human medicine in other countries, it is not currently an available control measure in livestock in the US.
Although Mycobacterium tuberculosis is the major cause of human tuberculosis, humans are also susceptible to bovine TB. Humans can be infected both by drinking raw milk produced by infected cattle, or by inhaling infective droplets. As recently as 100 years ago, tuberculosis was the leading cause of the death in people in the United States. It is estimated that up to 25% of these cases were due to infections with M. bovis. In some countries today, there are still estimates that up to 10% of tuberculosis cases in humans are caused by M. bovis.