Bovine tuberculosis (bTB) is a chronic, infectious disease that primarily affects cattle but can infect many other mammals. The infection is caused by the bacterium Mycobacterium bovis, which is closely related to the organism responsible for human tuberculosis, M. tuberculosis. Before widespread eradication programs began in the early 20th century, bTB was one of the most significant livestock diseases globally. Its ability to persist in livestock and spill over into other species gives it substantial importance in both veterinary medicine and public health.
Transmission and Manifestation in Animals
The primary method of spread for Mycobacterium bovis among cattle is through the respiratory route, via the inhalation of aerosol droplets expelled when an infected animal coughs. High animal density is a major factor, as transmission occurs easily in close quarters like barns or shared feeding areas. Indirect transmission is also possible when cattle ingest feed, water, or pasture contaminated by bodily discharges, such as saliva, urine, or feces.
The chronic nature of bTB means the disease progresses slowly, often taking months or years before outward signs appear. When symptoms manifest, they are typically non-specific, including chronic wasting, weakness, fluctuating fever, and loss of appetite. A persistent, moist cough may develop in later stages if the disease has extensively damaged the lungs.
The most characteristic sign of the infection is the formation of nodules, known as tubercles, which develop in the lymph nodes and lungs. However, in regions with active control programs, most cases are detected early through testing, before advanced clinical signs or extensive lesions have formed. This lengthy incubation period, combined with subtle or absent symptoms in the early stages, presents a significant challenge for detection and control.
The Zoonotic Nature of Bovine Tuberculosis
Bovine tuberculosis is classified as a zoonotic disease, meaning the infection can transmit from animals, primarily cattle, to humans. Historically, the main path of human infection was through consuming raw, unpasteurized milk or dairy products from infected cows. Ingestion of the bacteria leads to a form of tuberculosis that frequently affects sites other than the lungs, such as the lymph nodes or the gastrointestinal tract.
Airborne transmission can also occur, though less commonly, often posing an occupational risk to people in close contact with infected livestock or animal products. Veterinarians, farmers, and slaughterhouse workers face this increased risk through inhaling infectious aerosols. Historically, before widespread public health measures, M. bovis was a common cause of human tuberculosis in developed nations.
Mandatory milk pasteurization was a transformative public health intervention that drastically reduced human cases in many countries. Pasteurized milk and meat inspection programs ensure the bacteria are killed before products enter the food supply, making the risk to the general public in developed countries very low. However, in regions where bTB is prevalent and pasteurization is not routine, the disease remains an important cause of human tuberculosis.
Strategies for Disease Control and Eradication
The foundational strategy used globally to eliminate bovine tuberculosis in cattle is the ‘Test and Slaughter’ policy. This program relies on systematic surveillance and mandatory testing of livestock, followed by the immediate removal and culling of any positive animal. Financial compensation is typically provided to farmers who must cull infected animals to mitigate the economic hardship.
The standard diagnostic tool for large-scale surveillance is the tuberculin skin test, where a small amount of antigen is injected into the animal’s skin. A localized immune reaction at the injection site indicates a positive result, although ancillary tests like the interferon-gamma release assay are sometimes used to increase accuracy. Regulatory actions also include strict movement controls and quarantine measures to prevent the spread of infected animals.
A major complication in eradication efforts is the presence of wildlife species that act as reservoir hosts, sustaining the infection and transmitting it back to cattle. Examples include European badgers in the United Kingdom, brushtail possums in New Zealand, and white-tailed deer in parts of North America. Management strategies for these populations are varied and often controversial, including targeted culling programs, vaccination of wildlife, and biosecurity measures to reduce contact between livestock and wild animals.
Research is progressing on developing effective vaccines for cattle, such as the Bacillus Calmette-Guérin (BCG) vaccine, alongside advanced diagnostic tests that can distinguish between vaccinated and infected animals. These comprehensive programs, which include herd management, biosecurity protocols, and wildlife control, are continuously refined to achieve eradication.