Bovine tuberculosis (bTB) is a chronic infectious disease caused by the bacterium Mycobacterium bovis. It primarily affects cattle, challenging animal health and the agricultural economy worldwide. The disease’s slow progression and subtle signs make it difficult to detect and manage. The bacterium can also infect other animals, including wildlife, and poses a risk to human health.
How Bovine Tuberculosis Spreads
Mycobacterium bovis spreads most commonly between cattle through respiratory aerosols. When an infected animal coughs or sneezes, it releases bacteria into the air, which can then be inhaled by other animals in close contact. This route of transmission is particularly effective in densely housed herds where animals share the same air space. The bacterium can also be transmitted indirectly through contact with contaminated environments, such as shared feeding and watering troughs.
The disease is not limited to cattle and can be transmitted to a wide range of other mammals. This includes other livestock like goats and pigs, as well as various wildlife species. These wildlife populations can then become reservoirs for the disease, creating a persistent cycle of infection. For example, badgers in the United Kingdom and white-tailed deer in North America are well-documented wildlife hosts that can transmit M. bovis back to cattle herds.
Humans can also contract bovine tuberculosis. The most frequent pathway for human infection is the consumption of unpasteurized milk or other raw dairy products from an infected cow. Less commonly, individuals who work closely with infected animals, such as farmers, veterinarians, and slaughterhouse workers, can become infected through direct contact with infectious tissues or by inhaling respiratory droplets.
Impact on Animal and Human Health
In cattle, the clinical signs of bovine tuberculosis often develop slowly and can be subtle. Infected animals may experience progressive weight loss, a fluctuating fever, and general weakness. A persistent, low-grade cough is another common sign as the disease primarily targets the respiratory system. However, many cattle infected with M. bovis may not show any outward signs of illness for a long time, while still being capable of transmitting the bacteria.
The disease causes characteristic lesions, known as tubercles, to form in various organs, most commonly in the lymph nodes, lungs, and liver. These nodules are the result of the body’s attempt to wall off the bacteria. In advanced cases, these lesions can become widespread, leading to severe organ dysfunction and eventual death. The economic impact on farmers is substantial, stemming from reduced productivity, condemnation of carcasses at slaughter, and trade restrictions placed on affected regions.
When humans are infected with Mycobacterium bovis, it causes a form of tuberculosis clinically similar to the disease caused by Mycobacterium tuberculosis. Symptoms can include a persistent cough, fever, night sweats, and weight loss. A distinguishing feature of M. bovis infection in humans is its tendency to affect parts of the body outside of the lungs. The bacteria often infect the lymph nodes, particularly in the neck, as well as bones, joints, and the gastrointestinal tract.
Individuals with weakened immune systems are at a higher risk of developing active disease after exposure. Historically, before the widespread adoption of milk pasteurization, M. bovis was a more significant cause of human tuberculosis. Today, cases in developed countries are rare and often occur in older individuals or among those who consume raw dairy products.
Global Surveillance and Diagnostic Methods
The most common screening tool used in live animals is the tuberculin skin test. This test involves injecting a small amount of a purified protein derivative from M. bovis, called tuberculin, into the skin. If the animal has been previously exposed to the bacteria, its immune system will recognize the tuberculin and mount a localized inflammatory response, causing a measurable swelling at the injection site after about 72 hours.
To improve diagnostic accuracy, particularly in herds with a known history of infection or to clarify inconclusive skin test results, supplementary blood tests are often employed. The interferon-gamma (IFN-gamma) assay is one such test. This laboratory-based test measures the release of a specific protein, interferon-gamma, by an animal’s immune cells in response to M. bovis antigens. This method can detect infection at an earlier stage than the skin test.
Post-mortem examination of cattle at slaughterhouses is a key part of national surveillance programs. Trained inspectors examine the lymph nodes, lungs, and other organs for the presence of tubercles. This inspection catches infected animals that may have been missed by routine testing on the farm. Tissue samples from any suspected lesions are then sent to a laboratory for culture to confirm the presence of M. bovis.
Control and Eradication Strategies
Eradication programs for bovine tuberculosis rely on a “test-and-slaughter” policy. Under this system, entire herds are regularly tested, and any animals that test positive for the disease are culled to remove them as a source of infection for others. This approach, while difficult for farmers, is effective at reducing the disease’s prevalence within cattle populations over time.
To prevent the introduction of bTB into clean herds or regions, strict controls on the movement of cattle are enforced. Animals must test negative for the disease before they can be moved from one farm to another, especially if moving from a higher-incidence area to a lower-incidence one. These movement restrictions help contain outbreaks and protect disease-free areas from reinfection.
From a public health perspective, the pasteurization of milk has been the single most effective intervention in preventing the spread of bovine tuberculosis to humans. The process of heating milk to a specific temperature for a set period effectively kills M. bovis bacteria, making dairy products safe for consumption. This practice has made bTB in humans a rare disease in countries where milk pasteurization is widely practiced.
Managing the disease in wildlife reservoirs presents a significant challenge to eradication. In areas where species like badgers or deer are infected, they can reintroduce the bacteria to cattle herds. Strategies to address this include managing wildlife populations through culling or implementing vaccination programs for either the wildlife or for cattle, though effective vaccines for cattle are still under development. These wildlife management strategies are often complex, costly, and can be the subject of public debate.