Bovine TB

Clinical Signs

Bovine TB is an infectious and chronic disease of cattle caused by the bacterium mycobacterium bovis (M.bovis). Once an animal is infected with M.bovis, the rate of progress of infection is dependent on a number of factors, including the infectious dose and the immune status of the host.

The signs and severity of the disease vary depending on the body system most affected, but clinical signs of disease in cattle often only appear in advanced cases and may be non-specific. This makes clinical cases of TB difficult to spot. The signs include:

• weakness
• loss of condition & appetite
• swelling of various lymph nodes
• persistent cough and respiratory distress

Udder involvement is rare, but the disease can result in progressive hardening of the affected quarter and enlargement of the supramammary (top of the udder) lymph nodes. In such cases the organism can be detected in milk samples.

Humans and animals with TB develop an immune response which can be detected by the tuberculin skin test. Tuberculin is a sterile laboratory product made by growing TB bacteria, killing them with heat, removing them from the substance on which they were grown, then properly diluting & preserving the remaining mixture. About 72 hours after tuberculin is injected into the animals affected with TB, a characteristic swelling reaction appears at the point of injection. This reaction is a positive result, indicating exposure to one type of Mycobacteria.

A variety of diagnostic methods are used to help confirm the presence of bovine TB. These may include the comparative cervical tuberculin test, serological tests, post-mortem examinations and other laboratory procedures.

How bovine tuberculosis is spread

TB is spread primarily through inhalation of invisible droplets (aerosols) containing TB bacteria. This transmission usually happens when animals are in close contact with each other with the risk of exposure being greatest in enclosed areas. Bacteria released into the air through coughing and sneezing are inhaled by uninfected animals. Cattle are also likely to infect each other when they share common watering places contaminated with saliva and other discharges from infected animals. The disease may also be spread by contaminated equipment, feedstuffs and slurry.

The disease can be spread to calves through the mother's milk and humans can also become infected by drinking unpasteurised milk from infected cows.  In some parts of the UK there is a link between the spread of bovine TB and wildlife. Badgers are considered a significant source of TB infection for cattle in some areas; however, there is no evidence to suggest that badgers are a reservoir for the disease in Scotland.

Only a relatively small number of non-bovine animals have been identified as infected in previous years and evidence suggests that these species appear to pose only a small risk of spreading TB to cattle.