Bovine tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium bovis. It's a serious animal health problem, and a threat to our farming industry’s reputation overseas. It can affect practically all mammals, from farmed cattle and deer to wildlife such as pigs, deer, ferrets and possums. It can also infect humans, though that's rare.
Possums are the main wildlife carriers of bovine TB in New Zealand, and contact with infected possums is the main cause of herd infection. Wild deer and pigs can also get TB from possums, but they are ‘dead-end’ hosts — meaning they can’t spread the disease themselves.
Eradicating bovine TB is necessary to maintain the production and reputation of our dairy, beef and deer export industries.
In New Zealand, possums transmit TB to livestock, often through aerosol transmission — through interaction between cattle and deer and infected possums on farm land. When a farmed animal is infected, the disease can then spread to the rest of the herd, or to other herds through stock movements between farms. Our TB testing programme and movement control regulations help to minimise the risk of TB spreading.
In cattle and deer, TB is usually found as gritty, pus filled lesions in the lymph nodes of the throat and chest. The sores can vary from a cream or green abscess to white, gritty lesions and can be just a few millimetres across, or to up to 60mm in diameter. If not detected early the disease will advance further throughout the body and eventually cause an animal to waste away.
Advanced TB is now uncommon in farmed animals in New Zealand, as our TB testing programme usually detects the disease at an early stage, and infected animals are culled.
Possums are very susceptible to TB and the disease will quickly progress to an infectious stage, then death within 4-6 months. Possums with advanced TB can have visibly swollen lymph nodes at the groin or armpits. These then burst, causing weeping bacteria-laden areas on the body.
In pigs, TB is often found in the lymph nodes at the base of the jawbone. It can sometimes also be found in the intestines. In deer, the lymph nodes behind the throat and at the join between the windpipe and the lungs are the most common place to find TB.