In cattle and deer, TB is usually found as small lesions in the lymph nodes of the chest. In more advanced cases, lesions are filled with a gritty or pus-filled mass and can be found in the lungs and other organs. The disease can lead to chronic wasting. Advanced TB is now uncommon in farmed animals in New Zealand because TB testing programmes usually detect the disease at an early stage and likely infected animals are culled.
Possums are unusually susceptible to TB and the disease quickly progresses to an infectious stage. Possums with advanced TB can have visibly swollen lymph nodes at the groin or armpits which can turn into weeping, pus-filled sores, leading to eventual death. These sores can vary from a cream/green abscess to white, gritty lesions and can be just a few millimetres across to up to 60mm in diameter.
In pigs, the disease can usually be found in the lymph nodes at the base of the jawbone. It can sometimes also be found in the intestines.
TB can be transmitted to people via infected milk, although it can also spread via aerosol droplets from live infected cattle. Hunters sometimes become infected from handling infected possums. Human cases are rare in developed countries like New Zealand thanks to pasteurisation and testing programmes. However, in areas of the developing world where pasteurisation is not routine, bovine TB is a relatively common cause of human tuberculosis.
TB in humans is more commonly caused by a related, but different, form of bacterium, Mycobacterium tuberculosis, that also mostly affects the lungs. Despite presenting a very low human health risk nowadays, TB is still regarded as a problem for agriculture. High levels of TB would cause significant production losses for New Zealand farmers, and could put at risk the marketability of our meat and dairy exports.
TB can be transmitted from person to person and even from person to cattle, but this is rare.
If you, or anyone you know, has been exposed to TB, seek medical advice immediately. The most likely source of exposure is drinking unpasteurised milk from an infected cow. Close contact with infected livestock (such as during milking) or handling an infected animal carcass may also pose an infection risk.