Bovine TB is an infectious disease caused by the bacterium Mycobacterium bovis. It is one of New Zealand’s most serious animal health problems, affecting domestic cattle and deer herds throughout the country.
TB is a threat because humans can become infected with it, mainly through the consumption of milk or by handling infected animals or carcasses. In the developing world TB still causes thousands of human deaths annually. In the developed world the risk is greatly reduced because of high standards of meat hygiene and milk pasteurisation.
Despite this bovine TB is still regarded as an unwanted disease because of the negative consumer perceptions and adverse market reactions it could generate. High levels of bovine TB would also cause significant production losses for New Zealand farmers.
Bovine TB can infect most warm-blooded mammals but possums, and in some areas ferrets, are the main source of infection (or vector) of TB in domestic cattle and deer herds. In New Zealand, areas where TB is found in wildlife are classified as TB Vector Risk Areas.
In cattle and deer the disease is most commonly found as small lesions in the lymph nodes of the upper respiratory system. In more advanced cases, lesions filled with a pussy or gritty mass can be found in the lungs and other organs of the body. The disease can lead to a chronic wasting and eventual death, but the exact cause of death depends on which organs are most damaged (usually the lungs). Advanced TB like this is now uncommon in farmed animals in New Zealand because TB testing programmes usually detect the disease at an early stage.
Possums are unusually susceptible to TB and the disease progresses quickly to an infectious stage. Possums with advanced TB can have visibly swollen lymph nodes at the groin or armpits which can open into pussy, weeping sores.