Tuberculosis in cattle (bovine Tb) is caused by the bacterium Mycobacterium bovis that is related to the bacterium causing Tb in humans. Bovine Tb also infects humans but only rarely since compulsory pasteurisation of milk. High levels of Tb infection in cattle, prevalent in New Zealand earlier this century, were gradually reduced after Tb testing was made compulsory for all dairy cattle in 1961 and for beef cattle in 1971. Cattle suspected of having Tb were slaughtered (as they are still) so that the disease was gradually eliminated from most herds.
However, in some parts of the country the gradual reduction of infected herds was not maintained, suggesting a source of infection other than diseased stock. In 1967, 20 out of 25 possums trapped just north of Westport were infected with bovine Tb, and within 5 years infected possums had been found in several areas of northern Westland, in the Wairarapa, and around the western bays of Lake Taupo. It appears the Tb must have entered possum populations during the early 1960?s at the latest, and possibly much earlier during the 1950?s. Today, possums are infected in areas of New Zealand covering approximately 6.24 million hectares or 23.5% of the nation?s land area, from north of Auckland to the south-eastern corner of the South Island.