Possums are the main source of TB infection in farmed cattle and deer herds in New Zealand. In fact, over 70 per cent of new herd infections in at-risk areas are due to contact with TB-infected wild animals. But we didn't always know this...
The first TB-infected possum was found in 1967, but the link between possums and cattle was not proven until 1971. Initially, the route of transmission was thought to be via TB pus on pasture but it was later shown that direct contact was needed.
When possums are suffering from advanced TB, their behaviour becomes extremely erratic. They will often venture out in daylight and on to pasture land. A tuberculous possum will exhale bacteria and may have open lesions. Naturally inquisitive, cows will nuzzle, sniff and lick lethargic possums, potentially becoming infected. Deer are likely to be aggressive - kicking, headbutting and biting sick possums.
When funding for possum control was cut in the late 1970s the number of infected herds tripled over a 14-year period, from around 550 to over 1700. That number started to drop again when full-scale possum control was reinstated in the early 1990s.
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