Behind the numbers: Hawke's Bay TB infection

Date 14 July 2020

OSPRI's disease management leader Mark Neill explains how the TBfree programme is clearing infected animals from Hawke's Bay's farmed livestock, and removing possums from the disease cycle.

Most cattle and deer herds that become infected in New Zealand only ever have one or two TB animals. They usually go on to complete two clear tests and never return to the programme.  This trend has also been seen in the infected herds identified in the Hawke’s Bay in the past year.

In the middle of July 2020, there were 19 herds with a TB infected status in the Hawke’s Bay region of which 14 have completed their first clear whole herd test.

Of the 14 herds with one clear test, at least 50% were expected to achieve a clear status at their next test.

For ‘infected’ status herds to return to a ‘clear’ status, two clear whole herd tests were required no less than six months apart. The obligatory period between herds tests is to ensure any recent infection, or possible spread within the herd, is detected.

Of the two additional herds, one had been produced by splitting one existing infected herd into two infected status herds. This herd was split because it contained two separate units and TB has only been found in one of them.  Both units will have to complete the required testing regime and continue to be managed separately.  It was possible that the herds might get back to a clear status independently of one another.

The TBfree programme is working towards the eradication of bovine tuberculosis from New Zealand.  The process of disease eradication – where a wildlife host is involved – is progressive and takes time, effort and resources to ensure that wildlife population is cleared of infection. 

Good progress has been made nationally, but areas remain where the disease persists in wildlife.  This means TB can still show up in herds from time to time, as has happened in Hawke’s Bay.

Most herds become infected from a wildlife source. Comprehensive possum control will reduce and eventually remove this risk to domestic herds. However, during the period that animals are exposed to an infected wildlife source TB can continue to be transmitted. 

Herds in which TB is detected remain under restriction until OSPRI is confident that the infection within the herd has been removed.

For these reasons, an immediate reduction in infected herd numbers is not expected immediately following possum control.  It is possible that there will be additional herds detected over time but these are expected to be reduced significantly as the benefit of the possum control programme eventuates.