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The small but rising number of New Zealanders drinking unpasteurised milk supplied directly from dairy herds need to have as much protection from being infected with bovine Tb as it is possible to provide.
When an animal in the Mt Cargill area reacted positive to a Tb test in May, the Ministry for Primary Industries - New Zealand's food safety regulator - halted the supply of raw milk.
The ministry made the decision to prevent raw milk sales from Tb-infected dairy herds until they had tested free from Tb for five years. MPI's decision was based on historical data that showed there is a very low risk of a previously infected herd being found with latent infection up to five years after being Tb-free.
The TBfree New Zealand programme run by Ospri uses Tb testing, animal movement and possum control towards the goal of eradicating bovine Tb. It employs a small range of diagnostic tests to detect tuberculosis infection in our cattle herds.
All tests are based on measuring an animal's immune response to tuberculin, which is a standardised protein extract derived from killed Tb bacteria (Mycobacterium bovis). In general, this response is measurably different between infected and non-infected animals.
Tuberculin tests can be applied directly to an animal with an injected skin test, or can be carried out in a laboratory using a blood sample taken from the animal.
The Tb skin test on average detects 85% of infected cattle. Thus, if there is one infected animal in a herd, there is an 85% chance that the herd will be found infected and subjected to further testing. If there are two infected cattle in a herd, then there is a 98% chance of detecting the herd as infected and subjecting it to further Tb testing.
Once a herd is infected, it is subject to a regime of increased skin and blood testing to clear infection from the herd as fast as possible. This has proven to be very effective. About 60% of infected herds are clear of infection within two Tb tests and 95% of herds clear infection within five Tb tests. However, in rare instances, a cow may be infected with Tb, yet appear clinically normal and not respond to any of the Tb tests.
Following a chronic stressful event, usually associated with a combination of events such as feed shortage or introduction of new herd mates during late pregnancy or early lactation, a latently infected animal may quickly convert into one that becomes a major shedder of Tb bacteria. This can happen via a number of routes including the udder, leading to Tb infected milk.
Hence the importance of requiring five years of testing freedom before allowing raw milk sales, to guard against such an event.
The Tb testing programme for cattle in New Zealand is based on long and thorough research, proven in the field. In any one year, a large proportion of the national cattle population (4.41million animals in 2014-15) is skin tested for Tb.
During the decade to 2013-14, about 52,850,000 cattle were Tb tested nationally and 52,840,000 of these gave negative test results. That is a 99.98% accuracy (what we term "specificity'') rate.
Of about 9650 cattle that were taken nationally as Tb reactors out of that nearly 53million, 2630 showed gross lesions of Tb at slaughter. A further proportion of those not showing gross lesions (anywhere between 10% and 20%) will be infected but not showing visible lesions, due to early stages of infection. Such gross lesion rates are generally within the ranges seen in overseas countries.
There is high confidence that the testing regime is working.
New Zealand is getting into the latter stages of its Tb eradication programme, where a more intensive approach to eradicating Tb in herds is required to clear infection quickly. It is, therefore, not surprising in the latter stages of a programme that lesion rates in Tb reactors fall as the overall amount of Tb in the population is reducing.
As part of the Tb-free programme we are constantly looking at ways to improve how we our work, and Tb testing is no exception.
Research is under way into a new tuberculin used in the reactor that is much more specific at targeting bovine Tb. The research is now 18 months into a trial and is looking promising in helping to reduce the false positive incidence.
Dr Paul Livingstone is research leader for TBfree New Zealand.