Cows can be infected but not showing signs of illness and it is very difficult to detect. It is for this reason that the whole mob needs to be culled during an eradication programme.
How it came to NZ is not known. There is no live transport (apart from Australia) and the testing shows that it is not an Australian strain. Testing of the bacteria shows the DNA is very similar so it is from one incursion. A compliance investigation is ongoing and details can be found on the website. The disease was here for at least 18 months before MPI were aware of it. If this turns out to be incorrect, they will stop and reconsider their actions; it may lead to them stopping eradication. All farms supplying milk are currently being tested with bulk milk tests that should give a better picture. Tonsils are the best place to swab but even that is still not conclusive (the stress levels of the animal are important).
On the 28th May, a decision was made on eradication, based on economic modelling and modelling of the disease, as well as scientific assessment. A cost/benefit analysis was carried out to compare eradication vs. working with the disease.
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