If the cattle disease Mycoplasma bovis becomes established, research to find an effective vaccine for the New Zealand strain would become a priority, says the Ministry for Primary Industries.

In response to Herald inquiries about science and testing improvement initiatives in the wake of the government-agriculture sector decision to keep trying to eradicate the disease through a mass cattle kill, MPI said there was no suitably effective vaccine available globally, despite years of development efforts.

This was due to a number of complex reasons which meant vaccination was unlikely to become a useful tool in New Zealand's eradication campaign.

"However if the disease was to establish, then research into producing an effective vaccine for the New Zealand strain over the medium to long term would be a priority," MPI said.

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Vaccines were licensed for use against M. bovis in some countries but had not demonstrated guaranteed immunity or protected an animal from getting sick, MPI said.

The reasons multiple research groups around the world had failed to develop an effective vaccine included the lack of complete understanding about the immune response to M.bovis, the bacterium's ability to hide from the immune system and because M. bovis often caused disease in partnership with viruses and other bacteria.

The world-first attempt to try to eradicate the disease, which was first diagnosed on two South Island farms in July last year, will mean about 126,000 dairy and beef cattle will need to be killed, most within the next two years.

That's in addition to the cull of 26,000 cattle already under way or completed.

They will be animals from "known and future infected farms and also highly suspect farms, those under current restricted place notices", MPI has said.

Complete herds have been killed, many of the cattle have been, and will be, healthy at the time they go to the meat works.

MPI has said this action is necessary because the disease is very difficult to test for and can't be reliably identified at an individual animal level. Healthy animals can still carry the infection and infect others.

Asked what MPI was doing to find a more reliable new test, MPI responded it was "paving the way" in best utilising two existing tests to detect latently-infected herds, which had not been done overseas.

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MPI vet scientists had refined the tools so testing today was more efficient than earlier in the biosecurity response. Work to refine tests and test combinations was re-evaluated and improved based on test performance on farms, MPI said.

The two tests are the PCR DNA-based test and the ELISA test.

A new improved ELISA test developed overseas was better at detecting positive animals but was not yet commercially available. However, MPI's laboratory would receive test kits soon to start validating the test for use in New Zealand.

The PCR test was good at detecting M. bovis if it was present in a sample, but infected animals only shed the bacterium intermittently and often there was no bacteria in a sample. This resulted in false negative test results.

The ELISA test detected antibodies in blood or milk, which was part of the animal's immune response to infection. This test also commonly gave false negative results.

Many antibiotics commonly used to treat mastitis, a symptom of M. bovis, were not effective against the bacteria, because it did not have a cell wall.

"There are some less common antibiotics that can be used against this bacterium but unfortunately, although treatment can sometimes provide a cure for clinical illness, it does not necessarily clear the organism from the animal," MPI said.

Strains of M. bovis overseas were showing increasing resistance to antibiotics. The New Zealand strain was not one of the super-resistant strains, the ministry said.