Early results from nationwide bulk milk testing for Mycoplasma bovis suggests eradication remains possible, the Ministry for Primary Industries says.

To date, more than 51,000 of approximately 70,000 tests have been completed and only three farms have been confirmed to have the cattle disease. Testing to date reinforces the theory the country is facing a single strain of the bacterial infection that affects cows.

"All three properties were already part of our tracing programme and they all have previously known links to the disease. One of these properties was under surveillance and the other two were about to go under surveillance," said MPI Mycoplasma bovis director Geoff Gwyn.

The disease was first detected in a dairy herd in South Canterbury in July 2017, marking the first known outbreak in New Zealand. In May this year, the government announced it would attempt a 'phased eradication', at an estimated cost of $886 million over 10 years.

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Phased eradication involves ongoing depopulation and includes any new infected properties. If successful, it will be a world first.

At the time, the government said a further round of national bulk milk surveillance testing was scheduled in spring. It planned to assess the operation based on that and other information in late 2018 or early 2019.

Under the testing programme, milk samples from every New Zealand dairy farm are being taken shortly after calving, when cows are most likely to be shedding the bacterium. For each farm, the first sample was collected around four weeks from the start of milk supply. Samples are then collected every two weeks, with a total of six samples from each farm.

"The fact that we have confirmed Mycoplasma bovis on only three farms, and that all three of them were already on our radar, is encouraging," Gwyn said.

"It reinforces our belief that we are dealing with a single strain of Mycoplasma bovis, based on the available scientific evidence and our own work in the field."

Gwyn reiterated that he is confident "we are on the right path in terms of tracking down the disease and eradicating it from New Zealand."

Full results from the bulk milk testing are expected to be delivered to the majority of farmers in the North and South islands in early December.