Two farms in Northland have been placed under movement restrictions by the Ministry of Primary Industries for cow disease Mycoplasma bovis due to "high-risk" stock transfers from infected or suspicious farms.

A further 25 properties in the region have forward traces, meaning they have received cattle from a suspect farm. The disease means up to 22,000 cows in the country will have to be destroyed.

An MPI spokesperson said putting the two high-risk farms under a Notice of Direction does not mean they have infected cattle.

"This doesn't necessarily indicate infection - many farms under these notices have returned negative test results so far. This legal direction restricts the movement of risk goods, including animals, off the property.

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"Like most regions in the country, the Mycoplasma bovis has some effect on Northland – but it is not a major one at this time."

Dairy farmers in Northland are worried about the spread of the disease, Federated Farmers' Ashley Cullen said, particularly as it comes at the end of the season when farmers are looking to move stock.

"You're putting a lot of faith in where the cows come from, and where they've been. There's absolutely nothing you can do, just make sure your own paperwork's up to date."

The only other option, he said, was closing herds and not moving cattle.

"My big hope is that we can just close the gate at the harbour bridge."

There could be an opportunity for Northland, Cullen said, if the region remained free of the disease.

Mycoplasma bovis was confirmed in Waikato earlier this week, the furthest north it has been known to have travelled so far.

The discovery at the farm near Cambridge took the total number of infected properties throughout the country to 39.

MPI has decided all cattle on infected properties - estimated to be just over 22,000, although more properties have now been infected - must be culled.

The disease was first detected in South Canterbury in July last year. It is not known how it spread to New Zealand.

It was months before the disease was detected in the North Island, with the first case found in Hawke's Bay in December last year.

It was not until earlier this month that another case was discovered in the North Island, near Pahiatua.

Agriculture and Biosecurity Minister Damien O'Connor said a decision on the next steps to deal with Mycoplasma bovis would be made soon.

"We are all committed to make a decision about the next steps in the M. bovis response within the next couple of weeks. We talked about phased eradication and long-term management.

"It is a difficult choice that we will make together once we receive more advice from the Technical Advisory Group in the coming days.

"Over the next few weeks farmers who are not under controls are allowed to move stock, but they must adhere to their legal National Animal Identification and Tracing requirements and record animal movements.

"If you are concerned about moving your stock then be prudent, seek advice from your industry groups and MPI. The same goes for sourcing feed.''

Mycoplasma bovis is common in most countries, including Australia. It causes untreatable mastitis, abortions and arthritis in cows, and pneumonia and ear infections in calves.

It is spread from animal to animal through close contact, although there is no danger to humans. It cannot be transmitted to sheep, but goats are thought to be able to carry and transmit the disease despite showing no symptoms.

There is no risk from consuming meat, milk or milk products from infected cattle.

Notice of Direction (NoD)

• MPI issues NoDs to farms when an inspector or authorised person believes that movement of stock and other risk goods from a property poses a risk of spreading Mycoplasma bovis. For example, MPI may issue a NoD when animals from infected properties move to that property, but no testing has happened yet. We may also issue the NoD if test results are still pending.

• The NoD aims to prevent further spread and doesn't restrict movement of stock or goods onto the farm.

• Cattle can only move off the farm with a permit.

• Other steps may be required (cleaning and disinfecting of vehicles).