The Ministry for Primary Industries said it was working to assure South Canterbury farmers that control measures are in place to prevent the spread of cattle disease Mycoplasma bovis, which has already been found in two farms.

MPI advised on Monday that a second dairy farm in South Canterbury that was already under biosecurity control had been confirmed as positive for the disease.

This farm is linked to the initial property under investigation and is part of the 16-farm Van Leeuwen Dairy Group.

The detection was not unexpected given close connections between the two farms, the ministry said.

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All the Van Leeuwen farms are under legal controls restricting the movement of risk goods on and off the properties.

Cattle can only move between the other farms in the enterprise - no movement of stock is allowed to farms outside the group.

MPI, in its latest update, said it continues to build the picture of where the cattle disease Mycoplasma bovis is present and contain it to those farms.

The ministry said it is working to assure farmers with properties in the vicinity of the affected Van Leeuwen Dairy Group farms that the control measures in place are sufficient to prevent the spread of the disease.

MPI's director of response Geoff Gwyn said the main way the disease can spread is through direct contact between cattle, such as nose to nose.

"All the other potential factors such as on equipment, clothing and footwear, effluent and vehicles present a comparatively low risk," he said.

"We encourage farmers to pay attention to what are routine biosecurity measures such as on-farm hygiene, caution around stock movements and fencing stock back from neighbouring fence lines," he said.

"It is natural for people to be worried, especially those directly affected and living close by - but we don't want people to be unnecessarily concerned," he said.

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Gwyn says MPI is holding a further community meeting tomorrow to provide an update on progress in the response.

MPI is currently testing samples from the 16 Van Leeuwen farms as well as from neighbouring properties and a small number of additional farms that tracing has indicated may have received stock from the Van Leeuwen group.

These farms will receive visits and stock checks but they are not currently subject to "restricted place notices" from the ministry.

To date 25 animals on the first affected farm have been euthanised by the farmer for welfare reasons. A further group of affected animals have been checked by a vet and are fit for transport to slaughter.

Gwyn said people in the area will start to see stock movements, which he said is worrying some local farmers.

"I'd like to reassure them that any risk from transporting cattle from the Van Leeuwen farms to processing plants has been assessed as negligible and is being managed. This movement of the stock is being permitted by MPI and vehicles will be cleaned and disinfected after use under MPI supervision."

Mycoplasma bovis poses no risk to human health or the safety of food products - meat or dairy.

MPI is managing the situation "with urgency" because of the disease's impacts on farm management, animal welfare and production. "Ideally we'd like to contain it and get rid of it and that is our current focus," Gwyn said.