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The bacterial cattle disease was confirmed last month on two dairy farms owned by the Van Leeuwen Dairy Group, near Waimate.
It was the first time it has been detected in New Zealand, although it is present throughout the world's other cattle-breeding countries.
All 16 van Leeuwen properties have been in quarantine lock-down since the outbreak was identified on July22.
At the third public meeting hosted by the Ministry for Primary Industries, at the Waimate Showgrounds on Thursday night, ministry incursion investigator Kelly Buckle told about 100 people she and her colleagues believed farms outside the quarantine restrictions did not have the disease.
''At the moment, we're pretty confident it's just on those two farms.''
Ministry senior adviser Eve Pleydell said there had only been two positive identifications of M Bovis so far.
''There's no need to be hysterical at this point. Just be sensible.''
She advised farmers taking in bulls from other farms in coming weeks to segregate them as much as possible.
Of the 16 van Leeuwen farms, two had tested positive, 12 were negative from their first tests, and two had results pending, Dr Pleydell said.
Sixty-two neighbouring farms were also tested, 56 of them carrying cattle. Samples had been taken from 31 of those farms, and 16 were negative from their first test.
Two more batches of samples were needed from each property to confirm their negative status.
From this week, the ministry's Wallaceville laboratory would be processing 2700 tests a week. With a total of 33,224 tests to handle, that would take 12 to 13 weeks to complete.
The ministry's lab was the only one accredited for the work, but others were being brought up to standard and scientists from New Zealand and Australia were being enlisted, Dr Pleydell said.
Vets across the country were asked to call the ministry's hotline if they saw anything untoward. Regional laboratories were sending in milk from cows with mastitis, and Massey University was carrying out a nationwide survey to identify at-risk farms.
''We're trying to assess if the disease can be eradicated,'' Dr Pleydell said. ''We have to determine the extent of the spread to assess that. We need to build up a picture.''
Dairy NZ adviser Chris Morley, who has seen Mycoplasma bovis overseas, said it has ''silent spreaders'' - animals that are infected but not ill. They could have the disease for a year with no signs.
''Even trying to test for it is hard.''
On the up side, it did not affect humans or trade, he said.
''I believe we still have the option to eradicate it here.''
Oamaru vet Mat O'Sullivan said Mycoplasma bovis ''hasn't spread like wildfire'' elsewhere in the world. The biggest risk was close contact with cows from infected properties. Casual over-the-fence encounters posed a ''reasonably small'' risk.
Farmers should avoid feeding unpasteurised milk from at-risk properties to bobby calves.
They should also be ''really practical and pragmatic'' about biosecurity on their farms. Anything that did need to be taken on to the property should be kept away, and clothing and equipment should be cleaned then disinfected.
Citric acid at a 0.2% mix rate was cheap and effective. Other options were a 50g sachet of Virkon in 5 litres of water, or Trigene.
Milk tanker tracks should be a ''clean zone'', kept free of cattle faeces at all times.
Signs and guides were available at the meeting for farmers to take home.
-By Sally Brooker