A bulk milk test will help identify the level of pestivirus antibodies in a herd.
A combination of testing, monitoring and vaccination is recommended for managing pestivirus.
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That is the message from a workshop in Warragul recently, attended by dairy farmers, veterinarians and industry service providers.
A bulk milk test is used to identify if pestivirus has infected cows in the milking herd.
Veterinarian with ProDairy, Dr Rob Bonanno, recommends several ways of testing for pestivirus.
“Screen the herd by doing a bulk milk test,” he said.
“This measures the level of antibodies in the milk. From this, we know if the herd has had a low, moderate or high exposure to pestivirus.”
Rob said he would recommend only vaccinating the herd for a low exposure result.
“That herd is susceptible to an outbreak of pestivirus when the cows are pregnant, and that would be a disaster,” he said.
“I would recommend vaccinating about a month before joining begins. That would provide optimal protection for calves in utero.”
Heifers and cows that contract pestivirus while their foetus is 0-40 days old will often abort.
“If your herd’s exposure is low or none, I would recommend vaccinate every year,” Rob said.
“The smart manager has vaccination as part of their risk management plan.”
A medium or high exposure result in the milk test needed different treatment.
“A medium level of infection may be that all the old cows are immune and the young ones aren’t, or all the young ones are immune but the old cows aren’t,” he said.
“Or there’s a spattering of immunity throughout the herd.”
Rob recommends testing cohorts.
“I would test half a dozen heifers, half a dozen young cows, and half a dozen mature cows,” he said.
“That should identify if it’s one cohort or a mix of ages.
“If it’s the young animals, I would suggest a recent outbreak. If it’s the old cows, it could be the infection occurred years ago and the young animals are susceptible.”
Rob said high levels in the milk test indicates the exposure has been long term.
“So we wouldn’t necessarily vaccinate the herd, but we would monitor young cattle coming through,” he said.
“We would also test the herd to make sure there are no PIs. The PIs would be creating new infection in the herd, all the time.
“We would also vaccinate young animals as soon as they enter the herd.”
PIs are persistently infected animals that would have contracted pestivirus in utero, before their immune system identified the disease as alien.
The calf is then born contagious, and pathogens are shed continually because that calf’s immune system treats the disease as part of its normal state.
Ear notching heifers is a histology test that measures their level of exposure.
“If you’ve had a group of heifers running together, we only need to test about six to eight of them,” Rob said.
“If that sample size is immune, we know the probability is the whole group is immune.
“If none of them are immune, we known the probability is none of them are immune.
“If some of them are positive, there’s probably an historic infection, and there’s some other tests we can do which are more quantitative and can tell us if there’s been a recent infection of historic exposure.”
Buying bulls increases the risk of spreading pestivirus, as the disease is spread animal to animal from all mucosal production, including semen.
“If you bring in a bull and he’s a PI, he’ll infect the herd,” Rob said.
“So every bull should have an ear notch and be tested for pestivirus.
“Then he should be vaccinated, so he can’t infect the herd.”
Research indicates the contact time between a PI and other animals in the herd can be as little as one hour for transmission of infection.
Rob said bulk milk testing across the dairy industry would be a strong lever for eradicating PIs in the national herd.
Allan Murray, from IDEXX Laboratories, was one of the speakers at the Warragul workshop.
IDEXX undertakes the bulk of antibiotic testing for milk producers in Australia.
He recommends following good biosecurity practices on the farm to minimise potential contact between animals that have been brought into the herd, until test results provide a clear indicator of health status.
“The main economic impact is linked to milk yield, mortality rate and cull risk,” he said.
“There’s also a risk of other diseases because of the mass immunosuppressive effect of pestivirus – for example, mastitis increases in the milking herd.
“The risk of not testing a small sample of animals compared to needing to test the entire herd - there’s been an 80 per cent mortality in some herds.
“Vaccination is part of the solution and is a risk mitigator.”
Yannathan dairy farmer, Evan Campbell, called for the government to subsidise a national bulk milk test to identify the extent of the problem in Australia.
Calves that are born carrying the pestivirus disease can be healthy, grow out to be milking cows and infect every animal they interact with. (Photograph for illustrative purposes only.)
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