8 minute read
BVD: Disregarded disease could be eliminated
Disregarded disease
COULD BE ELIMINATED
By Karen Trebilcock
BVD Free New Zealand is frustrated there is still no nationwide BVD (bovine viral diarrhoea) eradication programme.
“BVD control has been put on the backburner over the past few years while farmers and industry have had to respond to other high priority issues like M. Bovis and Covid,” Massey University associate professor of veterinary epidemiology Carolyn Gates says.
“Because there are no human health risks from BVD and currently no international trade restrictions, it has been very difficult to get engagement around national BVD control.”
BVD Free, a group of mostly vets and university researchers, finished modelling last year a cost analysis and road map which showed the disease could be eliminated in New Zealand within seven years.
It’s estimated BVD costs the country $150 million each year in losses and another $42m for finding animals with the disease and vaccinations.
For infected dairy herds the cost is about $22 per cow per year on average over the five years of an outbreak and $44 per cow in beef herds.
With testing data still low in NZ, it’s thought between 15% and 25% of dairy herds and about half of beef herds are infected and there will be between 5% and 10% of new herds infected this year.
“The modelling we’ve done shows that if all dairy herds tested their keeper calves to get rid of persistently infected animals and all beef herds vaccinated their mixed aged cows and replacement heifers to prevent the creation of new persistently infected animals, national eradication could be achieved at a cost $350 million over the seven years with a total benefit of $645 million back to industry.
“Any additional measures that farmers take to improve onfarm biosecurity could make eradication even more financially attractive,” Carolyn says.
In comparison, the government, DairyNZ and Beef + Lamb NZ are spending $870m over 10 years to eliminate Mycoplasma Bovis.
“We are hoping BVD will eventually be added to the list of eradicated cattle diseases in New Zealand such as Brucellosis, Hydatid disease and EBL as well as M. bovis.
“However, in the last few years, whenever we have tried to get government or industry engagement in progressing the infrastructure development to support national control, there have always been other issues higher on the agenda. First it was M. bovis and now it’s also Covid.
“I think the drive to achieve national control really should come from farmers and veterinarians, but these groups have also been under significant stress from recent events,” she says.
“Having BVD certainly doesn’t help any farmer’s situation and some of our most-committed supporters are unfortunately folks who have witnessed the devastating effects of a BVD outbreak first-hand. These are the guys picking up the phone to ask us why we aren’t doing more as a nation to control BVD.”
Carolyn says simple and effective tools to support national control such as getting an animal’s BVD status integrated into NAIT has also not gained traction.
“In the UK, farmers can register their herds and when a calf is tested, its status is recorded on a national database automatically by the diagnostic lab.
“It means future purchasers of the animal can access that database and see it verified that the animal is not a carrier of the disease.
“In New Zealand, purchasers have to rely on vet certificates or more often just the other farmers’ word, but the BVD status could easily be an extra column on NAIT records.
Carolyn Gates, associate professor of veterinary epidemiology, Massey University.
“A lot of the infrastructure to support this is already in place from the M. bovis response, but we have run into challenges with getting key stakeholders from DairyNZ, Beef + Lamb, Deer New Zealand, OSPRI, and MPI to make a formal decision.”
Dairy farmers are doing better than beef herds controlling BVD, although dairy has an unfair advantage as only about 25% of dairy calves born each year are kept within the industry as the rest are either bobbied or sold to beef rearers.
About 70% of dairy herds now do bulk milk testing and positive results have dropped from 15% to 5% in the past 10 years. BVD was found in cattle in New Zealand in the 1960s after reports of it overseas. Although cases were affecting herds, farmers had little information on how to diagnose or treat it until the first vaccine became available in the late 1990s. In 2005, vets held a symposium on the disease and formed a steering committee, launching roadshows and information packs to educate farmers. With the introduction of NAIT, a bulk milk test for dairy herds and pooled serum antibody testing for beef herds during the past 10 years ago, the disease has become more discoverable and traceable. The virus attacks cells in the intestines, immune system and reproductive organs with cattle usually recovering in two to three weeks. Symptoms include diarrhoea and animals are more susceptible to other diseases including high worm burdens in calves. It can affect the fertility of bulls.
A persistently infected animal, (PI), like this calf on the right that survived infection in utero will then shed BVD all its life and infect others in the herd.
Calves highest risk
However, it’s unborn calves that are most at risk depending on what stage of development they are when the cow has the disease.
In the first 120 days of pregnancy, foetuses are usually lost and cows will show as having long returns, mummified calves, or abortions.
Infection from day 120 to 280 often causes birth defects with the calf being carried to full term. Calves that survive infection between days 40 and 120 of pregnancy are born with a persistent infection and are known as PIs. Their immune system does not recognise the disease as abnormal so they have no immune response to it and shed the virus through their bodily fluids throughout their lives.
They often have compromised growth rates and fertility but some can appear visually normal and have productive lives but every calf they have will be a PI and the virus will continue to be shed and cattle in the herd will be continually infected because of them. Unfortunately, having a PI in a herd does not give the cattle immunity due to them being exposed constantly to the virus as the rate of spread is too low, Carolyn says.
“If the ‘do nothing’ approach of leaving PIs in a herd worked to control BVD, we wouldn’t be in the situation we are in now in New Zealand.
“BVD spreads just enough to cause significant health and welfare impacts, but not enough to get rid of itself by natural immunity.”
In NZ, there is a very short gap between calving and mating so any PI calves born into the herd
have a high chance of infecting at least one susceptible cow during the early stages of pregnancy leading to the birth of more PIs the following season.
The financial impact of the disease is seen onfarm as higher empty rates, longer calving patterns, higher calf mortality, lower milk yields, higher incidences of respiratory diseases and scouring and lower growth rates.
Bulk milk testing game changer
Bulk milk PCR testing is described by Carolyn as a “game changer” for dairy herds because it allows farmers to screen their milking herd for PIs with a single bulk milk sample. The test is sensitive enough to detect a single PI contributing to a bulk milk tank with more than 1000 other animals.
While it is also useful to measure antibody levels as an indicator of exposure to BVD, high antibody levels could be from a previous BVD outbreak that has since been cleared or from vaccinated cows.
“What we’re looking for in bulk milk antibody testing is often trends. If there is a sudden spike that is not from vaccination, then it could mean BVD is getting back in somewhere. “One of the diagnostic challenges with eliminating BVD is that cows may be carrying PI foetuses, but we have no way of testing for this until the calf is born. That’s why purchasing pregnant replacement cattle is a big risk for BVD introductions.”
On the plus side, animals that test negative for BVD can be certified as non-PI for life since PI animals can only be created from infections during pregnancy.
“The tests are very accurate at detecting PI animals since these animals are constantly shedding large amounts of virus throughout their whole lives, unlike animals with TB or M. Bovis.”
Testing of calves using an ear notch sample while DNA testing for parentage is an easy and accurate way of making sure no PI animals were entering the herd.
Risk times for infection in dairy herds are when using bought-in bulls for mating which are not vaccinated, buying pregnant cattle and not testing the calf at birth and across-the-fence transmission from neighbours, during transport or grazing.
BVD CONTROL MEASURES ON DAIRY FARMS:
• Test replacement heifer calves for
BVD virus as soon as possible after birth and cull positive animals • Confirm all cattle brought onto the farm have been individually tested for BVD virus • Vaccinate all bulls before the start of the breeding season • Prevent cows from coming in contact with other cattle (over the fence or during transport).
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