THE latest thinking regarding one of the most costly diseases to the Australian cattle industry, bovine viral diarrhoea (BVDV) or pestivirus, advocates risk management based on a thorough knowledge of the mechanisms of the virus.
Recognition of the fact circumstances change over time is also a key element, according to leading veterinarian researchers.
Despite the fact the fertility and reproduction impacts of BVDV are well known amongst producers, only a third take any control measures.
Of those, 83 per cent are vaccinating but the message emerging is it can be erroneous to assume that means BVDV has been fully covered off in a beef operation.
Bovine reproduction expert Dr Alistair Smith, from Charles Sturt University’s School of Animal and Veterinary Sciences, said it had become clear there was not a ‘one size fits all’ solution to BVDV management in Australia.
Dr Smith presented a comprehensive overview of what is known about BVDV, what impacts it presents to herds and what research is indicating is the best approach for producers, at the 2016 Graham Centre Beef Forum last month.
Researchers and industry advisors in the field have concluded a three-pronged approach was needed to protect herds: Removing the main source of infection; protecting susceptible animals and preventing re-introduction.
“It’s about making informed decisions,” Dr Smith said.
“There are costs to control BVDV and so if how the virus works is not fully understood, money can be spent in the wrong place or there is the chance a producer may think they have mitigated all risk when in fact they have left the gate wide open.”
Pestivirus, which occurs worldwide, is estimated to cost Australian producers $114.4m a year.
Signs include severe reproductive loss, ill thrift and diarrhoea and respiratory disease.
There are costs to control BVDV and so if how the virus works is not fully understood, money can be spent in the wrong place or there is the chance a producer may think they have mitigated all risk when in fact they have left the gate wide open
- Dr Alistair Smith
The main impact of pestivirus is on the unborn foetus.
Dr Smith explains: “If you have a mother cow, non-pregnant, who meets the virus for first time, she may have mild symptoms but her immune system kicks in and she is protected.
“If she is pregnant when that exposure happens, plenty more can happen and that is dependent on the timing of that exposure.
“If it is after day 18 of pregnancy up to day 125, the results can be foetal loss or the virus gets into foetus and, because its immune system hasn’t developed, it becomes part of the normal make-up of the animal and that calf will always be a carrier.
“It will shed the virus for its entire life so it is a threat to the rest of the herd.”
If the virus meets the cow half way through pregnancy, some PI calves may result or some malformations may occur.
In the last third of the pregnancy, the calf is able to fight off the virus.
“Once those PI calves hit the ground they are a threat to rest of the herd,” Dr Smith said.
“So then it depends on the immune status of the herd - if your herd has never experienced the virus before, that is it is totally naive, and a PI animal comes in, either born in or purchased in, it can be devastating, depending on what stage of the year it is, in relation to the current pregnancy status of the herd.
The ‘weird silver lining’ is that the next year much of the herd has then become immune so it won’t be such a problem going forward.
Few producers know the pestivirus status of their herd.
A blood antibody test by screening a small sample of young stock, around 5pc, after nine months, is the best starting place.
“If you have a naive herd, your option could be to live with the risk and use biosecurity measures to keep it out, which means things like screening animals you purchase in, thinking about what opportunity exists for animals to meet other neighbouring animals and not agisting where commingling with third-party animals can occur,” Dr Smith said.
“The other option is getting your herd immune - you can purchase the vaccine or you can expose your breeding females before they get pregnant.”
Vaccination involves two injections four to six weeks apart at least, ensuring it is fully completed a minimum of two weeks prior to breeding, with booster shots needed annually.
The problem with using deliberate exposure via a known PI is it is generally a random event and the effectiveness can remain unknown unless a considerable amount of testing of animals is undertaken.
The bottom line, says Dr Smith, is that vaccination will achieve a lot but that is not necessarily the entire job done.
Vaccination can effectively be ‘value-added’ by going the extra mile with good biosecurity and removing PI animals, he said.
Research into the virus circulating amongst weaners has shown compensatory growth from good pasture allowed for those exposed to ‘make up’ on the weight front come market day.
His last piece of advice: “Talk to your vet. There has been an extensive operation by the Australian Cattle Vet Association, to skill up vets on the complexities of BVDV.
