On October 13 the scientific community clustered in Kansas City, Missouri, to tackle what has become a tired veterinary euphemism: “Bovine Viral Diarrhea Virus Eradication: Reality or Myth?”
Infections with bovine virus diarrhea virus (BVDV) are widespread throughout the world. Although the prevalence varies among surveys, BVDV infection is endemic in many populations, with a maximum one to two per cent of cattle being persistently infected (PI) and 60 to 85 per cent antibody positive.
Despite the depth of our understanding about this disease, bovine virus diarrhea (BVD) remains an enigma.
Is eradication possible? The answer is probably no. Not in the harum-scarum North American marketplace.
“BVD is one of those insidious, production-limiting diseases that affects many cattlemen and their livestock, even though owners may not be aware of it. Virus infections acquired during pregnancy produce persistently infected offspring that look normal, but spew tremendous quantities of virus, causing disease and production losses through the rest of the herd.”
He says the cost can run up to $100 per head per year in herds where the virus has a foothold.
One estimate put the cost of having one PI animal in a beef herd between $14.85 and $24.84 per cow per year. Once PI calves hit the feedyard, the costs skyrocket. Another calculated the cost for every animal going into a feedyard following exposure to a PI animal at US$47 per head.
This virus is capable of causing a variety of gastrointestinal and respiratory diseases by suppressing the immunity of infected contacts.
Persistently infected calves often don’t survive more than a year, although some survive up to five years. The survivors are usually poor-doers but remain a constant source of infection every day they remain in the herd.
In the U.S., total annual losses have been estimated at $20 million for every million calves when modeling for low-virulent BVDV strains and $57 million when modeling for a high-virulent BVDV strain. If we take Canada’s breeding herd at approximately 3.9 million beef cows, that would put BVD losses here in the range of $78 and $220 million.
The economic impact of BVD is affected by things like herd size, movement of animals in and out of herds, routine biosecurity precautions, vaccination programs and pasturing practices. Herd losses due to BVD accumulate steadily once an infection is established. It starts with mortality of PI calves prior to and after weaning, but the real damage is from non-specific production losses and secondary infections brought on through immune suppression of infected contacts.
Although PI suckling calves are considered to be the primary reservoir for BVDV in a herd, PI adults can also be present. It has been reported that 17 to 50 per cent of PI calves actually reach breeding age. PI breeding females and their offspring transmit BVDV to herdmates and spread it widely to contact herds in places like community pastures, or any situation where animals are comingled. Persistently infected females will always produce a PI calf themselves. Research has shown that about seven per cent of PI calves are born to persistently infected dams. The vast majority (93 per cent) of PI calves, however, are born to non-PI dams exposed to BVDV during gestation.
Male PI calves will occasionally be selected for use as breeding bulls. The amount of BVDV excreted in the semen of persistently infected bulls is very high. As well, circulating virus may exist in herds following removal of PI calves. Introduction of the infection into a totally susceptible population invariably causes extensive losses until a state of equilibrium is reached.
It is heartening to see the Saskatchewan industry taking a co-ordinated approach to BVD control.
The University of Michigan headed a five-year project to eradicate BVDV from Michigan’s Upper Peninsula herds, one of a very few examples in North America showing that regional eradication is possible.
VanLeeuwen says Norway’s experience with BVD provides an example of how a highly co-ordinated and regulated animal health system eradicated BVDV over a 10-year period.
It all starts with understanding BVDV and the impact the pathogen’s genetic diversity plays in its control. BVDV is not a simple pathogen, nor are the syndromes it causes, or the way it is transmitted. Knowing the enemy and monitoring its prevalence across a broad section of the industry is absolutely crucial. Along with effective surveillance to gauge progress, is the importance of a regimented protocol for diagnosing disease and freedom from disease. Other critical elements include BVD vaccination strategies and common-sense biosecurity practices faithfully implemented at all levels.
One weak link in the supply chain between calving shed and feedyard negates it all and the control pyramid crumbles.
It requires a commitment by the industry, veterinary community, and academia to put all the right pieces together and managing them in the long term. Identifying and eliminating PIs is the first basic step, and the single biggest obstacle in many control programs, especially in a marketing system that embraces the sale of high-risk calves at sharply discounted prices over the testing of all sale animals to eliminate PIs.
For now, the industry’s inability to accept presale testing is one major reason why BVD eradication remains a pipe dream.
— Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent by email to Canadian Cattlemen or to the WCABP.