For many diseases affecting cattle in North America, prevention is difficult and eradication near impossible. About the best we can hope for is to achieve some reasonable level of control. However, in the case of bovine viral diarrhea (BVD), we have the tools for effective prevention and, potentially, even eradication, at least on a local scale. These include effective vaccines, accurate and affordable diagnostic tests and a good understanding of how the disease spreads within and between herds.
And yet, in spite of those tools, BVD remains one of the most prevalent and economically important diseases of cattle in North America.
The term “bovine viral diarrhea” can be misleading, as the most common and damaging impacts of BVD occur in the form of reproductive losses and respiratory disease. The key to preventing and eliminating BVD lies in identifying and removing persistently infected (PI) animals, which act like “Typhoid Mary,” shedding the virus far and wide, on ranches and anywhere else they might travel.
Strategically designed programs of testing and biosecurity can block that proliferation and spread of the BVD virus, but as with many investments in animal health and risk-reduction, producers want to see an economic incentive. Increasingly, as cattle values rise along with understanding of the impact of BVD, the economic incentive for testing has become tangible and measurable. Data from over 400,000 cattle marketed through Superior Livestock Auctions in 2013 showed an average premium of $2.97 per hundredweight for calves identified as BVD-PI free. For a 600-pound calf, that meant an advantage of $17.82 per head. After subtracting the cost of testing, producers netted at least $14 per head. Those premiums are likely to grow as high cattle values pressure stocker operators and cattle feeders to minimize risk as much as possible. And from an animal-health and reproduction standpoint, cow-calf producers increasingly realize one of the best ways they can reduce risk is to keep BVD-PI cattle out of their operations.
Understanding the enemy
In mid-October 2014, two symposia focusing on BVD and related pestiviruses took place in Kansas City. The first of these, sponsored by Merck Animal Health, Thermo Fisher Scientific and the Kansas State Veterinary Diagnostic Laboratory, explored the possibility of BVD eradication. The next was the international symposium titled “Pestiviruses: Old enemies, new challenges,” which provided updates and discussions of the latest research into BVDV and related viruses.
During the symposium, John Vanleeuwen, DVM, MSc, PhD, from the University of Prince Edward Island in eastern Canada, outlined some of the impacts of BVDV in North American beef and dairy herds.
Most BVD cases in dairies, cow-calf herds or feedyard animals are transient infections (TI). They can cause considerable losses in terms of damaged reproduction, respiratory and gastrointestinal disease and suppressed performance but do not usually last long within these infected animals.
PI animals usually occur when the cow experiences a TI with a non-cytopathogenic BVDV strain during gestation, generally between days 30 and 110 post-breeding, prior to immune system development. The fetus becomes infected, does not mount an immune response to the virus, and sometimes survives to term. These PI calves often die before weaning, but some can survive to infect other animals at subsequent production stages, including other pregnant cattle, maintaining a “super-shedding” viral reservoir on the farm.
The impacts of BVDV can vary widely, Vanleeuwen notes, because of differences in virulence between BVDV strains, previous exposure and resistance levels within herds, and environmental factors. However, he cited numerous studies showing substantial economic clinical and subclinical damage in beef operations, including:
· Moderate to severe reproduction losses in the cow herd including reduced conception rates, embryonic deaths, abortions and congenital defects.
· A 43 percent higher rate of BRD treatment in feedlot calves exposed to a PI animal.
· Beef calves with direct PI exposure had 20 percent lower average daily gain.
Speaking at the conference, Kansas State University veterinarian Dan Thomson, DVM, PhD, said researchers estimate about 1 percent of cattle born in the United States are persistently infected with BVDV. About half of those die before weaning, leaving about 0.5 percent of older calves as PIs. About half of those die or are “railed” in the feedyard, leaving about 0.25 percent that can spread the virus to their pen mates for the remainder of the fattening period. A small number of PI animals also can survive to adulthood in dairies and cow-calf herds.
Back when Thomson provided veterinary services for Cactus Feeders, he and Texas Tech epidemiologist Guy Loneragan, BVSc, PhD, conducted several tests looking at the prevalence of PI cattle in the feedyards and their effect on outcomes. Tests revealed that of 1,900 chronic “railer” type cattle, 3.6 percent were PI. They also ran BVD tests on 1,500 cattle that died during the same period, and 4 percent of those were PI.
The group also BVD-tested 2,860 high-risk calves from 20 pens 10 to 14 days after arrival. They found 10 PI calves among the group, a prevalence of 0.34 percent, which doesn’t sound bad, except those calves represented five pens, meaning 25 percent of pens were exposed. Over the next few weeks, those PI-exposed pens experienced a 30 percent pull rate compared with 18 percent from the other 15 pens.
During an earlier conference with the Academy of Veterinary Consultants, Michigan State University (MSU) veterinarian Dan Grooms, DVM, PhD, DACVM, outlined research showing the impacts of BVD during the finishing phase.
Grooms says BVD is a known contributor to bovine respiratory disease (BRD), helping set animals up for infection with bacterial respiratory pathogens. He and his team have conducted several trials to explore relationships between exposure to PI calves, vaccination and BRD incidence in the feedyard. In one trial, the researchers shipped calves from Alabama to Michigan for feeding. Prior to shipment, the calves were tested and confirmed negative for BVD exposure. One of the trucks included two PI calves, providing exposure during the trip. The other truck had no PI calves. Upon arrival, half of each group were vaccinated for BVD and the other half were not. This created four treatment groups.
Through the finishing period, 25 percent of the non-exposed, non-vaccinated group were treated for respiratory disease, compared with 65 percent of the exposed, non-vaccinated group and 40 percent for the exposed, vaccinated group. Average daily gains were lowest in the exposed, non-vaccinated group, and feed efficiency also was numerically lower.
Results of the study indicate exposure to BVD-PI calves during marketing, transport or upon arrival correlates with higher incidence of respiratory disease, and that vaccination upon arrival reduces morbidity rates in PI-exposed calves.
Besides direct exposure to a PI animal, Thomson outlined other ways the virus could spread within herds. Tests at K-State, for example, have shown that BVDV can be spread within an operation through rectal palpation when a PI animal is in the palpation group, and from exposure in hospital pens following treatment of a PI animal.
Biosecurity, vaccination and testing
Results of these tests show the importance of biosecurity practices in hospital facilities and across beef operations. BVD vaccines, used as directed, provide protection, including fetal protection, but do not eliminate transmission to cattle exposed to large volumes of virus from a PI calf. Testing is critical for identifying and removing those “super-shedding” PI animals from herds.
In cow herds, Thomson recommends testing cows, calves and bulls in cases where producers know of or suspect the presence of a PI animal, see increased rates of abortions or open cows, or experience an increase in morbidity among nursing calves. Producers, with assistance from their veterinarians, can reduce the cost of testing by pooling samples for analysis and strategically testing calves first because the dams of test-negative calves must also be test-negative. Dams of PI calves should be tested to see if they are PI. Any PI animal should be removed from the herd, either going straight to slaughter or marketed ethically, with full disclosure. Cows that test positive can be isolated and re-tested in a few weeks, as the infection likely is transient.
A public website, bvdconsult.com, provides background information and decision-support tools to help cow-calf producers work with their veterinarians to create BVD-control, -prevention and -eradication strategies specific to individual herds.
Several discussions at the 2014 conference focused on the potential for eradicating BVD in the United States. Several European countries have used government-mandated and -funded eradication programs involving systematic testing, culling and continued surveillance to reduce or virtually eliminate BVD. In the United States though, we have more than 700,000 beef-cattle operations, most of them with fewer than 50 cows, and a market system in which cattle typically change ownership several times. In addition to the fragmented beef industry, there is still limited knowledge, testing and vaccinating for BVDV. Data from the USDA’s National Animal Health Monitoring System 2007 survey show that 15 percent of cow-calf producers had never heard of BVDV. And while 29 percent indicated they were fairly knowledgeable on the topic, only 4.2 percent had tested any of their cattle for the disease in the previous three years.
Participants in the symposium generally agreed that national eradication is unlikely, but a combination of education and market-driven incentives for testing and control at the cow-calf and dairy level could significantly reduce losses associated with BVD. Grooms outlined a project in Michigan demonstrating the feasibility of, and challenges in, BVD eradication. From 2007 through 2012, MSU, in partnership with Zoetis, USDA and the Michigan Department of Agriculture, engaged in a BVD-eradication program on Michigan’s Upper Peninsula.
The program included BVD testing, vaccination, biosecurity and ongoing surveillance, with a focus on removing PI cattle from herds. The effort began with communication and education, with the MSU Extension team, along with other project personnel, spreading out across the region to meet with beef and dairy producers. They sent out educational flyers and issued farm signs to identify participating farms. These efforts, Grooms says, generated excitement and built awareness of the value of testing, vaccinating and biosecurity among local producers. Once farms enrolled, the team worked with owners to customize a control program based on their production environments and BVD-risk levels.
Ultimately, the five-year project lacked time and funding to completely eradicate BVD from the region, but Grooms considers it a success. “What we really demonstrated is that this needs to be a cooperative project,” he says. He also notes that the program’s educational efforts helped build awareness about disease control in general, and helped producers improve their vaccination protocols, diagnostic testing and biosecurity efforts for controlling diseases besides BVD. This in itself, he says, might be the most valuable part of an industry-driven eradication program.
See this article and features on gearing up for growth, preparing for bull sales and incoming NCBA president Phillip Ellis in the January digital edition of Drovers/CattleNetwork.