While the bovine viral diarrhea virus (BVDV) is a complex and challenging problem in beef and dairy cattle, the industry possesses an array of tools for effective control and, potentially, eradication of this destructive pathogen. 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.
In mid-October, 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 such as classic swine fever virus.
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. Persistently infected (PI) animals usually occur when the cow experiences a transient infection with a non-cytopathogenic BVDV strain during gestation, generally between day 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 many cases, including:
- PI cows in dairies with a 23 percent higher rate of clinical mastitis.
- 3.5 to 7 percent of fetal deaths in dairy cattle and many congenital defects are attributable to BVDV.
- TI cows in dairies with a 10 to 30 percent drop in milk production for three weeks.
- 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.
In a poster presented at the symposium, researchers documented an outbreak in South Dakota, where a ranch purchased 136 bred heifers that apparently were exposed to BVDV during gestation. The following calving season, of 128 calves born, nine had congenital lesions consistent with BVDV infection. Diagnostic tests found 44 of the calves were BVDV positive. Five of the 44 infected calves died following weaning. Later tests identified 36 BVDV PI and three TI calves. The three TI calves developed strong BVDV type 1 and BVDV type 2 antibody titers. Of the remaining 39 infected animals, 23 died, two were euthanized and 11 were sold for immediate slaughter.
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.
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. Researchers also explored transmission in hospital pens by first housing a known PI calf in the pen. Two hours after removing the PI calf, the workers placed three non-infected cattle in the pen. Within 10 days, two of the three calves tested positive for BVDV.
In another study, researchers inoculated various materials common to livestock operations with BVDV in a mucus solution and found high survival rates after 48 hours on latex and in water. Survival rates were lower on rubber, wood, enameled metal, galvanized metal and soil. Thomson says salt blocks appear to effectively kill the virus. Researchers also applied nasal secretion from a PI calf to the lid of a vaccine bottle and allowed it to dry. Then they used that bottle to vaccinate two calves. One of those calves tested positive for the virus seven days later and both calves showed signs of exposure within 21 days.
Results of these tests show the importance of biosecurity practices in hospital facilities and across beef and dairy 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. Bulk tank milk can also be monitored for the presence of a PI in a milking herd.
Several presenters outlined European programs, such as in Scandinavia and Germany, where systematic testing, culling and continued surveillance — sometimes with and sometimes without support of vaccinations — have significantly reduced or virtually eliminated occurrence of the disease. In Norway, for example, beef and dairy producers, under a government-mandated program, used extensive testing in beef and dairy herds, culling of all PI animals and follow-up surveillance to virtually eradicate BVD. Researchers estimate the cost of the 10-year program at about $9 million, with annual savings to the beef and dairy industries estimated between $8.5 million and $34 million in productivity losses.
The United States, however, presents additional challenges to BVDV eradication. That message was clear from a presentation by Auburn University veterinarian Dan Givens, DVM, PhD. We have more than 700,000 beef-cattle operations in the United States, most of them with fewer than 50 cows, and a market system in which cattle typically change ownership several times as they pass through the cow-calf, stocker and feedyard sectors.
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. Among producers with 200 or more head of cows, the percent who had tested cattle for BVDV increased to 15.6. In that same survey, 28 percent indicated they vaccinated cows, 25 percent vaccinated replacement heifers and 33 percent vaccinated calves against BVDV.
Participants in the symposium generally agreed that government support, including either indemnity payments for culling of PI calves or regulations governing testing and culling, is unlikely and probably undesirable. Instead, we’ll need market-driven incentives for control at the cow-calf and dairy level, based on a recognition that tested, BVDV-free cattle carry a premium value in the marketplace.
More education will be required, but incentives have emerged in recent years. 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 the high value of feeder cattle and finished cattle pressures stocker operators and cattle feeders to minimize risk as much as possible. And from an animal-health standpoint, producers increasingly realize one of the best ways they can reduce risk is to keep BVD-PI cattle out of their operations.
Educating producers about those benefits must be a key component in an eradication or widespread control program. During the conference, Michigan State University (MSU) veterinarian Dan Grooms, DVM, PhD, 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 in Michigan’s Upper Peninsula.
The Upper Peninsula, Grooms says, provided a good regional model as it is a relatively small area mostly surrounded by the Great Lakes, other than part of its southern border shared with Wisconsin. The area houses about 760 cattle farms, with a mix of beef and dairy operations.
The program included several components necessary for eradicating or controlling BVD, including testing, vaccination, biosecurity and ongoing surveillance, with a focus on removing PI cattle from herds. The effort began with communication and education, which Grooms considers absolutely critical. The MSU Extension team, along with Zoetis staff and others, spread out across the region to meet with beef and dairy producers, explaining the program and the value of eliminating BVD from their herds. 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.
Once the program was underway, 294 farms submitted samples, representing more than 18,000 cattle. Analysis identified 22 PI cattle from nine farms, including four beef and five dairy operations. Of the dairies, four had received cattle from a single heifer operation, which appeared to be the source of the virus.
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. It’s not just a top-down kind of government-regulated eradication program. It’s basically a project where people need to work together — producers, veterinarians, industry leaders, universities, pharmaceutical companies, diagnostic labs all working together to really make a significant impact in eradicating this virus.” 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.
So could we eradicate BVD from the United States? The general consensus among conference participants was that eradication is possible but maybe not feasible in the short-term, given the scale and structure of the U.S. cattle industry. However, a systematic control effort aimed at long-term eradication on a local, regional or national level definitely could bring dramatic reductions in BVD outbreaks and economic losses. As Jeff Baxter, global senior product manager for Thermo Fisher Scientific, suggested during the conference, when we think of “BVD,” we should think of biosecurity, vaccination and diagnostics.
Veterinarians can use a web-based decision tool, BVD Consult, to help clients create BVD-control, -prevention and -eradication strategies specific to individual herds. The website includes background, peer-reviewed articles and decision-support tools to guide vaccination, biosecurity and testing protocols. Find this resource at bvdconsult.com.