This week in Kansas City, researchers, veterinarians and industry stakeholders are discussing the latest thinking on bovine viral diarrhea virus (BVDV), its pathology, economic impacts and control strategies.
The Tuesday symposium, titled "Bovine Diarrhea Virus Eradication: Reality or Myth," was sponsored by Merck Animal Health, Life Technologies and thee Kansas State Veterinary Diagnostics Laboratory.
While it is well known that BVD outbreaks generally are associated with persistently infected (PI) animals, which constantly shed the virus in all their bodily fluids, the level of risk and degree of transmission in various production systems can vary significantly.
Most BVD infections in dairies, cow-calf herds or feedyard animals are transient. They cause considerable losses in terms of damaged reproduction, respiratory disease and suppressed performance, but do not last long. PI animals occur when the cow experiences a transient infection during gestation, generally between day 30 and 110 post-breeding. The fetus becomes infected, and sometimes survives to term. These PI calves often die before weaning, but some can survive to infect other animals at subsequent production stages.
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. A small number of PI animals also can survive to adulthood in diaries.
BVD vaccines, used as directed, provide protection in cow herds and feedyards, but do not eliminate the creation of PI calves or transmission to other 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 in cases where producers know of or suspect the presence of a PI animal, see increased rates of abortions or open cows or an increase in morbidity among nursing calves. Producers, with assistance from their veterinarians, can reduce the cost of testing by pooling samples for analysis. If the pooled sample shows positive evidence of the disease, individual testing of the animals included in the pool can identify the actual PI animal. In high-risk herds, Thomson recommends testing all calves, cows without calves and all bulls. 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.
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 affect on outcomes. Cactus at that time had initiated a “re-start” program for non-responding “railer” cattle that normally would have gone to slaughter. Tests revealed that of 1,900 re-start 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. Looking at the pull records, they found that most of the PI cattle were pulled and treated for coccidiosis.
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. Researchers at K-State, for example, conducted a test on rectal palpations for pregnancy detection. A worker first palpated a PI cow, then palpated eight of a group of 10 non-infected females. All eight showed antibodies for the virus after 13 days and five of eight were positive for BVDV isolation. The two non-palpated animals, which were housed with the rest of the group, tested negative. This mode of transmission could be significant, Thomson says, even in feedyards where crews sometimes palpate large groups of heifers for pregnancy upon arrival.
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. The group repeated the same test, but waited four days after removal of the PI calf to place three non-infected calves in the hospital pen. None of those calves showed signs of BVD exposure or infection 10 days later.
K-State researchers also measured the survival rates of BVD virus, suspended in a simulated mucus solution and inoculated on various materials common to livestock operations. These included latex, rubber, pine wood, enameled metal, galvanized metal, salt blocks, soil and water. After 48 hours, they found high survival rates of the virus on latex (20.7 percent) and water (16.4 percent). Survival rates were low on the other materials and Thomson says salt blocks appear to effectively kill the virus.
Researchers 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. Throughout the conference, veterinarians stressed the need for an integrated approach toward BVD control, and potentially eradication. This approach requires vaccination to prevent infection, biosecurity to prevent the spread of infection and testing and surveillance to identify and remove PI animals from herds.