During the USDA National Animal Health Monitoring System Beef 2007–08 study, beef producers who expected to calve at least 70% of their cows and heifers from November 1, 2007, through June 30, 2008, were encouraged to collect ear notches from their entire 2008 calf crop. Ear notches were subsequently frozen dry until tested for indications of persistent infection (PI) with BVDV.
At the laboratory, notches were tested using an IDEXX antigen capture enzyme-linked immunosorbent assay test according to the manufacturer’s instructions. Overall, 205 operations collected ear notches from calves. The number of notches collected per operation ranged from three to more than 500. A total of 44,150 notches were collected and tested. The prevalence of BVDV among the tested notches was 0.12% (53/44,150). Within herds, the prevalence ranged from 0 to 16%. Eighteen of the operations had one or more positive samples, for a herd-level prevalence of 8.8%
Dan Givens, DVM, PhD, Auburn University, says the 8.8% prevalence is consistent with previous studies in which herd prevalence ranged from 4% to 17.2% in research involving more than 15 herds. “Interestingly, a study by Steve Bolin and others on samples collected in 1985 from 66 beef cow-calf herds resulted in detection of herd prevalence of 9%; thus, our management appears to have had a minimal impact on herd prevalence of BVDV in the last 25 years,” Givens says.
“Awareness of BVDV has increased,” Givens says. “However, I am not sure if this awareness has resulted in management that consistently implements effective and proven control measures.”
One PI can be a problem
The NAHMS study says despite the low prevalence of PI found in that study, approximately one of 12 operations had at least one PI calf. That one PI calf, however, can cause problems for the rest of the herd. “Due to the transmission methods and disease movement through the herd, one PI calf this year can lead to several next year,” says Brad White, DVM, MS, Kansas State University. “Although we tend to think of the disease as a low prevalence, the disease can have relatively high within herd prevalence in cow-calf herds due to the clustering associated with PI formation during gestation. Herds with PI calves will typically have lower reproductive performance measures, and a higher percent of calf health problems.”
The impact of a single PI on ensuring circulation of virus is really amazing, Givens adds. “These calves are literally viral factories. If all of the animals that a PI animal contacts are already immune to BVDV then the consequences of the virus that they consistently shed may be limited. However, if the course of their lives brings them into contact with naive animals, especially pregnant heifers, then the virus that PIs shed may result in a real train wreck.”
NAHMS data showed that 57.2% of operations believed that removing persistently infected calves affects the health of the remaining calves in the group. Only 7.7% of producers believed that removal of persistently infected calves does not affect the health of the remaining calves.
Vaccination like a “seat belt”
The NAHMS study reported that approximately 33.1% of operations vaccinated calves aged from 22 days to weaning for BVDV; 25.1% vaccinated weaned replacement heifers through breeding; 13.7% vaccinated bred replacement heifers through calving; 28.1% vaccinated cows for bovine viral diarrhea; and 24.3% of operations vaccinated bulls for BVDV.
Bulls are often overlooked during vaccination of animals. “Yet, on the cow-calf operation, effective immunization of heifers truly deserves the top priority to minimize the production of new PI calves,” Givens states.
Producers are more likely to vaccinate cattle than test cattle for BVDV, Givens adds. “Use of field-validated BVDV vaccines is increasing. In my mind, effective vaccination for BVDV is like wearing a seat belt and testing animals for BVDV is like having functional headlights on your truck out on the interstate. If you don’t wear a seat belt or have functional headlights, you are likely to end up in a wreck with terrible consequences. If you use your seat belt yet lack functional headlights, the wreck is still probable but hopefully the consequences will be limited. If you have functional headlights but refrain from using your seat belt, you are limiting the chances of a wreck but if one happens, terrible consequences are likely.”
Testing for BVDV
According to the NAHMS Beef 2007–08 study, about one of 20 operations that brought cattle onto the operation during the previous three years required testing for BVDV (4.5%). Givens believes more producers are testing for BVDV and/or observing herd productivity to see if BVDV might be causing reproductive losses in their cow-calf operations.
Testing can be complex depending on the situation. “When describing testing, it is important to distinguish testing the whole herd from testing only incoming animals as part of biosecurity program,” White notes. “I think there are many herds that could benefit from testing incoming cattle and calves to prevent introduction of BVDV into the herd.”
As for whole-herd testing, ear-notch testing is relatively easy, and BVDV is very important for the herds that have the disease as BVDV can have great effects in the cow-calf herd due to the impact on reproduction. “All this said, the majority — over 90% of herds — are negative and whole herd testing will have limited benefits,” notes White. “So finding characteristics of herds with a greater probability of being positive is important as this influences the economics of testing.”
White adds that identifying herds more likely to test positive is a great opportunity for veterinarians. Dr. Wittum’s paper (2001 Prev Vet Med) described that when veterinarians thought BVDV was in the herd, they found it 19% of the time and when herds were randomly selected, it was found 3% of time. “This shows that having veterinary input into potential herd status is a valuable tool in picking which herds to test,” he says. “I think most of the programs available (diagnostic testing, vaccination, biosecurity) are very solid; the question becomes when and if these programs are utilized.”
White believes screening incoming cattle is an important part of the disease prevention process. “One class of animals that has relatively high risk and sometimes slips through the cracks is the testing of calves that were imported as fetuses,” he says. “When pregnant cows or heifers are imported to the farm it is possible for these animals to be carrying a PI fetus at the time of arrival. There is no way to test these calves until after they are born and they should be tested as soon as possible after birth to avoid potential problems.”
And when you find a positive calf? “Appropriate handling of PI animals continues to challenge our industry,” Givens says. “If they are handled by euthanasia of the young calf or isolation and terminal slaughter, all segments of the industry reap the benefits.”
Givens believes our biggest hole in controlling BVDV is consistent application of a BVDV disease control program which involves appropriate surveillance and vaccination of cattle on the farm. “We have become more versed in talking about the effective BVDV control program, but we need to become more consistent in implementing it to reduce the prevalence of this disease causing agent.”
Givens says upcoming BVDV vaccines will soon contain more subtypes of BVDV such as modified-live vaccines that are likely to include a 1b strain. Also, diagnostic tests will soon be available to perform on the farm or at the vet clinic to quickly assess the PI status of an animal.
“Research will soon improve our tools for implementation,” Givens says, “yet the tool that will make the most difference in preventing and controlling BVDV is the sustained motivation to implement the reliable tools that are currently available.”