Editor’s note: second in a BVDV series
Cow herds at high risk for bovine viral diarrhea virus (BVDV) are in jeopardy of exhibiting abortions, weak calves, reproductive problems, acute disease and the creation of persistently infected (PI) BVDV calves.
Larson et al found the economic loss of the disease for cow-calf producers when evaluated over a 10-year period ranged from $14.85 to $24.84 per head in herds with known infection.
Evaluating whether your clients’ herds are at high or low risk for BVDV problems, however, can be a bit subjective. David Smith, DVM, PhD, DACVPM, University of Nebraska in Lincoln, says it’s difficult to put numbers to these designations. “Predictions based on the clinical intuition of veterinarians regarding the BVDV status of herds is poorly sensitive and poorly specific,” he says. Published studies show that BVDV has been found in a large proportion of herds believed to not be infected, and not been found in a large proportion of herds where the agent was expected to be present.
Developing a strategy to test calves, cows, bulls and incoming replacements can help your clients monitor and prevent BVDV in their herds.
“In my opinion,” adds Smith, “the most critical need for BVDV control is a validated strategy to determine the BVDV status of cattle herds. We cannot be sure which actions to take until we reliably know if the virus is in the herd, and without a reliable measure of herd status, it is impossible to measure progress.”
Dan Goehl, DVM, Canton Veterinary Clinic, Canton, Mo., defines a high- or low-risk herd based on the movement and exposure of new cattle and an assessment of the health status of the herd. “A herd with very little exposure but excessive open cow numbers in my mind is high risk,” he says. “It is important to note that incidental contact with the neighbor’s cattle across the fence is recognized as exposure even though they are not moving in and out of the herd in question.”
Dave Rethorst, DVM, South Central Herd Services, Red Cloud, Neb., says he’s not sure there’s any such thing as a low-risk herd in his area of south-central Nebraska. “Low risk, to me, would be a closed herd that is on a sound vaccination program,” he says. “There are no closed herds in this area. My strategy has been to develop vaccinations programs to minimize risk and implement testing on an as-needed basis.” Rethorst adds that lower-risk herds would be closed herds, and high-risk herds would be non-closed herds.
Bob Larson, DVM, PhD, University of Missouri, says that high-risk herds are those with confirmed PI cases or several non-PI BVDV cases. Also, those herds with higher than expected calf losses pre-weaning and poorer than expected pregnancy percentages can also be considered high risk.
Low-risk herds, according to Larson, are those that have a high pregnancy proportion and a low pre-weaning death loss and have not had any recently diagnosed PI cattle. Larson recommends to necropsy calves that have died pre-weaning to determine if they are PI or non-PI to help assess a herd’s risk status.
A sound BVDV testing program can still be undermined by fenceline contact with other cattle.
Goehl says how you approach a herd has to be based on the value of the animals and at what level the herd needs to be. “For example, a seedstock herd doing a lot of embryo work has zero tolerance for any exposure to BVDV. Some of the herds we monitor the closest are actually at the lowest risk.” But at this point, most herds aren’t monitoring for BVDV, says Goehl. “I’d say that greater than 95 percent of producers are not yet testing.”
In herds where Goehl suspects a problem, he begins by taking ear notches for IHC testing on the newest calf crop. So far, he has not found a PI animal older than 24 months.
Darrell Farmer, DVM, Tucumcari, N.M., is working to implement BVDV-testing strategies with some of his ranchers. This will be the first year to PI test calves. Unfortunately, due to a multi-year drought and ranching economics, he’ll have to start more conservatively than he would like. At branding time, he plans to vaccinate calves with killed BVDV Types I and II. At weaning, blood samples will be drawn from a representative number of calves to check blood titers for PI activity. Calves will be given modified-live BVDV vaccine and then placed in the “warm-up” pen for 45 days.
Farmer also plans to necropsy and ear-notch everything that dies on the ranch and test for BVDV if it’s suspected. In addition, all new cattle that come onto the ranch will be ear-notched. “Biosecurity-wise, we’re going to start a program with our cow herd at home to find PI and other things, and then test every new animal for BVDV, TB, brucel-losis, etc. Everything we bring in will be clean or will be quarantined until test results come back.”
Farmer has had good reception with clients with this approach. “By starting conservatively, I think we’ll introduce them to the idea of having a proactive rather than a reactive program.” In east-central New Mexico, one of the biggest problems is convincing clients that there is financial reward to be gained from having a good, proactive program. “I’m hopeful that within a year or two these ranchers will come around and really start accepting these types of programs,” Farmer adds. “Severe drought and financial consequences have made people take a close look at their operations and it’s making them aware that they are going to have to change what they are doing in a lot of ways.”
Rethorst encourages clients to buy certified PI-free animals when possible, but it’s not always easy because there aren’t many around. “The problem I see is that seedstock producers are reluctant to test and advertise as such because they feel customers will see it as admitting the seedstock producers have a BVDV problem.”
Eventually, Rethorst would like to get his herds that are purchasing replacement females on a testing program. “I am also working to get seedstock producers that I do work for to test their sale bulls each year.”
Most discussions regarding BVDV control focus on the importance of identifying and removing PI cattle. This is an appropriate action, says Smith, but in placing emphasis on finding PIs, we may fail to adequately communicate to the producer that the primary objective is to prevent PIs in the first place by protecting the fetus from exposure to BVDV. “Producers may better understand and execute biocontainment by thinking in terms of protecting their pregnant cow herds from exposure to cattle of unknown BVDV status – whether they are transiently or persistently infected – including bulls, newborn calves and the neighbor’s cattle.”
Rethorst says he is slowly getting clients educated about quarantining new animals. Most still put new animals with the resident herd immediately, which can wreak havoc with the health program in the existing herd. “We had a client purchase cows this spring and he ‘isolated’ them – across the electric fence – from the existing herd. When they started calving, the calves got under the fence and created quite a scours problem in the existing herd.” The good thing that resulted, says Rethorst, is he now has an off-site quarantine plan developed for the future and a BVDV-testing plan for purchased cows.
As far as purchased heifers, Farmer says he will insist they be PI-negative before delivery to the ranch. “If a client decides to take them if they haven’t been tested, they need to quarantine them from any other cattle on the ranch until they are tested.” Those animals need to be quarantined for at least 21 days with no fenceline contact with the original herd on the ranch. He is also recommending that clients PI test their bulls.
Goehl notes that if a herd is high risk and has zero tolerance, he ear-notches every new animal and quarantines every arrival for 30 days. “If the number of animals moving in and out is minimal or the animal is going to be exposed to bred animals, then it is a good idea to do comparative titers to rule out an acute infection.”
Goehl recommends that all replacements are certified PI-free and if not purchased that way are tested on arrival. “Even if the owner will not quarantine and the animals are already in the herd before you have the test results, that’s a better situation than not knowing at all.”
He admits, however, that in some commercial herds, quarantining new arrivals may be impractical. “It can completely derail a testing system if you think you have a BVDV-free herd with a quarantine in place and unknowingly an acutely affected animal is placed in the general population, especially if pregnant animals are exposed and a PI calf is born as a result.”
Larson advises to watch for those animals that may fall through the cracks, including pregnant animals brought to the farm that may be PI-negative but carrying a PI-positive fetus. To avoid this, some herds isolate those purchases until the calves are born and then test the calves prior to mixing the pairs with the rest of the herd. “Other herds go ahead and mix those purchased pregnant animals into the herd, but then gather the new pairs prior to the start of the next breeding season and test all the calves from purchased replacements.”
What to do with PIs?
As seen on the Academy of Veterinary Consultants’ BVDV Guidelines document and other BVDV programs around the country, it’s recommended to sell for slaughter positively identified PI heifers and cows, and to euthanize PI calves. However, this may be a hard sell for some producers when faced with a seemingly healthy-looking calf with a positive PI test.
Smith says euthanizing PI calves may be an economic disincentive to some producers in a BVDV program. “More to the point of the requirement is that PI calves should not be marketed, except directly to slaughter,” he says. While trying to fatten PI calves might be an attractive idea for some producers, it’s risky. Smith says in one particular case, a number of PI calves were weaned, fed and were gaining well until the group broke with signs of mucosal disease. “Almost all of them died after having eaten up a pretty good feed bill.”
Goehl says that’s why testing newborn calves is a good idea, because it’s easier for producers to euthanize a very young calf at that time. “It’s also much better to eliminate them early from the standpoint of stopping the perpetuation of BVDV in the herd.”
Consumers are looking for value-added products, and the livestock industry is no different. Being able to market dairy cattle as Johne’s-free or beef cattle as PI-free may be a financial incentive to some, and to some it may eventually be the price they have to pay to stay in the game at all. “Cattle producers recognize that they can add value to their cattle by marketing performance and genetics, says Smith. “BVDV-PI status of individual cattle is a health parameter that should also be marketable. This approach may be a better incentive for producers to consider BVDV control than what is accomplished by speaking in the traditional ‘disease control’ language.”
In Missouri, Goehl has one seedstock herd that is marketing its cattle as PI-free after undergoing testing programs. “I believe in the future it will become an accepted practice,” he says.
The Colorado State University BVDV program (see sidebar), for example, issues certificates to producers who are certified by a veterinarian and/or the CSU diagnostic laboratory as completing different levels of the Colorado Voluntary BVDV Control Program.
Goehl would like to see every client’s herd on some level of BVDV surveillance. “I don’t think it is necessary that every herd is ear-notching every animal born, but it is good to do a risk assessment and implement a surveillance program that is adequate for the needs of the herd,” he says. “Producers who are serious about remaining BVDV-free and realize the consequences will follow the BVDV guidelines.”
Next: BVDV surveillance programs
Colorado's voluntary BVDV program
Some states are encouraging cattlemen and their veterinarians to join voluntary BVDV programs. Not only can this help the individual producer combat BVDV in his or her herd, but the test results are added to a statewide database on BVDV- prevalence practices.
Jim Kennedy, DVM, MS, director of Colorado State University’s Rocky Ford Veterinary Diagnostic Lab in Rocky Ford, Colo., is heading a newly formed, voluntary BVDV program in his state. He’s initiated a database and will merge into it statewide BVDV test results from CSU.
The first part of the program is getting a handle on a producer’s current situation through a questionnaire (see sample). “The questionnaire is as much an educational tool as anything and is intended to stress quality assurance and to introduce some of the concepts of biosecurity to the producers,” says Kennedy. “We do score the questions and as long as the owner is willing to acknowledge those areas of discrepancy, we move on. If they don’t wish to make any changes, then we suggest they stop at that point and save their money for something else.” Once discrepancies are corrected to 90 percent or more, the producer gets a letter of completion, and implementation of a BVDV biosecurity program is undertaken.
Level Two involves serology on 14 unvaccinated (in the last 90 days) calves. If any titers are 1:512 or higher, the current calf crop is individually tested for PI’s with positives removed and dams of positive calves tested. It is important to not dwell on the cutoff titer of 1:512, but to use a titer that the diagnostic lab considers indicative of exposure, adds Kennedy. Then, a BVDV-vaccination program is initiated, and any new additions are tested. At this point, the producer is given a certificate of having a low-risk BVDV herd.
To obtain a BVDV PI-free certificate, all requirements of Level Two must be met, and individual BVDV tests must be done on all cattle on the premise, not represented by previously tested calves. This would include retained open cows, bulls and show stock. All PI-positive animals are removed as are positive dams of positive calves.
With just a few herds on the program in this early stage, Kennedy says they have found .25-.5 percent positive PI animals. “I think this will change as we are beginning to do testing in some herds where BVD abortions were diagnosed earlier this spring.”
For more information on Colorado’s testing program, visit the Web site at www.dlab.colostate.edu/BVDControlProgram/bvdcontrolprog_main.cfm.
Colorado Voluntary BVDV Control Program Questionnaire
1. Do you regularly consult with your veterinarian? Yes No
2. Do you pregnancy check your cows? Yes No
3. What was last year’s conception rate (number pregnant divided by number exposed)? >90% or
4. What was last year’s weaning rate (number weaned divided by number exposed)? >90% or
5. How many calves died after weaning? 5%
6. Do you annually vaccinate all of your cattle for BVD? Yes No
7. Do you record the brand, lot number and expiration date of the vaccines you use? Yes No
8. Do you submit aborted fetuses for diagnostic work-up to your local
veterinarian? Yes No
9. My veterinarian has never diagnosed BVD in my cattle. Yes No
10. Do you routinely have all new additions to your herd tested for BVD?
11. Do you routinely isolate all new additions for at least 21 days?
12. Over the past 3 years your calving rate has always been above 90%?
13. Greater than 95% of my cows calve within 90 days. Yes No
14. Do you see wildlife grazing with your cattle less than three times per month? Yes No
15. Do you spend less than $5.00 per animal per year to treat illness (excluding routine vaccinations, dewormers, pour-ons and implants)? Yes No
16. Your veterinarian has informed you of the different types of BVD viruses and their importance in vaccination selection. Yes No
17. Did you treat less than 10% of last year’s calf crop for pneumonia or other respiratory disease? Yes No
18. Do you treat less than 10% of last year’s calf crop for GI disease, scours, bloat, etc.? Yes No
19. Do you maintain production records that individually identify each
animal? Yes No
20. Have you monitored weaning weights over the past 3 years? Yes No