Although BVDV has the potential to inflict severe negative economic consequences throughout all phases of the beef and dairy industries, the reality of BVDV epidemiology in the cow-calf segment is that only approximately 8.8% of U.S. cow-calf herds are infected (National Animal Health Monitoring Service, 2008). “Publicized BVDV outbreaks, the complexity of the disease and the novel disease reservoir of persistently infected (PI) cattle has permitted BVDV to garner a high degree of exposure among veterinarians and producers,” says Jason Nickell, DVM, PhD, Dipl. ACVPM, Bayer Animal Health. “Additionally, numerous BVDV tests are currently available which easily facilitates implementation of testing regimens.”Whole-herd testing is expensive, and should be used strategically in herds where veterinarians feel the risk is high enough to warrant the cost.
As stated above, testing costs money. However, if it can identify individual animals whose removal from the herd can benefit it economically, it has a high value. If it’s implemented in negative herds that do a good job of biosecurity and have little risk, it’s money that may have been better spent to improve upon other disease control or production measures. In 2011 Nickell et al published a study in the Journal of Veterinary Diagnostic Investigation using stochastic models to look at the economic viability and value of different types of BVDV testing strategies in different herd situations.
The cost of individual-animal BVDV testing can range from $4-$7/head, and the producer’s initial financial investment can be significant in larger herds. “Our data suggests that as test cost increases, economic value of whole-herd VDV testing decreases,” Nickell says. “However, as the pre-test probability of a herd being BVDV positive or herd prevalence in a population increases, the negative influence of test cost on the overall value of whole-herd testing is reduced.” This is likely due to an increase in the probability of detecting a PI animal and reducing the negative influence of BVDV in the herd. The testing strategy paid for itself when herds are positive.
The low cow-calf herd prevalence coupled with a high variability of potential clinical outcomes calls into question the economic sustainability of instituting whole cow-calf herd testing programs across either large populations or in individual cow-calf herds without strong signs of BVDV infection.
“This study was designed to estimate the relative economic value of whole-herd BVDV testing among cow-calf herds based upon different levels of herd prevalence/pretest probability of disease and across multiple BVDV testing strategies,” Nickell explains. A “test strategy” was defined as all testing regimens that involved either a single test or a two-test system based upon currently available individual animal tests. “In other words, we were asking if it was viable for veterinarians to implement whole-herd tests across the range of their clientele or if that decision should be more critically assessed.”
Assess BVDV probability first
Nickell’s data suggests that when dealing with individual cow-calf herds, and before a whole-herd BVDV testing regimen is implemented, the herd veterinarian should assess the probability of the herd possessing a PI animal. “This is imperative as a whole-herd testing regimen among all cow-calf herds with a relatively small probability of being BVDV-positive will likely result in a negative financial outcome due to test costs and potential culling of cattle with false-positive test results. The veterinarian’s assessment is the parameter in which the entire decision to institute a whole herd testing regimen should hinge upon.”
Estimating the pre-test probability of BVDV can be determined by evaluation of clinical and reproductive history, animal procurement practices, grazing management such as communal grazing, and diagnostic test results on dead cattle. Additionally, knowing these same parameters for neighboring herds would be valuable as well. “With this information, veterinarians can then formulate their own opinion as to the likelihood of a herd’s BVDV status,” Nickell says.
Once the probability of the herd being BVDV positive is high enough to justify a whole-herd testing regimen, then a test strategy that will maximize value yet provide a realistic approach for the producer should be implemented.
Low-prevalence testing not profitable
In Nickell’s study, three different pre-test probabilities of a herd being BVDV positive (~ 0.10, 0.20, and 0.50) across three different herd sizes (50, 100, and 500 head herds) while evaluating 12 different BVDV test strategies, were assessed. Results suggest that when the pretest probability of disease is low (i.e. 0.10 and 0.20 in this case), implementing whole-herd BVDV testing strategies is not profitable on average regardless of the herd size or the test strategy.
If the pre-test probability of a herd being BVDV positive is high (i.e. 0.50) then the relative value of a whole-herd BVDV test is justifiable, on average, regardless of herd size and test strategy. Of the 12 test strategies evaluated, most performed relatively consistent to one another regardless of herd size and pre-test probability.
“Despite a potentially high estimated probability of a positive BVDV status, the herd may not actually have BVDV as many of these parameters discussed above are not necessarily specific for BVDV infection and recalling that most herds in the U.S. are BVDV negative,” Nickell explains. “Nonetheless, our data suggests that if the veterinarian does not have strong reason to think that the herd is infected, it is highly likely that whole-herd testing will not be profitable. Therefore, the herd veterinarian’s overall assessment of the potential for harboring PI cattle is imperative to ensure that implementation of a whole-herd BVDV testing regimen is economically justified.” Nickell says these findings were consistent across all of the herd sizes evaluated in this study.
Despite reports of BVDV causing production loss, other data suggests that the presence of PI cattle does not always induce a negative impact on production. “In other words, production parameters may be sufficient and one may not know that a PI animal is present unless identified by a test,” Nickell says. “We know that the herd prevalence of BVDV is low in the U.S., and the goal of BVDV testing should not be to simply identify and remove PI cattle, but rather to test a cow-calf herd in order to eliminate the negative production and economic effects of BVDV by finding and removing PI cattle.”
A whole-herd test for PI cattle may be implemented in order to establish that the herd is truly negative before starting a policy that all herd additions be tested for BVDV PI status and confirmed negative prior to contact with the breeding herd. If whole herd-testing is instituted in a low prevalence situationor in an individual herd with a low probability of being BVDV positive, the financial reward is likely to be minimal and more likely to be of negative economic value.
However, if the producer’s goal is to simply ensure that their herd is BVDV negative, then the relative economic value of a whole-herd test has limited impact on the decision to test. In this case, choosing a test strategy that minimizes false positive diagnoses should be the ultimate goal, Nickell says.
False positives can be a problem
Due to biological variation and human error, it is unlikely that a diagnostic test will be 100% accurate on all occasions. Therefore, in low-prevalence disease situations across a population (or in low pre-test probability estimations of individual herds), most “positive” test outcomes are most likely to be falsepositives-truly negative animals with a positive test outcome.
In the case of BVDV, it is recommended that test-positive (presumptive PI) cattle be euthanized. Therefore, if one is to implement a whole-herd testing strategy in a low-prevalence population, it is likely that false-positive cattle could be misclassified and euthanized.
Diagnostic test specificity can be increased by retesting “positive” animals a second time, or “testing in series”. “Our data suggests that an immunohistochemistry (IHC) test on skin tissue followed by a confirmatory IHC test on initial positive cattle maximized value regardless of the herd size or probability of a positive BVDV status when compared to other testing regimens,” Nickell says. “This finding was not consistent across all evaluated test strategies.”
In contrast to herds with a low risk of being BVDV positive, positive tests among herds with a high probability of being a positive BVDV status are most likely true positives.
Convincing clients to test entire herds, let alone retest cattle that test positive on the initial test, can be challenging. “If the pretest probability of a herd being BVDV-positive was deemed to justify whole-herd testing, a single IHC test or real-time rtPCR test would, on average, be financially sound. These strategies may not maximize economic value compared to the two-test IHC regimen, but would obviously reduce the time and labor of putting animals back through the chute a second time,” Nickell explains.
At the end of the day, a model is just that-a model. “A veterinarian must interpret the results and decide if the modeled herd is similar to herds of his/her clients,” Nickell says. By evaluating multiple herd sizes and herd prevalences in this study, the robustness of the model was maximized. “However, attempts to extrapolate these results to existing herds that implement different management practices should be done with caution,” he advises.
If a positive herd doesn’t test for BVDV and the PIs live long enough to be sold, then the producer is paid for the market value of that PI calf despite potential production loss attributed to the presence of PI cattle. If a producer tests and finds PI or “test-positive” animals, this model assumed that they euthanized the respective test-positive cattle and therefore forfeited any potential market value that animal may have. “If producers are committed to euthanize all ‘test-positive’ cattle, then healthy non-PI cattle (that may be incorrectly classified as PI) are at risk of euthanasia. This along with test cost is why whole-herd testing in low prevalence scenarios does not provide a positive economic value (on average).”
But some producers and their veterinarians who are risk-averse may still want to test, regardless of the likelihood of positive animals. In those situations, Nickell urges veterinarians to implement a two-test system such as IHC followed by IHC on test-positives in order to reduce the risk of euthanizing false-positive cattle. In those scenarios, the cost to the producer will likely only be the cumulative test cost without the added cost of euthanizing false-positive cattle.
“But given the low herd prevalence among cow-calf herds and the desire to optimize the value of whole-herd testing, the motivation to conduct whole-herd testing regimens among individual cow-calf herds should be based upon the herd veterinarian’s opinion as to the pre-test probability of disease,” Nickell says.
Testing herds built after the drought
Now that many cow-calf producers have been through a two-year drought where liquidation and then repopulation of herds has been occurring, BVDV testing may be a critical step for biosecurity. “Importation of replacement bulls and females increases the risk of exposing the existing herd to BVDV by either direct exposure at the time of purchase, or indirectly after giving birth to a PI calf,” says Jason Nickell, DVM, PhD, Dipl. ACVPM.
Currently, a commercial method to test the PI status of an unborn fetus does not exist. Therefore, although individual testing of mature replacement cattle will likely reveal no PI individuals and transient infections can be dealt with via isolation prior to introduction to the existing herd, pregnant females may still be harboring a PI fetus. “Therefore, the actual BVDV exposure to the existing herd does not occur until calving season,” Nickell says.
“Research generated at Kansas State University observed a 50% increase in the likelihood of importing BVDV into a negative herd when purchasing pregnant heifers over a 10-year period if the calves were not tested at parturition. Extreme care and detailed management must be observed when procuring replacement cattle.”
Bulls need to be factored in as well, and Nickell says it can be easy to forget about them. “The probability that a sexually mature bull is PI with BVDV is extremely low, however, it is possible and they should be incorporated into a whole-herd testing regimen or at the time of purchase and prior to introduction to the existing herd.” Once they test negative, there is no longer a need for future tests.