Bovine viral diarrhea virus (BVDV) testing has come a long way with the advent of the immunohistochemistry (IHC) test used to identify persistently-infected (PI) animals with just a piece of skin or other tissue.
As calving season has begun and veterinarians and their clients will be testing calves and their dams for persistent BVDV infection, Bruce Brodersen, DVM, PhD, Veterinary Diagnostic Center, University of Nebraska, offers some timely reminders about PI testing of beef herds.
IDENTIFY GOALS BEFORE TESTING
Helping clients decide which and how many calves to PI BVDV test depends on the goals of testing. If the goal of testing is to eliminate BVDV from the herd, all calves, dams not represented by calves, replacement heifers, and bulls should be tested. “If a calf is negative, one is assured the dam of that calf is negative,” Brodersen says. “If a calf is positive, the dam of that calf needs to be tested to determine if she was the source of virus as a PI animal herself.”
If the goal is to determine if BVDV is present in the herd, there are better ways than just ear-notch testing, Brodersen adds. “Testing a percentage of the calf crop or suspect animals has a good chance of missing any PIs if they’re present,” he explains. “This is because the number of PIs in any given herd is often very low. Of course, testing sick animals may increase those odds of finding a PI.”
If one wants to determine if BVDV is present in a herd, serologic testing for the presence of antibodies against BVDV is one strategy. This needs to be done on a number of non-vaccinated animals which have lost their maternal antibodies. “Additionally, testing specimens collected from calves which have died is valuable in finding BVDV and that means testing more than just a skin sample,” Brodersen says. “You are also looking for acute infections, so other tissues such as thymus, several lymph nodes, Peyer’s patch, and palatine tonsils are good tissues to test.”
Only testing a subset of animals and missing a PI calf is basically no different than not testing at all, plus you’ve wasted time and money. “That single missed PI calf will continue to serve as a source of virus to infect or re-infect dams, fetuses, and cohort calves who ultimately end up in feedlots,” Brodersen notes.
If one is going to test all calves in a calf crop, it is best to test them as soon as possible so they may be removed as soon as possible. Those PI calves which are allowed to remain in the herd serve as a source of virus to cohorts. “Herds with PI calves can have problems with increased morbidity and mortality due to diarrhea and pneumonia as well as other miscellaneous infectious diseases,”
Brodersen says. “There are other factors involved with this and certainly good levels of maternal antibodies can help, but that passive immunity from the dam can be overwhelmed in the calf.” Brodersen reiterates to test all open cows, replacement heifers, and bulls -- anything in the herd not represented by a calf.
TESTING DEAD CALVES
While the IHC test is a great way to find PI calves, it’s not limited to the live calf. Knowing that a dead calf is PI-positive can help your clients make management decisions about their dams and stop the spread in a herd. “Sick calves and calves which have died are a very good source of specimens for use in a BVDV surveillance scheme,” explains Brodersen. “You can use skin from these animals to get an indication if it was persistently infected. This is true for herds which have established a BVDV-PI free status as well as those herds where BVDV infection is suspected.”
Post mortem skin is a good specimen to collect for immunohistochemistry for several days after the animal has died as long as it has not become dried and “leathery” or as long as it has not undergone extensive autolysis, Brodersen says. “Remember, skin from anywhere on the body is useful in identifying a PI animal. If the ears are frozen and desiccated, other skin can be used.”
COLLECTING OTHER SPECIMENS
Other tissues that can be examined for BVDV include thymus, palatine tonsil, lymph nodes (from more than one region), Peyer’s patch (ileum is best because the Peyer’s patch is diffuse throughout the ileum), and spleen.
“It is important to collect other specimens in addition to skin,” Brodersen notes. “If skin is negative by IHC and the other tissues are positive, one can be quite sure that animal was acutely infected and not a PI animal. In acutely infected animals, there may not be positive staining in all tissues. If all tissues are positive (including skin), by IHC, then there is a very high likelihood that animal was PI with BVDV.”
SAMPLING AND TESTS
“We still prefer immunohistochemistry (IHC) of skin samples/ear notches,” Brodersen states. “Those who do not want to leave a notch in the ear should remember the skin sample can be collected from anywhere on the body.”
The Nebraska Diagnostic Center laboratory offers other tests, but there continues to be problems, especially with false positives with both the ELISA and PCR, Brodersen says. “It becomes a problem for the laboratories in terms of technician time and repeated tests to sort out those false positives.”
For the IHC test, sending specimens in buffered formalin in a leak-proof vial of any kind is the best. These specimens do not need to be chilled, but in winter months, they should be protected from freezing and generally an insulated shipping container is sufficient. Serum or a skin sample can be used for ELISA and PCR, but as Brodersen pointed out, there can be problems with false positive test results.
DISPOSITION OF PIS
What to do with PIs is the hardest part of the equation. “Ethically, the calf that is persistently-infected with BVDV should be euthanized in order to pre-Bruce Brodersen, DVM, PhD vent any chance of that calf exposing other animals to infection with BVDV,” Brodersen advises. “However, economics is a factor in the real world. Those PI calves need to be quarantined from the remainder of the herd if they are going to be kept. Their survival time is not predictable, but some individuals will try to rear these calves to slaughter weight.”
IS ERADICATION POSSIBLE?
Widespread testing with immunohistochemistry using skin samples, especially in Nebraska, only started in 1998, but Brodersen knows of herds that have eliminated BVDV from their herds close to 10 years ago and have been able to keep it out, using sound biosecurity, surveillance, and good vaccination programs.
“I’m not sure if we’ll ever eradicate BVDV from the United States,” he says, “because of the complexity of the beef and dairy industries. I believe we can create ‘cattle streams’ where there is intensive testing at the cow-calf level and we can keep those animals free of BVDV in that ‘stream’ all the way through the feedlot to harvest. This would require cooperation among all parties involved through the ‘stream’.”
CREATING A PI
Getting a handle on bovine viral diarrhea virus (BVDV), especially persistent infections (PI), requires remembering how a PI calf is created.
Bruce Brodersen, DVM, PhD, reminds practitioners that a PI calf is the result of an in utero infection. This infection can occur in two ways. First and most common is when there is an acute or transient infection of the dam. That infection can be passed to the fetus. “Cows not well-vaccinated have the greatest chance of creating PI calves when that infection takes place,” Brodersen says. “Many vaccines are labeled as offering fetal protection and that is the reason those vaccines were developed, to protect from PI calves being created.”
Second is when the dam is PI, she will always have a PI calf. “It is essential the dam and her calf can be identified as a pair, since the dam will need to be found after the results of the calf test are reported from the laboratory,” Brodersen advises.