Bovine Viral Diarrhea
Lee Jones DVM,MS
Recent articles have publicized the premiums paid for BVD tested calves in some internet auctions. According to an analysis of Superior Livestock Auction data of more than 400,000 head marketed during 2013, the average premium paid for BVD-PI Free status calves was $2.97 per hundredweight – a 23% increase from the previous year. Similar to what producers have experienced with premiums paid for value-added vaccination programs, this BVD-PI Free premium translates into more profit per calf – an additional $14 per head on a 600 pound animal. “What I’ve found looking through data on more than 6.5 million head marketed with Superior Livestock over the past 19 years is that buyers truly appreciate healthy calves backed by information documenting their health status,” says Glenn Rogers, D.V.M, Superior Livestock Auction Consultant. “For example, BVD-PI Free status is one factor that does affect sale price. In 2013, the 1.4% of calf lots identified and sold as BVD-PI Free averaged $2.97 per hundredweight more, creating a really nice return for their owners.” – See more at: http://www.thecattlesite.com/news/46373/new-analysis-shows-bvdpi-testing-pays-netting-producers-14-more-per-head#sthash.AKGPChvS.dpuf In 2013, the 47 lots identified as being BVD PI Free brought a $2.97-per-hundredweight premium compared to non-BVD-PI tested cattle. For a 600-pound calf, this meant a $17.82 advantage. After subtracting the testing costs, producers netted at least $14 per head.
What is BVD?
Bovine Viral Diarrhea is a viral disease of cattle and other ruminants that is caused by bovine viral diarrhea virus. The virus has 2 biotypes; cytopathic (causes cell death) and non-cytopathic (does not cause cell death, and most commonly isolated from infected animals) and 2 genotypes, type 1 and type 2. Some cattle infected with BVD virus do not show any signs of disease. BVD infections may be acute, temporary infection or permanent. Permanently infected cattle are referred to as persistent infected (PI). BVD transmission after birth is considered transient but may lead to clinical disease and in severe cases even death. The effects of BVD infection on the developing fetus vary depending on the stage of pregnancy when the cow is infected. Infection early in the pregnancy will usually result in abortion. Exposure of the fetus to the non-cytopathic BVD biotype prior to 125 days of gestation can result in the development of a permanently infected calf or PI. The calf’s immune system does not recognize the BVD virus as foreign so the immune system does not attack it. Although many PI calves are born unthrifty and die or are culled within 6 months, some become adults. PI’s shed large amounts of virus throughout their lifetime and continually infect other cattle with the virus. Transmission from PIs to susceptible members of the herd can be by direct contact or through contaminated veterinary and farm equipment or facilities.
BVD: The Disease
Diseases observed in infected herds include: reproductive failure including failure to conceive, embryonic loss, abortion, congenital defects and stillborn or weak, unthrifty calves, and also respiratory disease or diarrhea in young cattle. Though most infections are transient, a PI is often the source for recurring infections in many herds. In adult cattle, the clinical signs vary from no obvious illness to fever, nasal discharge, oral lesions, diarrhea, decreased milk production, pneumonia, infertility or abortion in pregnant females. Signs that a calf might have been expose before birth include weak or stillborn calves, failure to get up, uncoordinated or stumbling and failure to nurse. Exposure after birth may lead to diarrhea and pneumonia.
|Effects of BVD Exposure on the Fetus During Pregnancy|
|< ---- Infertility|
|Variety of Effects|
|Months of Gestation|
Risks of BVD
BVD is present in the state of Georgia and has been diagnosed in beef and dairy herds. A USDA report in 2007 found 13.6% of the cattle operations in the Southeast had at least one BVD positive sample in their herd. In a south GA herd, 1/3 of the replacement heifers were found to be infected with BVD. In another herd, 3 PI calves were identified out of 16 calves. The owner wanted to know why he only had 16 calves out of 45 cows. If your herd has a problem with high incidence of calf sickness, poor reproductive performance including abortions and poor weaning weights, it is beneficial to test for and rule out BVD.
Controlling BVD in the Herd
Disease caused by bovine viral diarrhea virus produces significant economic loss to the beef and dairy industries. BVD costs farmers through increased reproductive losses, poor calf health, lower weaning weights and weaning rates, cost of treating disease and increased susceptibility to other diseases. Management practices to control the disease include vaccination of all members of the herd, biosecurity procedures to prevent introduction, bio-containment to interrupt the spread of the virus and laboratory testing of samples for the presence of the virus in a herd, with the final goal of identifying and removing PI animals from the herd.
Many vaccines today are proven to protect against acute BVD infections in the herd as well as to protect the developing fetus from BVD infection when used according to the label directions. Modified live viral (MLV) vaccines stimulate the part of the immune system that kills cells infected with BVD virus and can stimulate immunity in one dose. MLV should not be given to pregnant cows that haven’t been previously vaccinated with at least 2 doses of MLV. Killed viral vaccines require at least 2 doses and stimulate only the part of the immune system that produces antibodies to BVD. The antibodies bind and inactivate circulating virus particles. Killed vaccines are safe to give to cows any time and require annual boosters to be most effective. There are many kinds of vaccines and vaccine combinations. It can be confusing when deciding on a vaccine product and strategy, so consult with a knowledgeable veterinarian to develop an effective vaccine program tailored to your herd. The most convenient time to start cattle on a MLV program is after weaning and before breeding. Replacement heifers can be easily vaccinated before the breeding season, preventing any concern about using MLV in the future.
An effective biosecurity plan reduces the chances of introducing disease into the herd. New additions are kept separate from the herd for 1-2 months and watched for signs of disease before being combined with the rest of the herd. During this quarantine period testing can be done to determine if the animals carry or have been exposed to BVD or any other pathogen. Also, this is a good time to vaccinate the new additions and get them on the same program as the main herd. If the new additions are pregnant, the calves will have to be tested soon after birth to see if they are BVD negative. Cows that have been exposed to BVD virus can pass it on to the developing fetus without developing any outward signs of infection.
Biosecurity focuses on preventing introduction, while bio-containment is focused on preventing the spread of a disease that might be present in the herd. Implementing a good general management plan to identify and manage known risks in the herd will help interrupt the spread of many known pathogens. One of the most effective management tools to improve animal health, performance and fertility is to have controlled breeding and calving seasons. Eliminating exposure of the breeding herd to any source of BVD prevents future PI calves. Calving is an especially vulnerable time for cows and calves, so reducing pathogen exposure during this time gives cows and calves time to recover from this stressful period. Shortening the calving season, moving dry cows onto fresh pasture to calve and calving first calf heifers separately from the main cow herd are strategies that limit the exposure of cows and calves to pathogens and facilitate management.
There are several situations in which laboratory testing is needed: diagnosing the cause of disease, identifying infected animals, herd surveillance and marketing of BVD free animals. Diagnostic tests need to be accurate, economical and rapid. Diagnosis: If an animal is showing clinical signs of illness, a whole blood sample (purple cap tube) needs to be taken and sent to the lab (see details on laboratory testing below). If abortions have occurred it is imperative to send in as much fresh tissue as possible from the aborted fetus (or the whole fetus with placenta). A blood sample from the dam may be helpful to see if she is acutely infected. Looking for antibodies in the serum could help make a diagnosis but a second sample 30 days after the first sample (referred to as a convalescent sample) is needed to compare the antibody levels in the blood. Accurate, rapid results are needed to know how to treat affected animals and protect the other members of the herd.
Whole herd screening: You want to buy or sell a calf and you want to verify that it is not BVD-PI. If you purchase pregnant, BVD-free heifers, you must still test the calf after it is born to ensure you will not accidentally introduce BVD into the herd. Individual samples can be blood samples or tissue samples like ear notches.
Testing strategy depends on the diagnostic purpose. Even after infected animals are identified it still might be worthwhile to test all cattle in the herd or all exposed to the affected animal(s). Not all BVD positive cattle are PI. It is recommended to retest the animals that have a BVD positive test 4-6 weeks later. In some cases, BVD positive animals may be acutely infected but not persistently infected. Animals with acute infection may be able to recover from the disease and are not BVD carriers. Whole herd screening tests all animals in the herd. This can be done if there is a suspicion that BVD might be affecting the herd’s health or reproductive performance or to keep a known BVD negative herd. Fortunately, by testing the calf we also know the status of the dam. If a calf is negative then his mother is negative as well, so we get the benefit of testing the pair for one test fee. However, if the calf is positive the dam needs to be tested because she may not be BVD positive. If the dam is negative (not a PI), the likely source of BVD fetal infection was dam exposure during gestation (acute BVD infection) to another infected animal in the herd. Having documentation of BVD test-negative animals aids in marketing cattle. As mentioned earlier BVD negative feeder calves have sold for premiums in some marketing programs. After a herd has been test-negative for a few years, it is only necessary to test the market animals and all additions to keep adequate surveillance on the herd BVD status.
There are 3 basic BVD tests commonly used for routine or diagnostic testing: antigen capture ELISA (or ACE) on ear notches or blood samples; polymerase chain reaction (PCR) on ear notches or blood, or immunohistochemistry (IHC) on ear notches. The ELISA or ACE test detects the virus in the fluid (ear notch in saline) and is used for testing individuals. This test is not approved for mixed samples (pooling) to reduce costs and may result in a false negative test (animal tests negative when in fact is infected). The individual ACE test is the most common test used because it offers rapid results with the fastest turnaround time. PCR can be used to run several samples pooled together in a single test. The PCR process takes small amount of virus RNA and multiplies the amount to improve the test sensitivity. That is why it can be used with pooled samples. For PCR, up to 24 samples can be combined in a single test which can reduce testing costs significantly. IHC is a tissue staining technique to identify PI animals. The tissue is fixed in formalin, stained to target BVD virus infected cells and examined under a microscope. This is used for testing individual samples and takes longer to do than either ACE or PCR.
Some cattle that test positive with ACE or PCR are not PI’s. It is highly recommended to retest cattle that test positive with the ACE or PCR tests 4-5 weeks later to determine if the animal is a PI or temporarily infected. Cattle with a temporary BVD infection often clear within 4 weeks.
Normally, the turn-around time for ACE and PCR is 24 hours. However, if a pool tests positive, then all the pooled samples have to be run individually by ACE to identify the PI animal(s), which requires additional time (1-2 days) and costs. If time is critical, we suggest that you request the ACE test upfront. PCR testing is recommended when the number of samples to be tested is higher than 96.
|# of samples received at once||PCR pool size||PCR cost per head||Time to results (PCR)||ACE cost per head||Time to results (ACE)|
|1-5||NA||NA||NA||$5.00 plus $10 Setup fee||24 hours|
|10-23||10||$3.20||2-3 business days||$4.50||24 hours|
|24 or more||10||$3.20||2-3 business days||$3.50||24 hours|
Fees may change without notice. Check website for most current fees; www.ugavetlab.org
There are a lot of laboratories from which to choose for BVD testing. It is important to select a lab that has a verifiable quality control process and uses the correct tests in the correct way. False negative tests are worse than false positive because it gives a false sense of security and increases the chances of infecting other cattle. The Georgia Veterinary Diagnostic Laboratories in Athens and Tifton undergo regular quality reviews to verify their lab procedures are as accurate as possible. They are the only laboratories in Georgia accredited by the American Association of Veterinary Laboratory Diagnosticians (AAVLD), the gold standard for veterinary diagnostic laboratory accountability. If you suspect your herd may be affected quick action to find the source of infection can help reduce the damage by this potentially devastating disease. If your herd is not performing to your expectations and you have higher than expected calf loss or too many open cows, it may be worth investigating whether BVD is the cause. As with many things, ‘an ounce of prevention is worth a pound of cure’. The UGA veterinary diagnostic laboratories provide diagnostic guidance and assistance to veterinarians and Georgia farmers that desire information about eradicating BVD or other potential disease agents from their herds. Further information about the tests, prices, and turn-around time can be found at www.ugavetlab.org or by calling either laboratory: Athens (706-542-5568) or Tifton (229-386-3340).
Importance of Testing
Cattle farmers have an unprecedented financial opportunity in today’s cattle market. With healthy calves bringing as much as $1200 or more, the costs of disease in the cow herd is higher than ever. Reproductive losses caused by BVD or other diseases are preventable. Farmers are encouraged to work with their veterinarians to develop a good herd health program and achieve optimal reproductive efficiency in their herds. Contact your local veterinarian, county extension coordinator or the UGA Veterinary Diagnostic Laboratories for more information if you think BVD testing might help you and your herd.