– Dr. Michelle Arnold, UK Veterinary Diagnostic Laboratory
“BVD” or “Bovine Viral Diarrhea” virus is one of the most common and costliest viruses affecting KY cow/calf herds and backgrounding operations. Control of the BVD virus is best accomplished through implementation of three equally important practices: 1) surveillance testing to detect and remove infected cattle, 2) vaccination to increase herd immunity and 3) implementation of biosecurity measures to reduce virus entry into the herd. But how would a producer know that BVD virus is circulating in his or her herd? This article, the first in a two-part series, is written to help understand how BVD virus enters a beef herd and how to recognize its effects, and targets for control. Part 2 will address diagnostic testing strategies, how to correctly interpret results, and how to implement BVD virus measures.
One of the initial problems with this virus is its name. Although BVD stands for “Bovine Viral Diarrhea”, rarely does an animal show any symptoms of diarrhea. Instead, cow-calf producers may observe one or more of the following disease manifestations in the herd:
1. Poor reproductive performance despite females in good body condition and fertile bulls.
- More open cows-Producers may find a decrease in overall pregnancy rate, including a reduced percentage confirmed pregnant after the first service. This infertility and “delayed breeding” are often blamed on the AI technician, a dud bull, hot weather, or fescue when it is actually the BVD virus causing failure to conceive and early embryonic death.
- Fewer live calves- BVD virus infection during pregnancy may result in abortions, mummies, stillbirths, neonatal deaths, and weak newborns that die shortly after birth.
2. Physical abnormalities including “dummy calves” that cannot nurse, eye defects, and cleft palates in newborns if dams are infected with the virus during mid-pregnancy.
3. An increased number of calf death losses pre-weaning due to pneumonia or scours.
It is important to realize that BVD virus in a herd may not have easily recognizable “classic signs” such as an increased number of abortions or birth defects. It may simply look like fewer mature cows pregnant at pregnancy check, finding cows open that should be calving, or more disease and death loss in pre-weaned calves than usual.
There are two types of BVD virus infection described in cattle; 1) “acute” or “transient” infection and 2) “persistent” infection (PI). An acute BVD viral infection usually lasts 10 days to 2 weeks and symptoms may range from severe to unnoticed, depending on the age of the affected animal and its level of immunity. In an adult unvaccinated (or poorly vaccinated) animal, the virus typically does not cause outward signs of sickness. However, the virus attacks reproductive tissues (ovaries of females, testes in males) resulting in infertility. Bulls may experience prolonged testicular infections. In pregnant cattle, the virus crosses the placenta to the developing embryo or fetus, causing several types of reproductive wastage or malformations, depending on the stage of fetal development when infection occurred.
Acute BVD virus infection in a susceptible calf may result in disease manifestations ranging from mild to severe disease and death. The BVD virus first attacks the immune system where it destroys the production of disease-fighting white blood cells, causing severe immunosuppression. Secondly, it can work synergistically with other viruses to make them more aggressive and deadly. This combination attack results in increased disease and mortality risk in pre-weaned calves exposed to the virus and a substantial risk of respiratory disease and death loss in calves post-weaning.
As mentioned previously, the outcome of acute BVD infection in the cow herd may be observed by a producer as infertility, delayed breeding, abortions, malformed calves, and neonatal death loss. Acute BVD virus infections in pre-weaned calves are recognized by the increased amount of sickness and death loss. What cannot be observed is the 2nd type of BVD infection, the development and birth of persistently infected (PI) calves. If an unvaccinated pregnant cow or heifer is infected with the BVD virus between 42-125 days (about 4 months) of gestation, the virus crosses the placenta and infects the fetus during a critical stage in its immune system development. The virus incorporates itself into the developing fetus, so the immune system does not recognize the virus as a foreign invader. When this calf is born, it is “persistently infected” with the BVD virus (known as a “PI” calf) and is a lifetime “carrier” and “shedder” of massive amounts of virus particles from all its bodily fluids including saliva, nasal discharge, feces, and urine. Those PI calves that survive past sexual maturity will also shed virus particles in milk, semen, uterine secretions, and aborted membranes. A PI cow will always have a PI calf although less than 10% of PI calves come from PI positive dams. One PI calf born on a cow/calf operation usually means there will be additional disease problems within that calf crop. Any fetus infected with BVD later in gestation (150 days (about 5 months) or more) while in the uterus will not become a PI but still will not be “normal”. The virus commandeers certain cell types to produce more virus particles that would normally be used for critical fetal development. This results in destruction of endocrine tissue and may destroy 20-80% of the thymus gland, an important driver of immune function in young calves. These calves will have increased respiratory disease, poor growth and performance, and if they reach sexual maturity, more reproductive issues. Therefore, the reason pre-weaned calves in the same field with a PI calf typically exhibit more sickness and death loss (scours, “summer pneumonias”) is two-fold; 1) due to ongoing immune system suppression from constant BVD virus exposure coming from the PI calf and 2) due to a damaged immune system from BVD virus infection while in utero.
The key to transmission of the BVD virus within and between herds and virus persistence year after year is the PI animal. Ingestion or inhalation of the virus by direct contact with body fluids or aerosols from PI cattle are the main and most important source for BVD virus transmission. In addition, any virus deposited in watering troughs, feed troughs, round bales of hay, cattle trailers-virtually everywhere the PI animal goes-can be picked up by the other cattle in the herd, either by mouth or nose. Importantly, a PI calf shedding virus in the pasture during breeding season will expose many (if not all) of the cows/heifers to the virus during the highest risk time for development of the next generation of PI calves that will be born during next year’s calving season. Acutely infected cattle shed virus particles too but at a significantly lower rate and shorter length of time. A normal calf infected with the BVD virus sheds approximately 10,000 virus particles per day and recovers in 10-14 days. In comparison, a PI calf sheds 10 MILLION virus particles every day of its life. This is why detection and removal of PI animals is crucial to BVD virus control.
Once a producer receives the diagnosis of BVD virus infection in the herd, the first question asked is how did it get here? Research has proven that the #1 cause of BVD virus entering a herd is through the purchase of pregnant females, especially first calf heifers, without properly testing for the virus. The testing strategy must include testing every purchased pregnant female for BVD and also testing her newborn calf for “PI” status. It is not enough to test the dam and ignore her unborn calf because it may not have the same BVD status as its dam and cannot be BVD tested until it is born! It is recommended to calve out purchased pregnant females away from the home herd and test their calves for BVD virus as soon as possible. In addition, don’t allow these cow/calf pairs to have contact with the home herd until each new cow and her calf has a BVD negative test result. Although a pregnant cow tests negative for BVD, always bear in mind she can still be carrying a persistently infected (“PI”) calf that will test positive. Over 90% of PI calves are born from BVD negative dams.
All newly purchased cattle, regardless of age or pregnancy status, should be tested for BVD-PI, vaccinated appropriately, and isolated away from the home herd at least 2 weeks. Other sources of the BVD virus in a cow/calf herd include introduction of new breeding bulls, a calf purchased from a sale to graft on a cow, or feeder calves purchased at auction and brought home to the farm have the potential to be acutely or persistently infected. Even show cattle that are vaccinated and have tested negative for BVD-PI may become acutely infected with the virus and can bring the virus back when they return from fairs and exhibitions. This is why quarantine for all animals arriving to the farm away from the home herd for at least 2 weeks with no nose-to-nose contact or shared water sources is crucial to preventing virus spread. In the same manner, herds with fence line contact with feeder calves or other cattle frequently traded may result in exposure to the BVD virus if allowed to touch noses or share water. There are other minor sources of transmission including fomites (needles, OB sleeves, nose tongs) and vectors including stable flies and horse flies. Other species including sheep, pigs, alpacas, deer, and goats can also carry the virus to cattle.
Although vaccination is a key component in BVD virus control, just vaccinating the herd annually without practicing excellent biosecurity and surveillance for PIs will not keep this virus from gaining entry to the farm. Vaccines against BVD virus (including those with Fetal Protection claims or “FP” vaccines) will reduce the chance of fetal infection and PI development but this is an extremely high bar for any BVD vaccine to achieve. The question of whether to use modified live or killed vaccine is not an easy one to answer. Many popular beef magazines offer articles concerning what types of vaccines work “the best” or are “safest” according to the latest research. The truth is, there are tradeoffs when it comes to vaccine selection. Modified live vaccines (MLVs) offer better and more effective pregnancy protection but the IBR portion of the vaccine can impact conception rates if given too close to breeding season. If using timed artificial insemination (AI), experts recommend administering MLV vaccines at least 45 days pre-breeding to allow 2 estrus cycles prior to insemination. In addition, MLV vaccines can cause abortions if given to pregnant cattle without strict adherence to label directions. Killed vaccines, on the other hand, are safer but are not as good at preventing fetal BVD infection. A herd with excellent biosecurity and at exceptionally low risk can err on the side of safety and use killed vaccine. However, herds that purchase animals including replacement females and/or bulls, herds near stocker cattle or unvaccinated neighboring cattle, show cattle herds, herds with frequent deer contact, or any other probable exposure should err on the side of efficacy and choose modified live. If breeding occurs year-round so MLVs are not an option, an alternative is to administer two doses of MLV vaccine to open heifers (at weaning and a second dose 6 weeks prior to breeding) with annual revaccination using a killed vaccine. This combination stimulates excellent protection without the risk of MLVs although this protective response will diminish after several years. Finally, and perhaps most importantly, cattle herds are unique entities with different risks for disease on every farm so work with a veterinarian to choose the right vaccines for the herd.