A lot of research has gone into developing better bovine virus diarrhea vaccines, stressing the ability to protect the pregnant cow and developing fetus.
We know vaccination before breeding is the best way to protect the fetus. It gives the cow maximum protection throughout pregnancy.
An unprotected cow exposed to the BVD virus during pregnancy faces three scenarios, depending on how far gestation has progressed.
- If more than six months into pregnancy, the fetus already has a competent immune system and can mount its own defence. It is born entirely protected.
- In middle gestation, four to six months, calves may be born with congenital abnormalities involving the brain or eyes. The eyes may be small and abnormal. Brain abnormalities involve the cerebellum, which results in co-ordination and balance problems.
- If early in gestation, less than four months, cows may abort or mummify the fetus.
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The greatest risk is if the fetus develops a persistent viral infection and sheds the virus when born. If exposed to BVD again later in life, these calves may develop the fatal mucosal diarrhea disease. If they survive to adulthood they can also give birth to persistently infected calves. We must work together to eliminate these persistently infected calves from herds, whether they be cow-calf, backgrounder or feedlot.
One would think persistently infected calves would be easily recognizable, but that is not the case. Many look normal yet shed vast amounts of BVD virus to the rest of the herd. Fortunately, an accurate skin test is available to determine persistently infected animals. The common method is to take a notch of skin from the ear. It is convenient and can be done on live animals.
The best way to control any disease, including BVD, is through prevention by vaccination. Previous research focused on protecting calves from developing the mucosal disease, diarrhea or respiratory diseases associated with BVD. We always recommend giving the live vaccines to breeding age heifers and cows before the breeding season because we want the best immunity during the all-important first and second trimesters of pregnancy. There is also the worry that giving live vaccines during pregnancy could cause problems such as abortions.
It is critical to protect the cow because we never know when or at what stage of pregnancy BVD may hit the herd. Usually the BVD vaccine comes combined with IBR, which means producers can simultaneously vaccinate for the two greatest infectious causes of abortions and fetal death in Canada.
Newer research has focused on whether vaccines actually protect the developing fetus. The only way to do that was to challenge the control group with enough virus so that a high percentage either aborted or delivered persistently infected calves. If the vaccinated group did not develop abortions or persistently infected calves, then its protection level could be measured. That is why producers are now seeing vaccines with protection against fetal infection stated on the label. Other vaccine manufacturers will likely run trials of their own to prove their BVD vaccines also have fetal protection.
For years veterinarians have recommended vaccinating for BVD and IBR to prevent abortions and reproductive losses.
The most important thing is to vaccinate so maximum immunity is achieved during the critical first five to six months of pregnancy.
If a producer or veterinarian suspects that cows or breeding bulls are persistently infected, they can easily be tested using the ear notch. If persistently infected, they should be eliminated from the herd.
If you can ensure your cow herd has good immunity, vaccination with live vaccines can be done late in pregnancy when scour vaccinations are given. This eliminates the inconvenience of vaccinating cows just before breeding when they have young calves at foot. Talk to a veterinarian if this is the plan.
Roy Lewis is a veterinarian practising in Westlock, Alta.
