PI Animals Allow BVD to Remain in Cow Herds
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Bovine viral diarrhea (BVD) can have devastating financial impacts on a beef herd. For years we have assumed that vaccinating the calf crop at weaning and the cowherd when fall processing would eliminate BVD from the herd. However, what we have found is that persistently infected (PI) animals can survive and allow BVD to remain in herds with traditional vaccination programs.
Figure 1. BVD Can be Transferred from Persistently Infected Cows to Their Herd Mates
PI (persistently infected) calves are produced as a result of BVD infection of a cow or heifer when their fetus is between day 45 and day 145 of gestation. This period is prior to the fetus developing an immune system that can recognize a foreign virus. If the fetus survives to birth and beyond, it becomes a PI animal that will continue to shed the virus in the herd. The immune system of the PI animal is compromised, therefore most will be lost to some disease early in life due to their inability to fight infection. However, some animals can survive to become members of the cowherd. Any calf born to these cows will be PI as well, so the cycle continues. It is very important to note that a PI animal will never respond to vaccination and develop immunity to BVD. The only solution to eliminate BVD from the herd is identifying and culling all PI animals. Industry average prevalence of PI calves is estimated to be from .05-1.0 % of calves born. When PI's are present in a herd there tends to be a much higher incidence than the average herd.
Vaccination of the cowherd at fall processing offers protection to the cow but not the fetus she is carrying. None of the killed type vaccines, which are commonly used in the fall, have label claims for protecting the unborn calf. The placenta is a barrier to transmission of immunity from the cow to the fetus. However, some protection may be provided to the fetus if the cow has another pregnancy. Many years of continued traditional vaccination does offer herd immunity but if any vaccination gaps occur (i.e. animals are missed, new animals introduced into a herd or PI's are present before a vaccination program is started), BVD problems will persist in the herd. Since vaccination never works on PI animals, if one is selected as a replacement animal the BVD cycle will continue in the herd.
PI's impact individual herds through death losses, reduced pregnancy rates, increased treatment costs and general poor performance leading to increased production costs. PI calves that survive and leave the herd for placement in feedlots can have a damaging impact on the health of all animals in the feedlot. The first 30 days in any feedlot is the biggest stress period for any calf's health. Add a PI calf that is continually shedding BVD virus to a group of stressed calves and the consequences can be major. The PI calf will probably die within the 30-day period and possibly others infected by it. The treatment rate for the pen will be higher than normal and affected calves will have decreased performance on feed.
The following steps must be followed to eliminate BVD from the cowherd:
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