Researchers at Oklahoma State University College of Veterinary Medicine have recently completed a study designed to determine how common BVD Persistent Infected (PI) individuals are in Oklahoma cow herds. The study involved testing all of the 2006 calf crops from 30 herds in Southern Oklahoma with the ear notch test. Herds included in the study ranged in size from 14 cows to over 500 cows.
The study showed that 16.7% of the ranches had at least one PI calf in the 2006 calf crop with some ranches having as many as 10 or 12.
PI claves result when a pregnant, susceptible cow is exposed to the BVD virus between 40 and 125 days of gestation. If she is exposed before that time she will return to heat, and after that time she will either abort or have a weak calf. During the 40 to 125 day range, however, the fetus is inventorying his tissues for the development of the immune system and he categorizes the virus as self tissue. Because of this, he will never be able to respond immunologically to the virus, and when born becomes a super shedder of the virus.
Normal calves that become infected usually get sick and can shed from 1 thousand to 10 thousand virus particles per day in the nasal secretions. PI calves, may appear perfectly normal, but they will be shedding from 1 million to 10 million virus particles per day for their entire life.
Vaccination will probably protect animals from exposure by acute carriers, but the virus shedding capabilities of the PI calves can overcome even the best vaccination program. For this reason, vaccination alone cannot control BVD if you have PI individuals in the herd. The solution also involves testing to recognize PI individuals and removing them from the herd. Estimates are that BVD losses cost cow/calf producers in Oklahoma from $50 to $70 per cow per year. Testing for PI’s is inexpensive, quick, and samples can be taken by the rancher as he works his calves.
Signs of BVD problems that cow operators may see in their herds include cows that are slow to breed or high percentage of open cows, abortions, congenital defects (especially involving the brain or eyes), weak calves at birth, and calves that die before weaning with no apparent cause. BVD is an insipient disease and rather than dramatic losses you usually see a few losses from all or most of the above concerns.
By causing reproductive problems in cows and respiratory disease in growing and finishing cattle, BVD is one of the most devastating and costly problems in cattle production. Recent research is now helping us to understand why we have had so much trouble getting a good handle on this problem. If you are having trouble weaning as many calves as you think you should, you might want to consider testing your calves. Your local veterinarian can help you further understand the problem and assist in setting up a testing program that is right for you.