In a recent study of cattle herds in southern Oklahoma, nearly 17% of the ranches had at least one BVD persistently infected (PI) calf in the 2006 calf crop; some had as many as 10 or 12.
Dave Sparks, DVM, an Oklahoma State University Extension food animal/quality and health specialist, says PI calves result when a susceptible cow is exposed to the virus between 40 and 125 days of gestation. During this time, the fetus in inventorying its tissue for the development of the immune system and categorizes the virus as self tissue. When this happens, the calf can't respond immunologically to the virus and becomes a super shedder.
Vaccination can protect herdmates from exposure by these super shedders, but they spread such large numbers of the virus that they can overcome even the best vaccination program. Estimates are that BVD losses cost cow-calf producers in Oklahoma from $50-$70/cow/year.
Sparks says PI calves shouldn't be sold. Since they do not pose a human health danger, many producers isolate them from the other cattle and feed them until they're big enough to salvage via the local locker plant.
If you are having trouble weaning as many calves as you think you should, you might want to consider testing your calves, Sparks says. It's not expensive and your local veterinarian can help you set up a testing program that is right for you.
For more on this handling topic read Kansas veterinarian Dave Sjeklocha's comments from the February issue of BEEF at: http://beef-mag.com/mag/beef_dealing_pi_calves/. For some tips on finding PI calves, read Clint Peck's story at: http://beef-mag.com/mag/beef_finding_pis .
-- Burt Rutherford