Tri-State Neighbor: South Dakota Farm Newspaper

Every day of warm weather this winter is one day less of cold for producers and their cows.

Most cows in this area are still in winter grazing and lots of stored feed has been saved for later feeding. Calving season is beginning for some and soon it will be here for everyone.

Yearly, researchers unravel more of the complexity of the Bovine Virus Diarrhea (BVD) infections in cattle. Our herds are relatively isolated here in South Dakota and we tend to believe our vaccination programs will handle any challenge facing our herds. We are realizing that we need more than good vaccination to control this common problem.

We understand that BVD has both a reproductive and respiratory form. The virus is commonly isolated at the diagnostic lab from samples submitted for feedlot respiratory problems. As for the reproductive form, it is the No. 1 agent seen in abortions and is indirectly diagnosed through blood serum levels in numerous bouts of infertility.

The feedlots have been complaining of persistently infected BVD calves (PIs) for several years. These are formed when the gestating calf is infected with BVD virus at certain stages of development. The cow is viremic (circulating virus) for a short time and then recovers. The calf becomes viremic, but is unable to rid itself of the virus and continually sheds throughout the calf’s life at levels up to 10 times that of normal infection. This PI calf is the source of infection for all the cows and calves in the pen.

In the past several years I have sent many samples to the lab from breeding bulls and heifers and have never found a positive PI. We were lulled into believing PIs were very uncommon and not to be alarmed. This fall one of our clients purchased 100 home-raised calves from northern South Dakota. These calves were “certified drug-free” and had vaccination records. We tried to maintain these as “drug-free” but found it impossible. Two of the calves were chronically ill and doing poorly. Tests from these calves indicated they were PIs and the source of the pen problem.

This poses one problem: “What do you do with known PI calves?” In a perfect world the animal would be euthanized, but many producers market them as singles at the auction markets. I am sure many known PI calves are marketed to unknowing buyers assuring a future of problems.

One feedyard in Kansas routinely tests all incoming animals for BVD PIs. The PIs are a constant source of BVD virus to infect healthy animals, which has a negative effect on the health and performance of the cattle they contact. Feedlot records on cost of gain show that a pen containing a PI calf is nearly twice as high as a pen with no PIs. Gain is also affected in adjoining pens to the PI, but not at such high levels. This increased cost of gain more than balances the cost of testing so the PIs can be removed.

Recently we have had some reproductive problems in herds that are adequately vaccinated for BVD. Our diagnostics have not nailed down an exact cause, but some researchers have proposed a theory requiring rotation of vaccine strains rather than continued usage of the old standbys. The jury is still out, but it is an interesting concept.

BVD is a common problem in our area and is very complex in the way it affects the cattle population. Visit with a veterinarian to see if testing or additional vaccination protocols are needed in a specific herd.

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