Bovine viral diarrhea virus was isolated from this calf with severe internal hydrocephalis. This calf had other congenital defects including scoliosis and pulmonary hypoplasia.
Photo credit: Bruce Brodersen, DVM, PhD
Cortese says because the death loss from BVDV is even greater in dairy herds, the economic losses are higher. “Assume on a dairy a prevalence rate of 4 percent PI animals in animals under 2 years of age. If I buy a group of 20 springing heifers, the risk of buying a PI animal is 66 percent. If I’m buying 10 yearling bulls, I have a 40 percent chance there will be one PI bull.”
BVDV biosecurity for calves
Vaccination alone won’t protect a dairy and its calves from BVDV. “A sound biosecurity plan is essential to obtain any amount of BVDVcontrol in our calf populations,” notes Smith. “Without proper biosecurity, we continue to allow BVDV a foothold in our calf populations that it will not yield.”
Cortese offers five areas of a BVDV control program:
Because of increased animal movement around the country and in and out of dairies, biosecurity is critical. It might be wise to encourage clients to close their dairies to outside visitors, limit visits to appointments only and hold meetings with suppliers, advisors, etc., off of the dairy premises.
2. Find if the herd currently has PIs
It’s important to know the current PI BVDV status of the herd. Cortese offers serological ways this can be accomplished.
Pre-colostral blood samples on calves. If a calf has antibody to BVDV, it indicates BVDV is circulating on the dairy because the calf is making antibodies and that the vaccination program is not preventing cross-placental transfer of BVDV. Five to six calves can be tested once every three or four months. Split the sample – send one to the lab and keep one. “If you get a high titer back, ask the lab to run IgG levels to tell if the calf has nursed,” says Cortese. “If the BVDV titer is higher than 1:16, you know in utero exposure has occurred if they have not nursed.”
Do serologic surveys at 6-8 months. This requires leaving some steers or heifers unvaccinated. Published work shows that if five calves are tested at 6-7 months and if three of the five calves run 1:16 or above, there has been exposure to a PI animal on the dairy.
Use sentinel animals, especially on larger dairies. This can be a slick tool to use if the dairy is having reproductive problems that might be associated with BVDV. It’s done by bleeding seronegative animals every three to four months to see what is circulating through the adult population, whether it’s BVDV, IBR or Neospora. Cortese says this is similar to what’s done in poultry and swine. “To do this, I recommend that the seronegative animals be home-raised,” he says. “If you purchase them, they might be PI but BVDV negative, and that will throw off the results. Make sure they are virus-isolated-negative, not just serology-negative.”
Polymerase chain reaction (PCR) tests on the milk can be useful, as they can pick up one PI out of 250 cows sampled. Cortese likes to narrow that number down and test in strings of 100. However, if you use PCR, keep in mind that if using modified-live BVDV vaccines in open cows or at preg-check, they can “pop” the PCR positive.
Of the PI animals, only 10 percent make it to the milking string,” says Cortese. “So all PI animals might exist in the youngstock, and PCR of milk won’t catch those.” It’s also important to remember that at any one time a percentage of the herd is not in the milking string, so PCRs will have to be done four to six months apart to catch those animals that were dry when the first animals were tested.
3. Cull PI animals