Thursday, July 17, 2014

The BVD-PI Test Can't be Positive!

A heifer aborted a late gestation calf. As part of our routine in this situation a tissue sample was sent for BVD-PI testing. The result was positive. The dairyman said, "The BVD-PI test can't be positive."

Let's step back for a moment. The  BVD-PI test is for an animal that was exposed to the BVD virus in utero during a window of time when the fetus incorporates the virus as a "normal" part of her body. Thus, she becomes "persistently infected" and though she is not ill she continuously sheds the virus as long as she lives.

Clearly we do not want BVD-PI animals on the dairy. This huge load of virus particles is a constant challenge to the immune system to all the other animals, both young and mature.

So, why did the dairyman exclaim, "Can't be positive" ?

He knew about  BVD and the challenge of BVD-PI animals. Before he buys springers he has them tested to be sure they are not PI positive. We had talked about testing heifer calves born to purchased heifers but he decided that he couldn't be bothered with that. Besides, he "knew" he would be able to tell by looking at a calf if she was a PI calf.

Our practice recommends that all replacement calves born to purchased dams be tested for BVD-PI status. This is not the first time a calf born to a purchased springer has come back positive at our practice. And, over the years we have seen heifers that look as normal as can be turn up positive when tested.

So where were the weak links in this situation. First, in spite of our explanation about how PI calves are created, he was so sure that if the dams were negative the calves had to be negative, too. False! Second, he was sure that even if a PI animal was born, he could reliably visually identify her. False!

What is this owner planning to do? The last I heard his regular vet and he are planning to collect samples for BVD-PI testing. Will they check all the animals? Just sample animals from purchased springers?

I have not had the opportunity to talk with this veterinarian this week to find out if this dairy has seen an increase in number of services per conception and abortions over the past year or two. If these have been increasing I would advise testing all the animals. There could be a "Typhoid Mary" out there in the freestalls!

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