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TESTS CAN DETERMINE IF CATTLE ARE INFECTED WITH BVD

by: Heather Smith Thomas

Ever since scientists discovered the BVD virus, researchers have been trying to find ways to help cattle producers detect and prevent the disease. A modified live virus vaccine was developed more than 30 years ago, giving ranchers their first good weapon to protect susceptible animals. Vaccination is not effective however, in PI (persistently infected cattle); since their immune system does not recognize the virus as foreign, they cannot produce immunity to the virus. Scientists also worked on ways to test cattle to find out if they are infected, in order to detect PI cattle so they can be removed from the herd.

Diagnosis and Testing -- Cattle can be blood tested to determine whether or not they are infected with the virus. If the virus is present, it can often be isolated from a blood sample or from tissues of an aborted fetus or from an infected "weak" or unthrifty calf. A small skin sample such as an ear notch can be checked for the presence of viral proteins. But a negative test did not always mean the animal is (or was) not infected with BVD virus. You might test a cow because she aborted or gave birth to a weak or abnormal calf, but the infection that caused the problem may have occurred much earlier and the virus or its proteins or genes may no longer be detectable in the cow or in the calf.

The blood can be checked for antibody levels, but this test is only useful if the animal has never been vaccinated or has never before been exposed to the virus. Most cattle have antibodies, due to previous vaccination or exposure. And if a calf has nursed its dam it will have obtained antibodies from its dam's colostrum.

When checking for BVD in a herd, animals that tested positive had to be checked again in 4 weeks (with the testing methods most commonly in use), to discern whether they had an acute infection or are persistently infected. The virus level will be dropping by the second test if the animal had an acute infection but will remain at the same level if the animal is persistently infected. All persistently infected (PI) animals should be culled, since they are the primary source of infection for most herds and produce calves that also carry the virus.

Commercial producers are usually reluctant to test their herds because of the high cost. Now there are improved tests that more quickly and accurately help with PI diagnosis, utilizing the reverse transcriptase polymerase chain reaction (PCR) technology that allows for pooling a large group of tissue samples such as ear notches. This is a more cost-effective method for screening a herd that may have just a few PI animals. A single PI animal can be detected in a pool of up to 250 samples. If the pool of samples comes up positive, retesting is necessary to find the one or more animals that are infected.

The first year of testing, all calves, open cows, replacement heifers and bulls should be tested. If tissue samples are taken from suckling calves before the cows are bred, you can test the dam of any calf that tests positive. All PI calves and their dams can then be removed from the herd before they come into contact with any pregnant animals -- thus eliminating the risk for passing the virus to a fetus. If you can keep BVD from reaching the fetus, there will be no more PI calves produced. In subsequent years, only the suckling calves and any newly purchased animals need to be tested for BVD. An individual animal only needs to be tested once; if it is PI positive it will always be infected, and if it's negative it will never become PI.

In addition to testing live animals, it pays to necropsy any calves that die, to check for the virus. This means calves that die of scours, pneumonia or any other cause, and any stillborn calves or aborted fetuses. If the calf is PI, this means the dam was infected.

The best way to eliminate a BVD problem is to check all animals in the herd and cull any that are shown to be persistently infected, and then keep the herd on a good vaccination program. If you can keep the virus from reaching the fetus (making sure all pregnant cows have strong immunity), your herd will never produce a PI calf. No new animals (cows, pairs, bulls, replacement heifers, calves for grafting) should be added to the herd unless they are known to be free of BVD. Diligent herd management is just as important as vaccination, and this may mean not grazing in common with neighbors' cattle.

Biosecurity: Keeping Your Herd Free of BVD -- Once you've tested your cattle and know you have no PI animals (or have culled any PI cattle from your herd), a good vaccination program and "closed herd" can ensure you won't have any future problems. Biosecurity measures such as isolation and quarantine of new arrivals (any purchased bulls, replacement heifers, calves to graft onto cows) until they can be tested, are crucial. You must protect your cattle from direct exposure to other animals that might have the BVD virus either as an active but temporary infection or as a persistent infection.

One of the pioneers in this type of biosecurity is Lucy Rechel (Yerington, Nevada), who runs Snyder Livestock, a 4000 head bull and heifer development program for breeders in several western states. A few years ago she decided to require all cattle coming into their feedyard to be tested to see if they were PI, and this made a significant difference in the health of the animals.

She ran ads featuring a photo of a notched ear, and called it the mark of excellence, or the mark of breeders who care. The very first year, her feedlot medicine cost dropped to 10 percent of what it was the previous winter. "In the herds that found PI animals, we were able to meet the problem head on and help them get the BVD virus out of their herd. After we required the test, Cal-Poly began requiring all cattle in their feeding program to be ear notched, so I feel we've had a positive impact in our part of the country," says Rechel.

The health issue is important, since Snyder Livestock develops and breeds 2500 heifers each year, synchronizing and AIing them. "I realized that a program like ours could be breeding heifers and inadvertently sending BVD home to a herd that never had it previously. So this was a major factor behind establishing our bio-security measures, and it has certainly made a big difference on animal health issues in the feedlot," says Rechel.

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