New skin test accurately diagnoses BVD in young calves.

The bovine viral diarrhea (BVD) virus can do a lot of damage to a cattle herd. From promoting pneumonia in calves to causing abortions in cows, the virus has been blamed for estimated losses of up to $150 million annually nationwide.

Traditional BVD virus testing involves blood samples taken from all cattle in a herd. The virus is detected by use of an ELISA blood test, which is highly accurate in animals over 2-3 three months of age.

Now a new skin test should eliminate the problem of diagnosing BVD in young calves.

Although the virus can be transmitted across fences and by commingling livestock, the main infection source in most herds is carrier animals that are persistently infected (PI). These PI animals are produced as a result of infection of the fetus during days 45-145 of gestation, before the fetal immune system has developed sufficiently to recognize the virus as foreign. The calves can spread the virus throughout a herd.

“As the fetal immune system develops, it thinks the virus is normal and no rejection attempt is made,” explains Donal O'Toole, Laramie, WY, a researcher with the Wyoming State Veterinary Laboratory at the University of Wyoming. “Thus, a carrier PI animal results and sheds the virus throughout its life.”

Normal blood tests often aren't accurate in detecting the BVD virus in calves. Calves absorb antibodies through colostrum in the milk that interfere with the ELISA test.

New Skin Test

Bruce Brodersen, veterinary pathologist at the University of Nebraska's Veterinary Diagnostic Laboratory, discovered an accurate test that can be used on calves soon after birth. He looks for the virus in a skin sample, rather than blood serum.

“It's really just a new application of a test that's been around for awhile,” says Brodersen. “Using the skin test, we can identify infected calves at a younger age and remove them from the herd sooner, so there's less chance of them infecting other animals.”

He read about a similar skin test in a European scientific journal in 1996 and decided to try it at the lab. The test, known as the BVD-IHC (immunohistochemistry) test, has been accurate in early research. Using formalin-preserved ear notches or biopsies, the tissue is sectioned and stained with an antibody against BVD virus.

Interest in the test is strong, Brodersen says. His lab has been testing 3,500-4,000 samples a month since the test came online late in 1999. Veterinary labs in Iowa, Oklahoma, Pennsylvania and Wyoming, among others, are processing samples.

“In PI animals, the virus is found in high concentrations in the skin,” says Lynn Woodard, Wyoming's Extension veterinarian. “Rather than use blood, veterinarians and ranchers may want to consider using ear notches from ear-marking activities.”

Eradication Possible

The skin test for the BVD virus will help a concerted effort to eradicate BVD. The Wyoming State Veterinary Laboratory is among those now testing skin biopsies for BVD. Woodard thinks it will help ranchers eliminate BVD on a herd-by-herd basis from the state.

“We're aware of several herds in Wyoming and elsewhere that have done so, although reintroduction is always a possibility,” says Woodard. “Eradication would be hard if you are a public lands rancher with mixed ownership of herds.”

For herds closed to other cattle, especially during the breeding season and early pregnancy, eradication may be an economic reality.

BVD eradication efforts were often unsatisfactory because only cows and replacement heifers were tested. New research reveals that calves nursing pregnant cows during the breeding season are the most likely carriers.

“This is the reason BVD continued to be a problem in some herds even though all adult carriers had been eliminated,” explains O'Toole.

The new skin test eliminates the cost and hassle of testing the bulk of the cowherd. Dams of any infected calves then need to be tested to see if they are PIs also. The traditional ELISA test on blood can be used on these individuals, plus the replacement heifers, the bull battery and cows without calves.

Monica Manton Norby, University of Nebraska, assisted with this article.