BVD virus is testing the cattle industry's patience

By Doug Rich

"I was in private practice for over 30 years and about every 10 years I thought I thoroughly understood BVD (bovine viral diarrhea)," says Dr. Arden Wohlers, DVM, now an Extension veterinarian with the University of Nebraska Panhandle Research and Education Center. "Then just down the line we would have a whole new concept of it."

Veterinarians, cow-calf operators, and feedlot owners all across the country can understand Dr. Wohlers frustration with BVD. The beef industry has been dealing with BVD for nearly 70 years and there are still more questions than answers. It has been estimated that the disease costs the beef industry $7 billion a year.

According to Dr. Wohlers, abortion, pneumonia, and lameness are often signs of BVD infection in a herd. Reduced weight gains and increased susceptibility to other diseases because of immune suppression caused by BVD are a problem across the beef industry.

Animals with an acute BVD infection will react a little like humans do when we have the flu. These animals will shed the virus for a few days, recover, and become immune. In many cases the virus is brought onto the ranch by replacement animals that spread the virus in the air, manure, and body secretions.

Persistently Infected

Persistently infected (PI) animals are harder to detect and control. PI calves are the result of a bovine fetus that is exposed between 30 and 125 days of gestation. Wohlers says this is prior to the time when an animal's immune system can eliminate the disease.

"These carriers look normal at birth and can live to be several years old, although that is rare," says Dr. Wohlers. "Usually they die before they are yearlings. Until then they can shed a tremendous amount of virus that can infect the animals around them."

"We have never really understood the BVD virus," says Dr. Wohler. "But we know a lot more about it now and this has come about with a lot of the new diagnostic techniques."

Quick, accurate tests for BVD have improved dramatically in recent years. In recent years Dr. Bruce Brodersen, veterinary pathologist with the Institute of Agriculture and Natural Resources, developed an accurate skin test for BVD that is faster and more accurate than a typical blood serum test.

"There have been some changes in the last few years to come up with less expensive and more rapid tests, but it remains the gold standard of BVD tests," says Dr. Brodersen. The tests involves collecting a small skin sample, usually an ear notch, which then is sent to a diagnostic lab for analysis. At the lab it is processed like a typical biopsy sample.

"A persistently infected calf that hits the ground in the spring and survives to weaning is exposing the other cows during that time," says Dr. Brodersen. "The cows can then infect their fetuses if they are pregnant. This can cause abortions or make new PI calves the next year."

In the last year improved testing for the BVD virus has been developed. Groups of calves are being tested which makes it much cheaper. If the test comes back positive then they can go back and test each calf individually.

Dr. Arden Wohlers says group tests begin with an ear notch which is placed in individual test tubes. In the lab a solution is placed in each tube which allows them to extract protein molecules from each sample. Samples from each tube are then mixed together and a single test is done to determine if the BVD virus is present.

"If you have PI calves in that group then you still have the original test tube with the ear notch samples and you can get tests on each of those," says Dr. Wohlers. "If not then you are done testing."

Ideally these test are done in the spring when calves are born. If you wait until the fall it is too late. That PI calf has infected the cow and possibly her unborn calf. "Test in the spring before you turn the bull in with them."

"Once those PI calves are removed problems with BVD go away," says Dr. Bruce Brodersen. "If they have a good sound vaccination program with it this helps a lot."

Although much of the emphasis on BVD detection and control has been done by cow-calf producers PI calves can survive long enough to get into feedlots and be a problem there. Pen mates and cattle in adjacent pens run an increased for respiratory diseases.

"In certain cases a PI animal can be born because the mom was exposed during gestation and he is born with the virus but does not recognize that he has the virus," says Dr. Bill Hessman, DVM, from Sublette, Kan. "That calf becomes a virus factory producing and shedding thousands if not millions of virus particles daily. That is an exposure risk for any animal that he comes in contact with. This has a substantial detrimental effect on health and performance in the feedlot phase."

Feedlot Test

In 2004 Dr. Hessman did an extensive test on 21,743 animals in 240 pens at Cattle Empire Feedyards in Satanta, Kan. These were five-weight cattle out of sale barns in the southeastern U.S. which were placed in a new 10,000 head starter yard. The test only followed the cattle through the first 60 days and not all the way to finish weights.

Dr. Hessman used an Antigen Capture Elisa by ear notch test. Positive animals were pulled within 48 hours of processing for re-sampling. "The Antigen Capture test can be run in the field, is a little less technical, and has the quickest turn around time," says Dr. Hessman.

"Turn around time is more important in a feedyard setting than with a cow herd," says Dr. Hessman. "If there is a PI animal in there cattle from different sources may not have protection and it will have a bigger impact."

Previous BVD trials at the feedlot level looked primarily at the health effect but they did not look at performance issues. His trial looked at the effect of varying degrees of exposure on cattle in the same pen and adjacent pens to a PI animal.

This extensive trial discovered that although the rate of PI animals is low, about 4 per 1,000, they have a dramatic effect on the total feeding population. Dr. Hessman said that even this small incidence rate can expose up to 61 percent of the feedlot population.

The biggest performance impact is on conversion rate. "Animals that were in direct contact with a PI animal had a much poorer conversion rate than animals in other pens," says Dr. Hessman. "There are more detrimental effects on performance than on health actually."

"Cost of gain and conversion rate are where the money is," says Nick Chesnut, operations manager at Cattle Empire Feedyards.

A year ago Cattle Empire Feedyards began testing every animal that comes into their three locations for BVD. They have a one-time capacity at three locations of 170,000 head.

When they began their 100 percent testing program, Chesnut said they expected to drastically reduce death loss and treatment levels. "But the big benefit is from removing those animals from the rest of the population," says Chesnut. "Our conversion rates are at record levels and the rate of return is very good."

The employees were initally aprehensive about the testing program, thinking it would create more work for them. "If we were to stop testing for BVD now I think we would have a revolt, especially in the grow yard," says Chesnut. "They are sold on it and they can see the difference in the cattle."

"Those PI calves are everywhere and if they are distributed properly they can affect your whole population," says Chesnut. PI calves are just as likely to come direct from ranches as they are from sale barns. "We have found them in 800-pound yearlings coming off grass."

Eradication of BVD is always a possibility, according to Dr. Hessman. But that is difficult because of the way we handle livestock in this country and because it is present in the wild in buffalo, white tailed deer and elk herds.

"We are trying to get people to understand that BVD is a complicated process," says Dr. Hessman. "The research we have done over the last four years has answered some questions, but unfortunately I probably have more questions about this disease now than I ldid four years ago. Which is kind of frustrating."

Doug Rich can be reached by phone at 785-749-5304 or by e-mail at richhpj@aol.com.

Date: 8/10/06

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