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Farm study raises doubts about new approach to swine-disease control

Sample sizes were small, but eyebrow-raising results from a study on a western Illinois farm have researchers and veterinarians taking a broader look at how swine producers battle an endemic viral disease that adds to their costs and threatens reproduction in their herds.

A new approach (acclimatization) has producers inoculating newly arrived pigs with the wild-type strain of the porcine reproductive and respiratory syndrome virus (PRRS) already existing on a farm. The hope is that the pigs will develop specific immunity to that virus and will recover prior to breeding, when the disease takes its toll.

The study found that the approach boosted development and strength of immunity against the local strain, but failed where it counted the most. Pigs exposed to the farm’s virus produced slightly more live births than pigs vaccinated two other ways, but many of these acclimatized animals never gave birth at all and had to be removed from the herd.

"At first we found it encouraging that animals exposed to the wild-type virus, regardless of whether or not they got a subsequent exposure to vaccine, mounted a faster and stronger immune response to the virus than did animals given the modified live vaccine," said Tony L. Goldberg, a professor in the department of pathobiology in the University of Illinois College of Veterinary Medicine.

No positive effect on production was seen when compared with the more traditional approach of inoculations with a commercially available modified live vaccine, he added.

"All things considered, exposure to the existing wild-type virus resulted in a net reduction of 2.45 piglets for each sow introduced onto the farm," Goldberg said. "At this point, though, because of small sample size, the most we can say is that the world still lacks an effective method for controlling PRRS virus in herds where the virus is endemic."

The privately owned farm involved in the study had suffered from chronic PRRS infection for more than five years before the project was conducted in 2003-2004. The study -- funded by the U.S. Department of Agriculture -- was published in the April 1 issue of the Journal of the American Veterinary Medical Association.

For the study, 30 healthy, 70-day-old pigs were brought to the farm and isolated. Soon after that, 20 of the pigs were purposely exposed to the farm’s existing wild-type PRRS strain, while 10 pigs received only the commercially available vaccine. After 42 days, 10 of the pigs that had received the farm-based virus also were inoculated with a killed-virus vaccine, which, unlike the live strains, promotes immunity but doesn’t cause illness.

All of the pigs were allowed to mingle with the other some 800 breeding sows and 7,800 growing swine on the farm. Researchers monitored the experimental pigs for both T-cell activity and antibody production to the virus, as well as recording the pigs’ reproductive outcomes at the end of the study.

"The sample size was small but the magnitude of the effect was large enough for us to detect it with good statistical confidence," Goldberg said. "Seeing 50 percent of animals exposed to just the wild-type strain dropping out of the herd was surprising."

There was no effect on reproductive outcome among the pigs that received both the wild-type live vaccine and the killed-virus strain, Goldberg said.

USDA funding is now covering a larger study on several Illinois farms. "We may be able to generalize more accurately or make some viable recommendations after we’ve have analyzed the new data," Goldberg said.

Jim Barlow | EurekAlert!
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