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Genetic Clue for Fighting Swine Virus

By Ann Perry
October 18, 2007

Agricultural Research Service (ARS) scientists are part of a team that has found a vital clue for battling a disease called porcine reproductive and respiratory syndrome virus (PRRSV), which costs U.S. swine producers about $560 million annually.

PRRSV-infected pigs are susceptible to pneumonia and reproductive losses, and infected sows give birth to weak piglets. It can take weeks or even months for them to recover from the virus, which evolves and adapts quickly to environmental challenges like vaccines and medications.

Scientists Joan Lunney, Patricia Boyd and Daniel Kuhar conduct research at the ARS Animal Parasitic Diseases Laboratory in Beltsville, Md. Working with animal scientist Rodger Johnson and graduate student Derek Petry at the University of Nebraska-Lincoln, they evaluated two lines of swine for genetic resistance to PRRSV. The Nebraska Index line (I) was chosen because of its improved reproductive traits, and the Hampshire by Duroc cross (HD) was selected for its high growth rates.

All pigs in both groups became infected after exposure to PRRSV. However, I pigs generally recovered more quickly, maintained higher levels of weight gain during their illness and had lower body temperatures. In addition, samples of blood, lung and bronchial lymph node tissue showed that virus levels cleared more quickly in PRRSV- resistant I and HD pigs.

The scientists then looked at the tissue expression of 11 genes and one "housekeeping" gene involved in the immune response to PRRSV. Both I and HD swine showed significant activity in 11 of the 12 genes, but the type of activity differed between the two groups.

High pre-infection blood levels of one protein, interleukin-8—IL8—was found to be significantly associated with PRRSV-resistant pigs. Low levels of another protein, interferon-gamma—IFNG—in blood and in RNA samples was also correlated with PRRSV resistance.

These findings support existing research that indicates animal breeds with high growth rates devote less energy to immune and disease traits. This information will facilitate work into developing genetic tools for increasing swine resistance to PRRSV.

ARS is the U.S. Department of Agriculture's chief scientific research agency.