PRRS Corner: Reduced Susceptibility to PRRS

March 14, 2012 — 

A genetic marker for reduced susceptibility in pigs to porcine reproductive and respiratory syndrome (PRRS), the most economically significant disease in pigs, has been discovered by a research team that includes scientists from the Agricultural Research Service (ARS).

Worldwide, PRRS affects pigs at all stages of growth and is easily spread. It costs the United States alone an estimated $664 million per year. In 2006-07, PRRS virus infected 2 million pigs in China, killing 20 percent of the animals. The disease continues to expand throughout eastern Asia and threatens the pig industry in Malaysia.

The discovery of the genetic marker—called a quantitative trait locus (QTL)—associated with resistance to PRRS virus infection is a collaborative effort that targets the elimination of PRRS. Partners include scientists at ARS, Kansas State University (K-State) and Iowa State University (ISU). Research is funded by the Coordinated Agriculture Products (CAP) program, supported by the National Institute of Food and Agriculture (NIFA); the PRRS Host Genetics Consortium (PHGC), a nationwide effort originally funded by the National Pork Board; and the U.S. Swine Genome Coordinator for the National Animal Genome Research Program.

Importantly, the QTL discovered on swine chromosome 4 (SSC4) is also associated with improved growth of pigs that are infected with the PRRS virus, according to Joan Lunney, a chemist at the ARS Henry A. Wallace Beltsville Agricultural Research Center (BARC) Animal Parasitic Diseases Laboratory in Beltsville, Md. Results indicate a positive effect of the favorable SSC4 allele for both PRRS resistance and higher weight gain. An allele is an alternative form of a gene that controls specific traits.

For the PHGC trials, groups of 200 pigs donated by six commercial pig breeding companies were experimentally infected with PRRS virus in 10 separate trials at KSU’s biosafety level-2 research facilities. During 42 days of infection, Bob Rowland, K-State diagnostic medicine and pathobiology professor, and his team collected blood, ear and tissue samples, measured weight gains, and stored the more than 45,000 samples from 2,000 pigs.

Ear notches were used to prepare genomic DNA at BARC and sent for genotyping using the Porcine 60K SNP Beadchip—generating genotypes on more than 60,000 genetic markers across the genome for each pig.

Working with ISU animal science professor Jack Dekkers, ISU graduate student Nick Boddicker used this data to search the entire genome of all pigs from the first three PHGC trials to identify chromosomal segments common to pigs that had lower viremia levels and faster growth after infection. A QTL was found on chromosome 4.

Now that scientists have found a chromosomal segment, the next step is to pinpoint the gene and determine whether it shows the same effects for other strains of the PRRS virus. This would allow producers to genetically select pigs for PRRSV resistance and increased growth following infection.

Results of this research were published in the Journal of Animal Science.

ARS is the chief intramural scientific research agency of the U.S. Department of Agriculture (USDA). This research supports the USDA priority of promoting international food security.

SCIENTIFIC CONTACTS: Joan Lunney, joan.lunney@ars.usda.gov

Source: USDA-ARS