Scientists at USDA’s Agricul­tural Research Service (ARS) have discovered a genetic marker for reduced susceptibility in pigs to porcine reproductive and respiratory syndrome (PRRS), the most economically significant disease in pigs.

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

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. The research team includes scientists at ARS, Kansas State University (KSU) and Iowa State University (ISU). The research is funded by the Coordinated Agricultural Project (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), also is associated with improved growth of pigs infected with the PRRS virus, says Joan Lunney, an ARS chemist at the Beltsville Agricultural Research Center (BARC) Animal Parasitic Diseases Laboratory, Beltsville, MD. Results indicate a positive effect of the favorable SSC4 allele for both PRRS resistance and 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, KSU 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 post-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 PRRS resistance and increased growth following infection.

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

For more information, contact Lunney at USDA-ARS Animal Parasitic Diseases Laboratory, 10300 Baltimore Ave., Bldg. 1040 BARC-East, Beltsville, MD 20705.