While the porcine reproductive and respiratory syndrome virus has been adept at changing course and surfacing with new strains, researchers may have finally gained the upper hand.
A consortium of scientists has discovered a genetic marker in pigs that identifies whether or not the animal has a reduced susceptibility to PRRS. The marker is a quantitative trait locus, or QTL, on swine chromosome 4 that is associated with the animal’s resistance to PRRS virus infection. Of particular importance is that the discovery at this location is associated with improved growth of pigs that are infected with the PRRS virus.
The research results show that there’s a positive effect for PRRS resistance and higher weight gain associated with the QTL, says Joan Lunney, a research scientist at USDA’s Agricultural Research Service in Beltsville, Md., who is involved in the collaborative effort that discovered the genetic marker.
She and researchers Jack Dekkers from Iowa State University, and Raymond “Bob” Rowland, a virologist and professor of diagnostic medicine and pathobiology at Kansas State University, discovered the QTL. This breakthrough is a first step in controlling and eliminating the virus that, according to 2011 Iowa State estimates, costs the U.S. pork industry $664 million annually in direct costs.
“This discovery is what you call a first-first,” Rowland says. “It is the first of its kind for PRRS but also for any large food-animal infectious disease.” He adds that in the 20 years that he’s worked in this area, “this is one of the biggest advances I’ve seen.”
The research began with PRRS Host Genetics Consortium, or PHGC, a nationwide effort involving the National Pork Board, USDA’s National Institute of Food and Agriculture, the Coordinated Agricultural Project program and the U.S. Swine Genome Coordinator for the National Animal Genome Research Program. Genome Canada, multiple universities and allied industry members also played a role. PHGC initiated the research and provided more than $5 million to the effort. Rowland is co-director of PHGC as well as director of the USDA-funded PRRS Coordinated Agriculture Project — PRRS CAP — which has been instrumental in funding and advancing numerous PRRS research projects over the years.
Beginning in 2007, the researchers collected blood and tissue samples, along with animal growth, weight, performance and virus measurements over time. More than 2,000 pigs that were housed in biosecure facilities at Kansas State University were involved in the study, producing more than 100,000 samples.
The samples were shipped to ARS for genomic DNA preparations to identify differences among more than 60,000 genes.
The data was then transferred to Iowa State University for genetic analysis that led to the discovery of the QTL common to pigs that had lower levels of PRRS virus circulating in their blood and that grew faster after PRRS infection.
The Iowa State researchers created a common database so that all data collected during the project is accessible at multiple locations by researchers as well as the swine breeding sector for decades to come.
According to Chris Hostetler, NPB’s director of animal science, the identification of the marker gene that’s responsible for increasing resistance to PRRS will allow genetics companies to more easily place selection pressure on PRRS resistance which, in turn, could allow producers to introduce new “PRRS-resistant” breeding lines into their herds.
Now that scientists have found a chromosomal segment that can signify resistance to PRRS, they work to pinpoint the gene and determine whether it shows the same effects for other PRRS virus strains. The effort also should help identify which genes contribute to the best PRRS vaccine response.
“We’re not only making healthier animals, we’re also understanding the fundamental biological relationship between a host and a pathogen,” Rowland notes. “This has direct applications to human medicine as well, because the same type of science and relationships applies to humans.”
The research led to the discovery of pigs with severe combined immunodeficiency. This should enable researchers to better study the syndrome and apply its use to the study of human cancer and anti-cancer drugs, Rowland points out.
There also are spin-off benefits for other swine diseases. “This could be one of the tools used to help eliminate PRRS, but more importantly, this work may provide the platform for finding similar marker genes responsible for conveying resistance to other economically devastating diseases,” Hostetler says.
And that could include application to animal health in general. “We have been looking at genes that may provide long-term resistance to a lot of infections,” Rowland notes. This is important for animal health because there are many diseases without a vaccine or a cure. “Now we have a tool to study these diseases,” he adds.