Outmanoeuvring PRRSv in pigs requires a better understanding of genetic diversity

But building a better understanding of that genetic diversity - how it evolves, spreads and maintains itself in pig populations - is the next step to making real progress against the disease.

“PRRSv is among the most diverse viruses out there,” Kim VanderWaal, PhD, assistant professor, University of Minnesota’s Department of Veterinary Population Medicine, told Pig Health Today. “Its diversity is a result of recombination, genetic drift, which is simply the slow accumulation of mutations, and selection pressures on the virus to change so that it can be more successful at spreading in pigs.”

Since the virus was first discovered in the US in the late 1980s, VanderWaal said there’s been about a 20 percent increase in genetic diversity of PRRSV Type 2, which is the North American version of PRRS. In fact, PRRSv has a general pattern where a significant new type or strain appears every 3 to 5 years. The most recent one occurred in 2014.

To dig deeper into PRRSv, researchers at the University of Minnesota College of Veterinary Medicine and the University of Edinburgh’s Roslin Institute received a $3 million grant for a 4-year collaborative project, for which VanderWaal will serve as the lead investigator.

“PRRS is not a unique problem to the US,” she noted. While PRRSv costs the US pork industry more than $560 million annually, it is a constant drag on pork-producing areas throughout Europe and Asia as well. Having a broad base of expertise and experience with the virus that scientists from the two universities bring is particularly beneficial.

“We’ll tackle PRRS evolution across multiple scales, through conducting in vitro and in vivo experimental studies all the way up to analysing population-level trends in the industry,” VanderWaal said.

Exploring a new concept

Among the investigative tools that the researchers will apply is a concept called multi-strain dynamics. While it has been used in human medicine for such issues as human influenza, it has been explored to a limited extent in animal medicine. However, because of the rich data available within pig production systems, there is a potential for research on pigs to lead to a better understanding of multi-strain dynamics, VanderWaal pointed out.

For example, researchers can pinpoint the location of farms and track how animals move between farms. Also, because significant virus sequencing has been done, scientists have information on where and when PRRSv strains occur.

An added benefit for researchers at the University of Minnesota is the Morrison Swine Health Monitoring Project (MSHMP) database, which tracks PRRSV in about 50 percent of the US swine breeding herd, and the Veterinary Diagnostic Lab (VDL) that houses genetic data on circulating viruses going back 20+ years. MSHMP coordinator Cesar Corzo and VDL assistant director Albert Rovira are on the project team.

While one component of the study is to evaluate how the virus is adapting, evolving and spreading, another component is to learn more about the role that host immunity plays. In theory, the most impactful PRRSv strains at any given time depend on the host population’s immunity, VanderWaal said. So, a virus that is significantly different from what the population has encountered previously would likely be more successful at spreading.

“We’re hoping to look at how these different virus lineages behaved through time. How different lineages interact and compete with each other might help us understand and maybe better anticipate what the virus is going to do next,” she added.

The end game

The genetic data generated from this collaborative research could be used to guide future vaccine development. Diagnostic benefits can be expected as well. Also on the team is Declan Schroeder, PhD, virologist at the University of Minnesota, whose laboratory developed a new technique that can outline the complete profile of a genomic strain of PRRSv within 24 hours of sampling. This will provide veterinarians with an effective tool to rapidly detect PRRSv infection in pigs.

At the end of 4 years, VanderWaal hopes to better understand how PRRSv evolves and adapts to immunity both within the individual host as well as at the population level. “We want to know how these components interact, be able to anticipate a herd’s susceptibility to different PRRSv strains and customise mitigation efforts accordingly,” she said.

Ultimately, the end game is to help producers and veterinarians outmanoeuvre PRRSv.