It is hoped that the study’s findings will help scientists and producers anticipate a herd’s susceptibility to different strains of PRRSv and customise mitigation efforts accordingly. The data generated could also be used to inform future vaccine designs.

The current study evolved from previous research conducted by Kim VanderWaal, an assistant professor in the College of Veterinary Medicine, and fellow researchers at the University of Minnesota.

Kim describes how as she began working on the PRRS virus, she realised that studying the spread of the virus would require getting to grips with the way it evolves.

“You can’t really understand how it’s spreading because there are different variants of PRRS that are more successful at spreading,” explains Kim.

“Until you understand the immunology of this disease and how it interacts with the host, you can’t accurately predict its evolution.

“Answering the questions that came from previous work has really spurred the research line that we’re pursuing for the next few years.”

The challenge of preventing and treating PRRS

The biggest challenges to working with PRRS are the genetic diversity plus the fact that the virus’s genetic diversity is constantly expanding, says Kim.

Homologous protection – immunological protection – is far more effective than heterologous protection. Essentially, past exposure to a virus that’s very similar to the one currently being experienced is very good in terms of preventing clinical disease in animals.

This said, if the virus you have now is sufficiently different, both genetically and antigenically from what the host has encountered previously, then the prevention and treatment is not as effective.

The two main ways that pig industry tries to control the PRRS virus is by live vaccine or live virus exposure. The success of both of those strategies depends on how well the virus strains used during exposure protect against the field strain.

“Developing an efficacious PRRS vaccine is kind of like hitting a moving target – the virus is constantly evolving which means that you generally can’t target two strains in one vaccine nor stimulate a robust immune response with closely-related virus DNA,” says Kim.

Identifying patterns of mutation

“We focus on looking at the genetic sequence of the virus,” Kim explains. “One of the wonderful things about working with the US pig industry is that veterinarians and practitioners are already submitting samples for sequencing on a routine basis.

“This means there are huge databases already available on which viruses are occurring on which farms and when.”

Kim describes how this data is a wonderful opportunity to start looking at patterns of outbreaks and mutation of the virus. Researchers identify these patterns primarily though looking at genetic sequences and assembling genetic trees, tracking how the virus has evolved and how it is evolving. In the past, this has been done on the OR-5 section of the genome - a very small section - but in the future, it will be possible to efficiently analyse the entire genome of PRRS.

One of the breakthroughs in PRRS research was observing the sequential turnover of the dominant PRRS variants in the population. In Kim’s research, she’s investigated PRRS evolution back to 2009 and analysis indicates that every three to four years a new sub-lineage of PRRS emerges or re-emerges. The most recent mutation was traced back to 2014 with the emergence of the 174 sub-lineage, which has been a particular challenge to the industry due to its patterns of mutation and virulence.

Predicting, preventing and responding to future outbreaks

“We’re looking back through the patterns of evolution and at the emergence or re-emergence of different variants through time, we can then start to understand the cycle of how often this occurs.

“From strain to strain and lineage to lineage that follows one another, we can start to see a pattern of how this particularly new, emerged PRRS or sub-lineage of the PRRS virus differs from whatever was previously circulating in the population.

“Through this project, we’re trying to determine a predictable pattern in any of the antigenic changes that have allowed those PRRS variants to be successful at any particular period in time.”

It is hoped that the work of Kim and her team will also further the understanding how the virus evolves in response to host immunity, as well as the antigenic regions of the genome that are important for cross protection.

This research is valuable to pig industries globally as there are common antigenic regions on the virus that are repeated in strains found in different continents. Equally, even if two strains found in different regions are not identical, it is expected that the dynamics and the evolutionary change will be similar between regions so any trends identified in the study will be widely applicable.

A welcome boost for the research team

The funding granted to the project is indicative of the importance of the research and the world-class team working on it. Kim says each team member brings a unique skillset that’s going to be essential in pursuing these questions.

“Part of the strength of this project is going to be the team that we’re working with,” she adds, “every person that was part of that grant is critical for its success.”