PRRS: Deciphering the Mystery Disease

( National Pork Board and the Pork Checkoff )

Everywhere veterinarians turned, the “mystery disease” was devastating sow herds and wiping out pigs. It was causing incredible losses in reproductive herds — breeding failure, abortion storms and non-viable pigs at birth. The disease didn’t stop there, triggering major respiratory issues with wean-to-finish pigs. Diagnostic sampling continually came up negative.

“I couldn’t solve it. Nobody could,” says Scott Dee, DVM and current director of research for Pipestone Veterinary Services. “Nobody could figure out what was happening. It was just a mess. I can’t think of anything worse as far as just how bad you felt for the poor producers and their animals. You couldn’t do anything, and you were the person who was supposed to bring the solution. It was mentally deflating.”

It was even more discouraging because of the lack of cooperation among universities to solve the problem, he adds.

“Everybody was fighting to determine what the agent was so they could make a vaccine,” Dee says. “And in the end, they were all wrong.”

In February 1991, Dutch researchers at the Central Veterinary Institute in the Netherlands recovered the causative agent of porcine reproductive and respiratory syndrome (PRRS).

It was a huge turning point for the good, Dee says. But, as a young swine vet, those were discouraging days.

Dee’s saving grace? A decision to return to graduate school at the University of Minnesota to pursue a Ph.D. and study this new disease. 

“It was wonderful for me,” he says. “My goal was to just get better at science and become a better thinker, a better practitioner through a scientific way. It saved me.”

With newfound knowledge of the disease, the research culture began to change. Information began to flow more freely as universities worked together to find answers. 

“We got a lot of people involved and shared information. It was very different – purposely – than what I experienced earlier,” Dee says.

Understanding PRRS 
In addition to the work taking place in the Netherlands, U.S. researchers were also isolating the PRRS virus (PRRSV) in the early 1990s. Scientists, including Jim Collins at the University of Minnesota and Dave Benfield at South Dakota State University, were able to isolate and identify the virus. 

“It was important to finally be able to identify the virus and know the cause for this impactful, clinical disease,” says Reid Philips, DVM and PRRS technical manager for Boehringer Ingelheim. “We had a huge learning curve from that time forward.”

Researchers discovered this tricky single-stranded RNA virus could change and mutate significantly over time. 

“Since the early ’90s to now, the virus continues to change and diverge,” Philips says. “New strains have emerged over the years, and these continue to diverge. The ability for the virus to change can make management and control of the virus challenging.”

Unlike some viruses, once PRRSV infects a population of pigs, the period of infection, viremia and shedding/transmission can be prolonged, which makes it challenging to control. For instance, infected pigs can be viremic for weeks and the virus can persist in tissues for months.

As understanding of the etiology of PRRS grew, researchers could begin digging into the hard questions producers were facing. How do you manage replacement gilts and nursery pigs that are infected? How do you deal with breeding herds with subpopulations of positive and negative animals?

“We finally had some answers to our questions and could start giving people management tools to use and build on,” Dee says. “That was a big step.”

Detection strategies
Up to this point in the process, diagnostics tests were antibody-based. The development of PCR for PRRS virus detection allowed the industry to more accurately discover the antigen or the virus itself in blood or tissue, Dee says. 

Soon after, scientists were able to sequence the virus genome and further characterize the strain producers had in their herd, Philips says. 

“All of these advancements and diagnostics were incredibly important and helpful for us to understand the status of the virus within populations of pigs, and even the status of the virus by phase of production,” Philips says. 

Today, these diagnostic tools are used in a holistic and systematic approach to understand where the virus is by phase of production. Being able to identify the PRRS virus status and pattern of movement or transmission, within and between these stages of production is critical as scientists develop control strategies, Philips adds. 

Routes of transmission
Still, big questions continued to plague researchers. How did PRRSV get here? Where did it come from? How is the virus transmitted? Understanding the routes of transmission was a big question, Philips says. 

Continued on Page 2.


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