Is This Year’s PRRS Better or Worse?
Is this year’s porcine reproductive and respiratory syndrome (PRRS) season better or worse than the last one? That perennial, challenging question drove Bob Morrison, former professor at the University of Minnesota College of Veterinary Medicine, to develop a system for gathering information. The Morrison Swine Health Monitoring Project (MSHMP) is the result.
Today, MSHMP is led by Cesar A. Corzo, DVM, associate professor and Leman Chair in Swine Health and Productivity at the University of Minnesota. It now includes 38 producers that represent 50% of the U.S. breeding herd. The project is 100% voluntary and is funded by the Swine Health Information Center (SHIC).
A Look at the 2020-21 PRRS
“PRRS tends to have cycles. We know that every few years, there's a new strain emerging. The last one had occurred about three to four years ago, which was the 1-7-4. Last fall, we were warned of a situation where a lot of finishing pig farms and some breeding herds were breaking simultaneously with a new strain which was baptized by our group as a 1-4-4 Lineage 1C strain,” Corzo says.
After analyzing the data, they discovered that PRRS sequence had been around for awhile in 2020, but it was not until the fall of 2020 when the viruses started manifesting themselves.
“Last fall we started seeing the outbreak in a very specific region of Minnesota,” he says. “It started moving little by little to other areas of the state and then went out of the state. We saw this huge outbreak, and then we didn't see more cases for a period of time. But now, we're starting to see a second wave of cases that continues to scare the industry.”
Is it a surprise? Corzo says maybe it is, but maybe it’s not.
“I think the surprise comes because even though we know that there's a virus cycle, that every few years we're going to see a new strain emerging, this one is different in that it is infecting a lot of finishing and breeding herds quite rapidly and simultaneously,” Corzo says.
Several factors may be contributing to the sense of disparity between the MSHMP PRRS data and the sense around the industry that this PRRS season has been a more difficult one compared to previous years. To date, MSHMP monitors disease in breeding herds only, but Corzo says this strain of PRRS is also being discovered in growing pig farms, which is where most of the pigs in the industry are housed.
During the 2020-21 season, several participants have reported Lineage 1C PRRS outbreaks in which losses have surpassed what they have usually seen. Such virus-generated losses prompted rapid communication among MSHMP participants and the industry, Corzo says. At the same time, another two unrelated 1-4-4 viruses classified as Lineage 1A and 1C were generating a similar degree of losses, but these occurred 600 and 400 miles away from the first identified outbreak, respectively.
“This PRRS virus is also getting to breeding herds located in remote areas. These are herds that haven’t seen PRRS for many years, or possibly ever,” Corzo says. He believes that this is what’s generating an important level of frustration and concern.
“It's telling us about the capacity of this virus to infect pigs, but also that it may be better equipped to survive outside the host,” Corzo says.
Seeing outbreaks of PRRS in areas where the industry normally doesn’t see PRRS outbreaks could signal that the virus was able to go through biosecurity measures, he says.
That’s when the questions start. “It couldn’t have come through the air because there are no farms nearby, right? Did we bring it in through positive semen? A contaminated truck? Gilts? Feed? Tools?” Corzo asks.
He believes there are plenty of options to rule out.
“That will take us some time,” he says. “I'm sure we can go and swab a lot of surfaces out there. And we may not have a very rewarding set of results.
However, we can learn where the virus may be present and perhaps build hypotheses on potential ways of indirect transmission, just like COVID-19.”
Why Does Lineage Matter?
Understanding the diversity in PRRS viruses is important. Corzo explains that in addition to RFLP (restriction fragment length polymorphism), which is used to designate different PRRS strains, diagnosticians are starting to use lineages and sub-lineages to further classify viruses.
One of the reasons why lineages matter is that it helps researchers understand virus evolution. It answers questions, such as: Is this family of viruses more virulent than another? Is this family changing faster than another family of viruses? What are the factors that lead a virus to change over time?
All of these answers can help create better vaccines, he says.
Lineage is also helpful when conducting outbreak investigations and understanding where viruses may have originated from. Knowing the PRRS lineages helps make sure you are dealing with the same virus when you work to eliminate viruses from the herd as sometimes there may be multiple strains co-circulating, he adds.
“By understanding the characteristics of the virus and the genetic components of the virus, we're going to be able to clean the data a little bit better, so that we can do the epidemiological investigation in a cleaner manner,” Corzo says.
With the Lineage 1C (RFLP 1-4-4) virus impacting many herds in 2020-21, knowing what to look for has helped make a big difference and simplify communication.
“Now that we're seeing a second wave of this virus, people already know what to look for,” Corzo says. “People can start assessing whether they need to fine-tune biosecurity compliance and their protocols or whether they need to avoid specific regions if they are moving gilts or recently weaned pigs.”
Awareness helps producers better assess risk, he says.
“At the end of the day, the fewer PRRS-positive pigs we have, the fewer viral particles that are going to go out into the environment, the fewer the chances that we contaminate trucks, tools, etc.,” Corzo says. “The fact that we saw this second spike leads me to believe we’re going to see a little more of this virus in the coming months. I hope this is not true, but it’s very likely bad news for the fall.”
Read more:
Put PRRS Strain 1-4-4 Lineage 1C in Perspective
PRRS Strain 1-4-4: The Most Dramatic Strain I’ve Seen, Yeske Says
