Revised herd classifications expand the PRRS road map

Since its introduction in 2011, the system to classify breeding herds according to their porcine reproductive and respiratory syndrome virus (PRRSV) status has provided swine veterinarians and producers with a well-defined common language. So much so that the system has been adopted worldwide by veterinarians, producers and researchers alike.

“Our original objective was to have a clear set of definitions that everyone agreed to,” Derald Holtkamp, DVM, professor of swine production medicine, Iowa State University, told Pig Health Today. “To develop a roadmap — a way for veterinarians to communicate with each other but especially with their producers — to get everybody on the same page, so everybody understood where they were going.”

Another important industry-wide benefit is that the classifications have assisted the Morrison Swine Health Monitoring Project at the University of Minnesota in tracking the PRRSV status of US breeding herds.

With years of in-field application, as well as an evolving virus and technologies, it became increasingly apparent that the PRRSV herd classification system could use some adjustment. A PRRS working group of American Association of Swine Veterinarian (AASV) members reviewed the classifications and drew up proposed changes. The AASV board of directors approved the revisions in fall 2019, with the final publication in the Journal of Swine Health and Production expected by early 2021.

Why the modifications?

“We’ve had several challenges from the 2011 classifications,” Holtkamp noted. “There was debate all along about the PRRSV-positive stable category (II) and the ability to consistently wean PRRSV-negative pigs.” He noted that it’s not uncommon for a herd to wean PRRSV-negative pigs for a period of time and then suddenly start to record some PRRSV-positive results. That challenges the decision-making process going forward.

There is some evidence that evolving PRRSV isolates are making it more challenging to stabilize sow farms, and that needed to be addressed. The working group also wanted to reinforce the definition of stability. “That stable means virus shedding has stopped and you consistently wean a PRRSV-negative pig,” Holtkamp added. “It had gotten watered down over the years.”

Among the modifications was to outline supporting evidence or criteria to maintain a herd in a category once it was placed there. “In the original system, we only developed criteria to move a heard into a category. The only thing that would knock a herd out was an outbreak,” Holtkamp said. “Now there are continuing diagnostic requirements outlined to maintain a herd in a category.”

The working group wanted to expand sampling options to include recent developments such as processing fluids and family oral fluids. Both are more cost-effective and easier to collect than serum.

Another factor driving the revisions, Holtkamp noted, is the development of new diagnostic tests. “Molecular tests, whole-genome sequencing and clamping PCRs (polymerase chain reaction) are used a lot more today than they were back when this started,” he added. “The consensus was to come back and address some sampling and testing.”

What has changed?

To be clear, the revisions are more of a natural evolution. “We didn’t want to have to go out and teach or train everybody on an entirely new system,” Holtkamp said. “We felt it was working pretty well. So, one of the objectives was not to reinvent the wheel.”

While there are some new sampling and testing protocols outlined, the most significant changes came in realigning the categories. In the original system, PRRSV-positive unstable (l) was a single category. The new version has split it in two: PRRSV-positive unstable, high prevalence (l-A) and PRRSV-positive unstable, low prevalence (l-B).

As Holtkamp pointed out, for many sow herds PRRSV control is the goal.  “Some breeding herds seem to get stuck in the PRRSV-positive unstable (I) category but with very low prevalence,” he said. “They just can’t get to PRRSV-positive stable. But when you look at the production, both in the sow herd and downstream pigs, it’s very acceptable.” So, the working group wanted to establish a destination for PRRSV control and gave those producers two options to feel like they met their goal.

The second change is in category II. Previously this was PRRSV-positive stable (II-A), with the goal of PRRSV elimination, and (II-B) without PRRSV elimination. In the new system, there is a PRRSV-positive stable (II) category and PRRSV-positive stable vaccinated (II-VX).

“In the old system, there was no distinction for vaccinated herds,” Holtkamp noted. “Now there’s a category for modified-live vaccines to be used in the breeding herd or even weaned pigs.”

The two remaining categories — provisional PRRSV-negative (III) and PRRSV-negative (IV) — remain unchanged.

Looking ahead

Although the new categories are just now moving into practice, it’s likely that revisions could materialize as technologies and the virus change and veterinarians, producers and researchers learn more.

With the fairly rapid developments in sampling types, “who knows what’s coming next,” Holtkamp said. In the end, the bottom line remains the same: Anything that reduces the cost of diagnostic tests and makes sampling easier will expand the supporting evidence needed to continue to make progress against PRRSV, he concluded.