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Virus persistence challenges PRRS management strategies

22 October 2021, at 11:58am

Eliminating porcine reproductive and respiratory syndrome virus (PRRSV) is a promising achievement for some commercial herds and a goal for others. Among the challenges is that PRRSV causes a persistent infection in pigs, which can be difficult to detect. “In terms of profitability and animal welfare, we would like pigs to clear the virus as quickly as possible,” Marcelo Almeida, clinical assistant professor at Iowa State University College of Veterinary Medicine, told Pig Health Today. However, there are many factors sabotaging that goal.

The PRRSV persists for different periods of time in different locations within the pig. “Under experimental conditions, it has been shown that infectious virus can be recovered from tonsil tissues for up to 157 days,” he noted.

Of course, PRRSV detection by polymerase chain reaction (PCR) does not necessarily equate to transmission. “If there is transmission from infected to susceptible animals, that’s worrisome,” Almeida said. “We have reports that can happen for up to 175 days post-infection. That is a very long time to be worried about PRRSV and having all the bio-containment management practices in place to avoid that situation.”

There are many unanswered questions about the variation in PRRSV persistence. So far, the research has shown that only 5% to 10% of animals are persistently infected for long periods of time — up to 175 days — and are able to transmit the virus to other pigs. “We do not understand the risk factors and why certain animals will have persistent infection and others are able to clear the virus,” he added. “Obviously, we need more studies to understand the individual risk factors that lead to persistent infection, both at the individual pig level, but also at the herd level — especially for sow herds,” he said.

The cycle of PRRS

It is that lingering virus persistence that challenges PRRS management in the breeding herd. It is essential that sows are clear of PRRSV so they don’t transmit it to the piglets, which then leads to the continued circulation of the virus on the farm.

The tricky part for the breeding herd, as Almeida explained, is that young replacement gilts may not have been exposed to PRRSV, so they don’t have the immunity to fight off the infection. “As soon as they get infected, it’s another chance for the virus to persist in the breeding herd and pigs,” he added.

On the plus side, some diagnostics tools — new sampling types and sensitivity — have improved, which has enhanced producers’ ability to detect PRRSV in piglets and the breeding herd. When it comes to herd closures, that increased ability to detect infected animals has extended herd-closure times — and maybe explained why some “negative” herds re-broke with the “same old virus.” However, in the long run, better sensitivity will provide a more accurate assessment of farm status.

“It’s taking a lot more time to achieve PRRS stability — where the virus cannot be detected in the nursing piglets,” Almeida noted. “But we want to be sure that the virus is not there before we end that closure period in farms.”

Part of a herd-closure program involves exposing breeding animals to live PRRSV or modified-live vaccine to build some level of immunity and to stop virus shedding in piglets at birth and during lactation in an attempt to end the PRRS cycle.

Future surveillance prospects

The key to maintaining a breeding herd’s PRRS stability is establishing a surveillance and monitoring program of the piglet population.

Processing fluids, collected from castration and tail docking, have significantly increased the number of piglets being tested each week. “Processing fluids have helped us increase the sensitivity of PCR testing because the sample represents a large number of animals and has likewise given us a practical tool for effective and inexpensive surveillance,” Almeida said.

Another practical tool is “family oral fluids,” in which both the sow and her piglets chew on a rope hung in the farrowing crate. Their combined oral fluids provide a sample from a population known to be key to PRRS control. “At the herd level, family oral fluids and processing fluids have far better sensitivity than serum samples for PRRS detection,” he noted.

Looking ahead, there are additional sampling strategies under investigation that will refine PRRSV surveillance and monitoring even further. Almeida pointed to “two-stage stratified sampling” and risk-based sampling that could further increase the efficiency of PRRS detection and control costs.

“Our research to date shows that these sampling approaches are far more efficient than individual pig sampling for detecting PRRS in farrowing rooms,” he said.

For now, processing fluids offer a screening option early in the piglet’s life, but you also need to follow up at weaning because a lot can happen between processing at 3 days of age and weaning at 21 days of age. For that reason, the best approach to accurately monitoring suckling pigs is to set up a combination of sampling strategies.

Asked whether herds can ever entirely clear PRRSV, Almeida said, “Because none of the testing techniques are 100% specific or sensitive, we can never be 100% sure. But when we do comprehensive sampling and testing, covering different ages of animals and following the herd over time, we can determine when the sow herd is no longer producing viremic piglets.”

The key is to be consistent. “Don’t stop when you believe the sow herd has started producing negative piglets. A continuous monitoring program is essential to knowing your herd status and avoiding surprises,” he said.