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Linhares: Digging deeper into PRRS outbreak management

Porcine reproductive and respiratory syndrome (PRRS) has challenged producers and swine veterinarians for decades. During that time, outbreak management and strategies have evolved along with the virus.

“If you ask 20 different people how they are managing PRRS outbreaks, you may get 20 different answers,” Daniel Linhares, DVM, associate professor at Iowa State University (ISU), told Pig Health Today. “People are still trying different things to succeed in their management program.”

To learn more, Linhares is collaborating with Giovani Trevisan, DVM, ISU research assistant professor, in a program called the PRRS outbreak management program (POMP). The effort involves a study similar to one Linhares conducted 10 years ago that looked at how approximately 60 herds managed PRRS breaks between 2009 and 2012. The ISU researchers are now following a similar-sized group of herds that broke with PRRS between 2020 and today.

Three components

 Linhares outlined three different components involved in the POMP study. These include:

  1. Benchmarking how sow farms are managing PRRS outbreaks. “Are they closing the herd? Are they exposing sows or gilts to PRRS virus (PPRSV)? If so, how they’re doing that — with vaccine, live-virus inoculation (LVI) or different approaches,” he noted.
  2. Comparing how long it takes farms to reach a low-PRRS prevalence or start producing negative pigs again, as well as the productivity impact of an outbreak.
  3. Whole-genome sequencing to try to determine how many PRRSV strains were present on farms. Also, whether there were recombinants with the wild-type virus or vaccine. “We looked at diversity — how many unique strains and how does that correlate back to clinical measures, time to stability and time to live production,” Linhares said.

Extended time to stability

The most basic finding was that there remains a wide range of PRRS outbreak-management strategies and outcomes. “We saw a lot of variation between herds that close, don’t close, use the LVI with the wild-type virus or vaccine or a combination of those,” he noted.

A significant finding was that the median time to consistently produce PRRS-negative piglets at weaning takes about 10 weeks longer than previously observed. “The median time to negative is now about 36 weeks,” Linhares said. “If you look at the whole distribution, we had farms that more than a year after the break they were still producing positive piglets.”

An influencing factor was whether the management scheme included herd closure, which is defined as stopping gilt introduction until the infection pressure is low and transmission has ended. “Farms that do herd closure achieved stability much quicker,” Linhares pointed out.

Farms that had some prior immunity through mass exposure via vaccination or LVI also tended to achieve stability much sooner. Batch farrowing, at either 2-week or 4-week intervals to create an all-in/all-out flow, reduced the time to achieve negative piglets as well.

One caveat to the longer time to stability may relate to more sensitive monitoring options today. Linhares noted that 10 years ago, PRRS monitoring involved collecting 30 serum samples once a month. “Today with processing fluids, familial fluids, people are monitoring hundreds of piglets on a weekly or biweekly basis,” he added. “You now have much more sensitive methods to detect at lower prevalence.”

Productivity metrics

Another important metric in determining PRRS-outbreak severity was pre-weaning mortality. “In general, the worst impact in terms of piglets not weaned after a break was almost twice as much as what we observed 10 years ago,” Linhares said.

Back then, the median number of piglets lost was about 2,500 per 1,000 sows. “Today we’re observing 4,700 as a median number of piglets not weaned. So, a much more severe type of break,” he added, citing PRRSV 1-4-4 L1C variant and PRRSV 1-7-4 as influencing factors today.

Different variants

For the third component of the POMP study, Linhares and Trevisan asked the farms’ veterinarians to submit virus samples at the beginning of a PRRSV outbreak, 10 weeks later and just before pigs turned negative. The point was to do whole-genome sequencing of the virus and evaluate genetic properties of the virus associated with clinical expression following the outbreak.

The researchers wanted to know, “was there something in the virus that would help explain that time to stability or time to baseline productivity,” Linhares said. They observed that it was the norm, not the exception, for farms to be infected with more than one PRRSV strain. Upon closer review, it was common to find recombinants between some viruses that are part vaccine virus, part wild-type virus or two different wild-type viruses.

Linhares relayed a very preliminary finding in the dataset of this ongoing study was that when a farm had less virus diversity, it progressed toward PRRS-negative status and recovered baseline productivity more quickly. When multiple strains surface or recombination occurs, it resulted in mortality waves, abortions and other PRRSV-associated clinical signs.

His take-home message is to characterize the PRRSV strain early in the outbreak and check again in 2 to 3 months to verify if it’s the same strain. If there’s evidence of other variants within the herd, “consider additional immunization and closing the grip on internal biosecurity, biomanagement and biocontainment measures to prevent virus spread from litter to litter, pen to pen, room to room,” Linhares said. “You want to make it hard for the virus to transmit and gain ground.”

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Porcine reproductive and respiratory syndrome (PRRS) has challenged producers and swine veterinarians for decades. During that time, outbreak management and strategies have evolved along with the virus.

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Posted on February 2, 2022


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When a sow doesn’t reach her full potential, the cost to the farm and the income stream of the sow herd is often “grossly underestimated,” said John Deen, DVM, PhD, a professor at the University of Minnesota.

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