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PRRS diagnostics not always straightforward

March 21, 2024
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When it comes to porcine respiratory and reproductive syndrome (PRRS), high sow death rates, abortion storms and poor nursery performance grab the headlines. But less familiar are milder cases and even those where a positive test is hard to find. Bottom line, there are many PRRS virus strains, and a strain can behave differently in different settings.

“It’s easy to forget that there are a lot of PRRS virus strains that aren’t as bad as the ones that tend to make the headlines, which can complicate or make it harder to diagnose when there are not those significant clinical signs,” says Daniel Gascho, DVM, Four Star Veterinary Service. “But that’s why PRRS diagnostics are important. If it’s not the dramatic PRRS like what you may be used to, don’t assume it’s something else and move on.”

That was the exact scenario that Gascho recently faced on a client’s farm.

Real-world case study

The farm was a recent start-up that was negative for typical pathogens of concern and maintained high-health status. Farm personnel reported an increase in abortions to Gascho, who had them send in recently aborted fetuses and placentas, as he was unable to reach the farm that day for more thorough diagnostics. However, those tests came back negative for PRRS. The next day, the farm sent in more samples and test results were the same. Gascho was able to get to the farm to collect serology from sows that recently aborted litters, as well as processing fluids and blood samples from weaned pigs. Again, all tests were negative. The abortion rate was running about 20% across all gestation groups.

The farm was on its fifth round of diagnostics when Gascho happened to be on-site when a sow aborted, and he collected a blood sample from her within the hour. It finally tested positive for PRRS. “We did not find a single other positive on the farm,” he noted. The only other place it was identified was in one group of weaned pigs from a down-stream nursery that produced a 100% match.

The virus had a 1-7-2 cut pattern, which isn’t necessarily a new strain, but there are many mutations and variants within a single cut pattern, and this version just acted differently, Gascho said.

From that single positive test, the farm began the elimination process, as it was able to hold enough gilts on-site to close the herd for 6 or more months, if needed. Even though weaned pigs had tested negative from the beginning, the farm monitored processing fluids and weaned-pig serology during the elimination period. To date there have been no PRRS-positive tests. “After closing the herd and seeing no more abortions or any impact on conception rates, everything went back to normal,” Gascho said.

His take-home message: “If we wouldn’t have dug deep and finally gotten proof that it was PRRS, it’s unlikely we would have taken the time to undergo an elimination protocol, and I suspect the abortions would have lingered and we would have been left with sub-par health,” he added.

Two approaches to PRRS

PRRS has a way of keeping producers and veterinarians guessing, but there are two broad approaches to PRRS diagnosis, Gascho pointed out.

  1. While every PRRS is different, in general, if it’s a new sow farm, a PRRS-naïve farm or in an area with limited PRRS exposure and not vaccinating for PRRS, acute infections often produce an “abortion storm.” “With little warning a huge wave of disease goes through the system,” Gascho noted. “In those scenarios, if PRRS is there, it’s not hard to find.” In this scenario, for quick turnaround and answers, especially if the veterinarian is not able to get to the site that day, farm personnel can collect aborted fetuses and placental tissue and usually get a positive test. The collection process simply involves bagging the sample with a cold pack and getting it to the lab. “It’s a simple sample because any live or dead PRRS virus is a significant finding. Serology remains the confirmatory or gold standard test, but if the farm isn’t able to draw blood themselves, I would rather use abortion material and get some diagnostics happening right away than have to wait a couple of days for the vet to get there,” he added.
  2. For a farm in a pig-dense, high-disease-pressure area, or that has had periodic PRRS outbreaks, is vaccinating, or may be working toward stability, the objective is less about eliminating PRRS. “In some parts of the country that is just not an economically feasible reality,” Gascho said. “In those farms, it’s less about diagnosing an outbreak and more about monitoring the farm status and maintaining baseline production.” In this case, processing fluids or umbilical-cord blood are good diagnostic samples. The important thing is to find a user-friendly method that farm personnel can collect on a routine basis. For pigs that are sold at weaning or if it’s necessary to ensure they’re PRRS-negative or haven’t seroconverted, serology is the best sample to check pig status when they leave the farm.

Make monitoring routine

Whether to make PRRS monitoring part of a farm’s routine depends somewhat on its location and surrounding disease risks. “If it’s a high-health, PRRS-naïve herd, you’re unlikely to overlook a virus exposure,” Gascho noted. “But it can never hurt to monitor your herd for PRRS.”

It goes back to the farm case study and understanding that there can be times when a herd contracts PRRS and you don’t immediately know it. The first signs may be poor nursery and/or finisher performance, so you test and find PRRS. “If you’re checking processing or oral fluids, you could have found PRRS weeks to months earlier and addressed it,” he said. “It’s an easy, lower-cost way to monitor what’s going on.”

How often to test samples depends on the flow of the farm. A batch farm has a schedule built into it with processing completed for the farrowing group within a few days, so a monthly test is simple.

Continuous-flow farms should tailor their sampling based on their health challenges and needs. Generally, the approach is to collect processing samples weekly but submit a sample for testing once a month. Very large farms that move a lot of pigs may prefer weekly tests. Collecting and freezing weekly samples offer the added benefit of letting you go back and test if further investigation is needed.

Time to baseline production

For many farms today, PRRS diagnostics may not be about the time to a true PRRS-negative status; it’s about time to baseline production. “It boils down to how many pigs are we putting out of the sow farm and how many are full-value pigs at market,” Gascho said. “Because in some areas of the country, PRRS will pop up in processing fluids every year, and that doesn’t necessarily mean decreased production or a major health issue.”

A baseline lets you monitor trends to determine such things as if you are getting positives more often, whether CT values are running lower, if the farm is still PRRS-negative or if the elimination process is going in the right direction. Also, if it’s a PRRS-stable farm, is it leaking positive pigs out when it usually doesn’t?

That information can be used to guide nursery- and finishing-pig performance. “Don’t get me wrong; it’s not that PRRS doesn’t matter, but diagnostics are more of a monitoring tool to track status and trends,” Gascho said. “At the end of the day, even if you get a positive test on processing fluid, if the pigs have no challenges, they don’t have any clinical signs, they perform well and are full value at market, and especially if you’re not trying to sell them, then it may not always matter if they tested PRRS-positive. What does that piece of information tell us in the bigger picture?”