SHIC/AASV PRRS 1-4-4 Lineage 1c Webinar Provides Information on Recent Outbreaks

Midwest veterinary diagnostic labs (VDLs) began diagnosing porcine reproductive and respiratory syndrome virus (PRRSV) 1-4-4 in late 2020 with practitioners and producers reporting dramatic PRRS-like clinical signs on farms. Reacting quickly to this new PRRS strain, the Swine Health Information Center, with co-sponsor American Association of Swine Veterinarians, offered a webinar on PRRSV 1-4-4 on February 4, 2021. Expert presenters on the webinar provided context from the practitioner, diagnostician, and monitoring perspectives. While 1-4-4 is not a new strain, on-farm experience with this new lineage 1c variant has shown it results in higher farrow to finish mortality, abortions, mummies, and slower growth in finishing pigs compared to other PRRSV strains.

Webinar presenters included Dr. Paul Yeske of the Swine Vet Center, St. Peter, Minnesota, Dr. Stephanie Rossow, University of Minnesota (UMN) VDL, Drs. Giovani Trevisan and Daniel Linhares, Iowa State University (ISU)/Swine Disease Reporting System (SDRS), and Dr. Mariana Kikuti, UMN/Morrison Swine Health Monitoring Project (MSHMP). MSHMP is a SHIC-funded program focused on monitoring disease occurrence. The webinar was conducted by the Iowa State University Swine Medicine Education Center.

Dr. Yeske said he and his colleagues at the Swine Vet Center have had the opportunity to see the virus more than they would like in sow farms and grow-finish facilities. While the clinical signs of this lineage are consistent with other PRRS outbreaks, this one is more dramatic. Sows go off feed, abortions begin, and other consequences follow, moving quickly through the herd once clinical signs are noticed. Their experience includes upwards of three to five weeks of production lost to abortion, sow mortality up to 20% within a two to three week period of time, and high piglet mortality in the farrowing house. In the nursery, piglet mortality remains high – 50% in some cases and some up to 80%. In finishing pigs, average daily gain dropped from 2.4 pounds per day to 1 pound per day following a drop in water consumption proceeding the outbreak of PRRS 1-4-4.

Dr. Rossow had spoken with several clients of the UMN VDL to get their input on how this PRRS 1-4-4 lineage 1c variant was impacting their businesses. She said the VDL first became aware of the issue in nursery and grow-finish pigs. Client experience showed the strain is moving laterally easily and is occurring on some farms that have been managed as PRRSV-negative for over 20 years. In some groups, mortality as high as 30% was reported.

Clinical signs reported to the UMN VDL include high fever as well as respiratory disease and it was noted secondary diseases – viruses and bacterial infections – are causing additional problems in affected herds. Without exception, Dr. Rossow said response to treatment – both water medication and injectables – has been poor.

Under the microscope, Dr. Rossow said they are finding really severe lung lesions, which are expected, along with a standout amount of necrosis, or tissue death, in the thymus and lymph nodes of affected pigs. Both the thymus and lymph nodes are important for a proper and coordinated immune response by the pig. “So there’s a marked dysfunction in those tissues and certainly going to relate to a poor response to other viral and bacterial challenges that pig is going to encounter, especially in the nursery/grow-finish environment,” she stated. 

Beginning in October 2020, the SDRS began showing increased PRRSV RNA PCR detection from data provided by cooperating VDLs at ISU, UMN, Kansas State University, and South Dakota State University. Then in December 2020, more than 7000 cases were tested by PCR in the cooperating VDLs, gaining attention of SDRS staff and its advisory board. RFLP typing also confirmed the increase in PRRS 1-4-4 occurrence in late 2020.

Dr. Rossow said PRRS 1-4-4 lineage 1c variant is detected by ThermoFisher and the Tetracore PRRSv PCR tests, the more common ones used in diagnostic labs. She noted fairly consistent viral myocarditis in these pigs which was the case with past PRRS pigs. While the UMN VDL has tested submitted spleens for African swine fever and classical swine fever as well, neither has been found, increasing confidence that this is a uniquely severe PRRSV strain causing these problems.

Across states, there is considerable genetic diversity for the PRRSV strains classified as RFLP 1-4-4, per Dr. Trevisan. More frequent detection of RFLP 1-4-4 strains in Iowa and Minnesota was observed after September 2020, in review of ISU and UMN VDL data. A cluster of 106 sequences was detected in Iowa and Minnesota after October 2020.

Dr. Kikuti said MSHMP participating production systems started independently reporting the occurrence of a virulent PRRSV RFLP 1-4-4 strain, particularly in grow-finish sites where high mortality was observed. The systems noted this was a different virus than the one used in gilt acclimation processes as well as different from others in the systems.

As of January 17, 2021, a genetic cluster of 72 sequences from 60 farms was identified by MSHMP. There were nine MSHMP participating production systems involved in their investigation. Of those 60 farms, 45 were growing pig farms with herds ranging in size from 960 to 8000 pigs. In addition, there were 11 affected breeding herds with an average size ranging from 750 to 4000 sows. Of the 72 PRRS sequences obtained, one was from 2018 (classified as RFLP 1-4-3), 67 were from 2020 (all RFLP 1-4-4; most occurred in October to December 2020), and four did not have a date available (all RFLP 1-4-4). 90% of cases within this genetic cluster occurred in a 50-mile radius. In a phylogenetic analysis, Dr. Kikuti said they were all cases were genetically very similar with identical sequences occurring in different production systems, and it appears to represent the emergence of a new clade within lineage 1c. Dr. Kikuti also said sequences from this cluster of PRRS are being submitted to GenBank to aid on-going investigations.

Herd stabilization is the goal. Dr. Yeske recommends producers identify greatest areas of risk on their farms, asking, “How likely is something going to cause the virus to enter your herd?” Then, once that question is answered, follow-up by asking how many times a week that risk activity takes place. “Review herd biosecurity plans,” he recommended. “Make sure things are being done like they are supposed to be. If possible, improve on any weaknesses, doing so sooner than later.”

A lot is known about biosecurity practices to prevent or minimize the probably of PRRSV spread, per Dr. Linhares. He described a recently completed study by ISU veterinary student Broc Mauch, et. al. which included data from 188 sow herds in 13 systems in 15 states of the US. Based on a 20-question survey, the work applied to biosecurity scoring systems. The objective was to see if a consistent 20-question format can be used to screen for PRRSV introduction into breeding herds, and if they can be good predictors. Dr. Linhares said the 20 questions relate to obvious things, mentioning the number of risky events on farms, annual employee turnover and number of employees which compounded with herd size, distance to main roads, density of sites around the premises, transport biosecurity, and more.

In a summary of work done by Dr. Derald Holtkamp of ISU on 19 investigations of PRRS outbreaks from 2015 to 2017, Dr. Linhares pointed out entry of employees, removal of culls, and repairs were risk events rated high most frequently on those farms. Dr. Linhares shared a reminder on SHIC’s Rapid Response Program designed for situations where outbreak investigations are needed to determine the pathways of entry onto the farm.

Area spread was also facilitated by favorable weather conditions in southern Minnesota and northern Iowa where many overcast days, fog, and warmer than usual winter temperatures were the norm, per Dr. Yeske. Dr. Kikuti also noted winter is the high transmission season for PRRSV. And she concurred with Dr. Linhares who shared the need for further epidemiological investigations, also suggesting molecular studies are needed as well.

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