SHIC-AASV Webinar Delves into PRRS 1-4-4 1C Incidence and Possible Pathways of Entry

Porcine reproductive and respiratory syndrome virus (PRRSV) 1-4-4 1C appeared in southern Minnesota and northern Iowa in late 2020. Dramatic clinical signs on farms raised concern and the Swine Health Information Center (SHIC), with co-sponsor American Association of Swine Veterinarians, offered a webinar on PRRSV 1-4-4 1C on February 4, 2021. A second wave of outbreaks in April and May 2021 in a wider geography with positive cases in upper Midwest states keeps this virus at the forefront. A second webinar, PRRS 1-4-4 Incidence and Response, took place July 20, 2021, featuring discussion on SHIC Rapid Response Teams’ investigations into possible pathways of entry of the virus plus updates on incidence and geographic distribution.

July webinar presenters included Dr. Giovani Trevisan, Iowa State University (ISU)/Swine Disease Reporting System (SDRS), Dr. Mariana Kikuti, University of Minnesota/Morrison Swine Health Monitoring Project (MSHMP), Dr. Paul Yeske of the Swine Vet Center, St. Peter, Minnesota, and Dr. Derald Holtkamp, ISU Veterinary Diagnostic and Production Animal Medicine. MSHMP is a SHIC-funded program focused on communication of disease occurrence. The webinar was conducted by the ISU Swine Medicine Education Center.

While 1-4-4 is not a new strain, on-farm experience with this new lineage 1C (L1C) variant has shown it results in higher farrow-to-finish mortality, abortions, mummies, and slower growth in finishing pigs compared to other PRRSV strains. Dr. Trevisan said information gathered from upper Midwest veterinary diagnostic labs shows PRRSV lineage detection over time is very dynamic with the L1C variant already becoming the fourth most detected. Data submitted to the Swine Disease Reporting System (reported by SHIC in the monthly Domestic Swine Disease Monitoring Report) shows the L1C variant is most frequently detected in the wean-to-market category in Minnesota and Iowa.

Dr. Trevisan said diversity is inherent to PRRSV and this remains true for the L1C variant which is evolving and forming different groups. While it remains most frequently detected in Iowa and Minnesota, at least seven other states have reported the L1C variant is present. More epidemiological work is needed for better understanding of this variant.

Dr. Kikuti presented information from MSHMP showing this variant is unusually similar at the ORF5 level between different sites. She also detailed their case definition saying sequences that were at least 98% similar to any other case sequence were included. As of June 30, 2021, a cluster of 259 sequences were reported. There were 227 sites affected, including 64 breeding herds, 126 growing pig farms, and 37 without information defining their type. But one concern Dr. Kikuti mentioned was lack of standardized nomenclature for whole genome sequences which assures the same case definition is used throughout the industry.

Dr. Yeske has seen the variant’s effects first hand as a practitioner. Clinical observations include abortions which continue to be the hallmark of the variant though the frequency was less in the spring outbreak than last fall. Mortality rates were about the same with both outbreaks. In the farrowing herd, stillborns and mummies were common. Preweaning mortality was as high as 100% for a few weeks this spring. They also noticed piglet quality deteriorating rapidly, and postweaning there was higher than normal mortality.

The SHIC Rapid Response Team outbreak investigation took place on two farms that broke with the variant in the spring. Dr. Holtkamp set up the Team’s work with the farms and emphasized the goal is not assigning blame to anyone on either farm or the system it is part of, rather, to understand weaknesses in biosecurity and then develop plans for improvement. 

The Rapid Response Team hazard analysis uses available epidemiological information including sequencing information as well as timing and location of the first clinical signs of the variant. The analysis looks for where the farm is vulnerable. With the high area incidence and so many potential factors at play, Dr. Holtkamp described the process as conducting a murder investigation in the middle of a gang fight, meaning there are an extraordinary number of suspects.

A review of production activities launches the investigation, looking at people, equipment, tools, supplies, animals, locations, timing of activities, and procedures including the who, what, where, and how of each. The Rapid Response Team’s form used for the investigation is available on the SHIC website.

Both farms being reviewed were commercial farrow-to-wean farms. On farm one, the first clinical signs were noted on May 9, 2021, where the first signs of the outbreak were scouring and rough hair coats on piglets in farrowing rooms. On farm two, the first clinical signs were seen on May 29, 2021, where sows were off feed in part of the gestation barn.

Four weeks of each farm’s records were analyzed. On farm one, there were 575 risk events noted. On farm two, there were 580 risk events. The Rapid Response Team rated employee entry and removal of weaned pigs as the highest risk factors on farm one. On farm two, removal of cull sows was the highest risk event. The Team noted employee transfer between farms in the system was a concern as well.

Because PRRSV 1-4-4 1C is more virulent and highly contagious, Dr. Holtkamp remarked any lapse in biosecurity could be an entry point onto a farm. Dr. Yeske observed the magnitude of the outbreak waves increased from the fall of 2020 to the spring of 2021, which creates concerns for a wave to occur again this fall. Both Drs. Yeske and Holtkamp are encouraging review of biosecurity plans to identify the potential areas of entry on farms.

SHIC, launched in 2015 with Pork Checkoff funding, continues to focus efforts on prevention, preparedness, and response to novel and emerging swine disease for the benefit of US swine health. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. SHIC is funded by America’s pork producers to fulfill its mission to protect and enhance the health of the US swine herd. For more information, visit or contact Dr. Sundberg at [email protected].