According to 2009-2019 data analyzed by the Minnesota Swine Health Monitoring Project, the incidence rate of porcine reproductive and respiratory syndrome showing up in PRRS-free herds was at its lowest in 2019 in the U.S.

Overall, PRRS incidence rate in status IV farms — farms that are PRRS naïve or clean — varied significantly from 2009 to 2019.

During 2019, 20 status IV farms had a PRRS outbreak. Incidence rate in this groups was five PRRS outbreaks per 100 farm-years. Another way to look at this is that five PRRS outbreaks can be expected in a population of 100 status IV farms during a year at risk of PRRS.

The highest PRRS incidence rate was observed in 2011 and 2012 with 19 and 22 cases per 100 farm-years, respectively. The lowest PRRS incidence rate prior to 2019 was observed during the emergence of porcine epidemic diarrhea in the U.S. in 2013 and 2014 with 6 and 5 cases per 100 farm-years, respectively.

In PED pre-epidemic years 2009-2012, the average PRRS incidence rate in status IV farms was 17 cases per 100 farm-years, while in post PED epidemic years 2015-2019 the average PRRS incidence rate was 9 cases per 100 farms-years.

Keeping pig herds free of the PRRS takes a lot of effort as producers, veterinarians and researchers continue to work on ways to contain the disease.

When asked why the PRRS incident rate was lower in 2019 and not spreading to clean herds, Cesar Corzo, veterinarian at the University of Minnesota’s College of Veterinary Medicine and Allen D. Leman Chair, offered several reasons, acknowledging that it is hard to pinpoint one factor responsible.

He noted that scientists and veterinarians have offered five theories:

“There are many possibilities and perhaps there's a multi-factorial response to this event,” Corzo says. “What is clear is that both producers and veterinarians’ progressive attitude and willingness to go the extra mile on the prevention arena is part of this event since we are seeing the same for PEDv.”

The project report also noted: “While the population of status IV farms changes year after year due to inclusion of new participants, monitoring PRRS occurrence in this select population of farms is helpful as it reminds us of the capability of this virus to affect farms that may have higher-than-average biosecurity standards.”