The porcine reproductive and respiratory syndrome (PRRS) virus has been challenging pork producers and veterinarians alike for nearly three decades. Today, two University of Minnesota veterinary researchers are working to determine whether PRRS season is a real or imagined phenomenon.

For nearly three years, Bob Morrison, DVM, and Steve Tousignant, DVM, PhD student, in veterinary population medicine at the University of Minnesota, have worked on a database to study the incidence and pattern of PRRS cases. It will soon have 12 participating systems, representing more than 300 sow herds for at total of about 1,000,000 sows in 13 states.

“This first of its kind effort was born from the frustration of veterinarians struggling to understand PRRS within their own systems,” says Tousignant. “They decided to come together and share data in a manner that will promote a greater understanding of the virus for all.”

Each week, the veterinarians report the PRRS status for all of herds. Tousignant and Morrison generate reports that are sent back to participants. The operations are not identified within the database, which will be used to provide answers for the entire pork industry.

The researchers use a statistical method known as Exponentially Weighted Moving Average (EWMA) to monitor the number of new infections each week (incidence) against a control limit they call an epidemic threshold.  “When the line is above this threshold, we might conclude we are experiencing a PRRS epidemic-- or the beginning of the PRRS season,” Tousignant says.  “Conversely, when the line drops below this threshold, we might conclude we have reached the end of the PRRS season.”

The researchers like the EWMA because it is sensitive to small increases in incidence, which help detect the beginning of an epidemic.  “It is possible to have some false alarms with this method and we set the threshold line high enough to minimize these events during the summer months where, by convention, we would not expect a PRRS epidemic,” Tousignant notes.

So far, the data shows that mid-October has tended to usher in “PRRS season.” Exactly why is not yet know, but there are numerous hypotheses from the corn crop’s natural bio-filter disappearing due to harvest to manure application to the perfect combination of environmental conditions for the virus and more.

“This project will be the first opportunity to investigate this on a large scale and we are now beginning to collect data on these variables,” Tousignant points out.  “This fall season, however, has been a bit of an anomaly for most of the country and this data will need to be collected for a few years before making a meaningful conclusion.”

For now, the data suggest a PRRS season that runs from mid-October/early November to mid-March “is eminent.” 

Worth noting is that there is a second brief epidemic in June/July that lasts only a few weeks. “I am a bit reluctant to speculate what might be occurring, but it is something I am working to understand and hope to have a better explanation for in the future,” Tousignant says. “It is intriguing how repeatable this mini-epidemic has been across the last three years of data.”