Paul Yeske, DVM, St. Peter, Minn.
Paul Yeske, DVM, St. Peter, Minn.

It’s clear that the porcine reproductive and respiratory syndrome virus is a formidable opponent. For nearly three decades the virus has evolved, retreated, resurfaced and, some might say, outsmarted attempts to gain control over it.

When it’s you staring at ailing sows and pigs, counting up the losses and trying to figure out how to put out the fire, it certainly seems like there are more questions than answers. Still, much progress has been made, and more will come.

Prevention and control techniques are among the things that are clearer today. While you can encounter a PRRS break most anytime, the virus definitely favors a seasonal pattern. University of Minnesota researchers Bob Morrison, DVM, and Steve Tousignant, DVM, and PhD student, are collecting and evaluating PRRS incidence data to determine when cases surface and how long they persist. It’s no secret that the PRRS virus favors cool temperatures and damp conditions, but the researchers found that late October/early November repeatedly ushered in “PRRS season,” which runs into March.

A surprise, however, is that another blip surfaces in late June/early July. Exactly why is not yet known, but having the information is the first step.

So knowing when PRRS virus exposure is likely to occur can help you batten down the hatches, so to speak.

Where do you start?

With hogs being the greatest source of PRRS virus spread, it’s important to know the status of neighboring herds. While it can be touchy to approach fellow pork producers about PRRS status, some of those obstacles are receding as more regional PRRS elimination projects (now more than 20) get underway. Paul Yeske, DVM, Swine Vet Center, St. Peter, Minn., suggests talking to producers with herds up to 5 miles around your site. “Understand the PRRS status of the pigs, what is being done for virus control and the biosecurity program — if they will share that information,” he says.

The PRRS virus travels well via air (as much as 7 miles), as well as on muck and slush. “Cooler weather and lower amounts of daylight may account for virus being able to survive better in the environment, allowing for greater herd-to-herd spread,” Yeske says. With fall becoming analogous with PRRS season, now is the time to brush up on biosecurity practices.

Increasingly, veterinarians point to PADRAP — or Production Animal Disease Risk Assessment Program — site assessments as a “must do,” especially in hog-dense areas. Conducted through a veterinarian, the process evaluates your production system for risk factors associated with PRRS virus exposure; it flags concerns and provides insight into needed changes. The American Association of Swine Practitioners manages the program, and while information is confidential by site, PADRAP also provides information to an industry database to identify answers for everyone. (For more, go to

Once you have a PADRAP assessment, periodic updates allow you to spot breakdowns or changes that have occurred within your system. If PADRAP is not within your fall schedule, have the herd veterinarian walk through the barns and review protocols. “This is a good time to retrain the existing staff and any new employees to make sure everyone is up to speed with procedures,” Yeske says. Some areas he says might need re-emphasizing include:

  • Steps for people — employees and visitors — entering the site and the procedures associated with the clean/dirty lines; also showering, hand washing, boot cleaning and garment changes.  
  • Product entry into the site, which includes double-packaging medications, vaccinations, boar semen and equipment. Also disinfection steps when necessary for tools and materials entering a facility.
  • Breeding stock isolation and the daily chore plan for those animals.
  • Procedures associated with the sale and transport of animals, including vehicle flow.
  • Movement and disposal of mortalities. Also garbage disposal.

Tousignant says that within the farrowing room, workers need to pay attention to needle use and frequency of changing needles as they can spread the PRRS virus. There also needs to be restrictions on the order of performing chores, moving from the youngest to older animals and never retracing.

Fall is naturally the time to check ventilation systems because facilities will be “closed up,” but PRRS underscores the importance of this task. Evaluate ventilation controls — set points, bandwidths and dead bands — for accuracy. Check curtain-sided and tunnel-ventilated barns for leaks and holes. “Inlet set up in a tunnel barn can make a difference in barn temperature,” Yeske notes. While some leakage around tunnel curtains occurs regardless, closing some ceiling inlets at the tunnel end of the barn will limit cool air concentration there.   

You may need to adjust temperature controllers to ensure that all animals are at the right temperature — not too cold at one end and too warm at the other.  “Temperature management is always tricky in the fall when we have warm days and cool nights,” Tousignant notes, “but a little effort to ensure animal comfort and minime temperature stress can go a long way in disease prevention.”

If your barn is filtered, now is a good time to conduct audits and determine what maintenance needs to be done, he adds.

See that the wire netting to keep birds out is intact. Trim and remove weeds around barns, and set up fresh rodent bait stations.  

Fall manure pumping and application have been suspected in PRRS transmission. “We know that virus can survive in manure,” Yeske says. “There is much debate whether it’s a risk to have a neighbor pumping manure next to your farm.”

If you use a custom manure applicator, work to establish a pumping order, starting with the highest health herds to the lowest. But even the best plans are likely to change, which is why it’s important to maintain open communications with the applicator and area producers. In the end, about the only way to ensure that application equipment doesn’t cross between hog farms is to have your own. 

Finally, as foliage drops in the fall it allows for greater air and virus movement. Corn fields no longer serve as natural bio-filters. Yeske says planting evergreens as windbreaks may filter out PRRS virus in the winter, as well as provide energy savings, improve snow management and offer public eye appeal. 

“Even with the best risk assessment, procedures and audits, there will be PRRS breaks,” Yeske notes. “The goal must be to reduce the incidence and ensure everyone understands the expectations.” By understanding risk factors and patterns that contribute to PRRS outbreaks, you can better identify solutions and procedures to be ready for PRRS season.

Following a Pattern

There is plenty of real-world experience with porcine reproductive and respiratory syndrome. It’s understood that cool temperatures and wet conditions increase the exposure risk. But exactly when the virus begins to take hold, for how long and when infections might trail off called for a more precise investigation.

For more than two years, Bob Morrison, DVM, and Steve Tousignant, DVM, PhD student, in veterinary population medicine at the University of Minnesota, have developed a database to study the incidence and pattern of PRRS cases. “The effort was born out of the frustration of veterinarians struggling to understand PRRS within their systems,” Tousignant relays.

The database will soon have 12 participating systems located in 13 states, which include more than 300 sow herds, for a total of nearly 1,000,000 sows. Each week the veterinarians report PRRS status for all herds, which the researchers use to generate reports back to the group. “We also have PRRS status dating back to July 2009, from which we are conducting an extensive retrospective analysis,” Tousignant says.

The researchers use a statistical method known as Exponentially Weighted Moving Average to monitor the number of new infections each week against a control limit they call an epidemic threshold. So, when the line rises above this threshold, it suggests a PRRS epidemic — or the beginning of PRRS season, Tousignant points out. Conversely, when the line drops below the threshold, it suggests the end of PRRS season.

Morrison and Tousignant researchers like the EWMA because it’s sensitive to small increases in incidence, which allows for quick detection of an epidemic. 

The chart shows that mid-October typically begins the epidemic — or PRRS season — which tends to run into mid-March. You will notice that there tends to be a brief hiccup in late June/early July. The researchers are not ready to speculate why that summer break might occur. “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.”

It’s important to note that the herds were not randomly selected for the project; rather they volunteered, which could present some unknown bias. “It should be noted, however, that the data for each system is strikingly similar, and each new system that joins the project shows the same patterns as the others,” Tousignant says.

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