When porcine reproductive and respiratory syndrome surfaced in a dozen or so boar studs across southern Minnesota and parts of Iowa this past winter, things certainly heated up.

The hot question: "How could the virus find its way into the biosecure world of these boar studs?"
Certainly PRRS virus transmission has kept researchers, veterinarians and producers scratching their heads for years. Pig to pig and infected boar semen are known transmission routes, but beyond that few concrete answers exist.

"We have to look outside the box in terms of these viruses, and the possible transmission roles that non-porcine vectors and fomites play," says Scott Dee, swine veterinarian and PRRS researcher at the University of Minnesota.

Indeed, that's what a team of University of Minnesota Swine Disease Eradication Center researchers set out to do. Directed by Carlos Pijoan, professor of swine medicine, the Center's quick response was based on input from an advisory board of pork industry representatives. Clinical disease signs among herds ranged from mild to severe, with extreme cases seeing high mortality throughout the farm. The outbreaks occurred in PRRS-positive farms as well as negative farms.

Based on laboratory diagnostics, "it appears that a few cases were caused by a virus that was different than what had previously been seen across the state," notes Dee.

Some of the spread had been traced to contaminated semen from affected boar studs, but there was more to the fast-paced spread. "There seemed to be other ways that didn't have as clean a connection," says Dee.

In general, PRRS cases increase in the fall and winter because the virus thrives in cool, wet environments. This year's warm Midwest winter with its freeze-and-thaw cycle offered a perfect setting.

Dee set up a study to look at the mechanical transmission of the PRRS virus through the course of a working day (12 hours). He wanted to look at common events and behavior patterns.

That focus centered on snow and truck washing. Dee inoculated snowballs with the PRRS virus, attached them to a vehicle's wheel-well and drove 30 miles to a truck wash. He washed the vehicle and then drove to a simulated farm. There, he delivered containers including a Styrofoam semen cooler, toolbox, plastic pail and cardboard box.

"My concern was that in practice, I had seen a lot of containers that enter into farms...they set on the floor, get wet and dirty and then are passed through the office that is supposedly the clean area," notes Dee. "We'd proven previously that if you get the virus on your hands, it's live and infectious and you can infect pigs."

He replicated the test 10 times – eight of those times the PRRS virus made it on to the "farm." One time the virus was found on the truck-wash floor, but didn't make it into the vehicle. The other time, it was in the vehicle, but didn't enter the "farm."

The virus had remarkable survivability throughout the 12-hour day as well.

Dee intends to replicate the study with PRRS-inoculated mud balls to see if that cool, damp medium presents a concern as well.

"The good news is, I think it's pretty easy to plug these holes without a lot of cost," he adds. Here are a few basic tips:

  • It starts at the truck wash. "Everyone goes there, and they think they're cleaning the exterior, but they may be spreading virus around and inadvertently dragging it into vehicles," he notes.
    Wearing disposable boots as you wash the vehicle, then removing them before entering the cab (without touching your shoes to the wet floor) is a start.
  • Review how you handle packages and equipment coming into the farm. Can they be disinfected, or double boxed? Perhaps you need to redesign your entry point to the farm or office.
  • What biosecurity practices does the delivery person practice? This is especially important if he or she goes from farm to farm.
  • Keep the entry-room floor clean. Boots that come in should not simply be left on the floor where ice and snow will melt and pool, creating a virus haven.
  • Emphasize the need for everyone to wash his/her hands thoroughly. This means with hot water, soap and for more than a few seconds.

Dee intends to look at other commonsense steps that can break the cycle.

Exactly how the PRRS virus spread through the upper Midwest this winter is not fully proven, but Dee's study sheds a new light on possible virus transmission.

"I used to think once the virus was out of the host, it was going to die immediately and pose no risk to susceptible herds. Now, I don't think that's true," says Dee. "It appears if the virus is allowed to remain moist and cold, it's pretty easy to drag around."

Other Modes of Transmission
Pig-to-pig transmission is of course the most common way to spread disease in a herd. In terms of porcine reproductive and respiratory syndrome, boar semen offers another vehicle. Beyond that, there remain a lot of unknowns.

Here Satoshi Otake, veterinarian and PRRS researcher at the University of Minnesota, offers additional insights into non-hog vectors based on recent research findings.

  • Contaminated needles can transmit PRRS from infected to non-infected pigs during vaccination and medical treatments. "Producers should change needles between sows, litters and pens of growing pigs," says Otake.
  • Fomites (such as coveralls, boots) and people, following direct contact with PRRS-infected pigs can transfer the virus to other pigs. Research shows that changing coveralls and boots as well as washing hands between pig groups that differ in PRRS status on-site or between farms can reduce or prevent virus spread.
  • Aerosol spread of PRRS remains a controversial and debatable issue. "Published data indicate that PRRS spread can only occur over short distances (0.46 to 1 meter)," notes Otake. "Aerosol transmission of PRRS between farms seems to be an infrequent event."

    Carlos Pijoan, veterinarian and director of the University of Minnesota's Swine Disease Eradication Center agrees. "Aerosol transmission would need a lot of animals shedding the virus at the same time," he notes. "In reality we see periodic and staggered shedding."

  • Mosquitoes are raising questions, and Otake, Scott Dee, Pijoan and other researchers are investigating this prospect. So far, they've found that PRRS virus can be transferred from infected pigs to mosquitoes. They've also found that mosquitoes can serve as mechanical vectors of the virus, but more research is needed to determine whether they are able to spread the virus throughout a pork producing area.

    Otake points to other areas that need future investigation, including insects such as houseflies, the bird/insect link, vehicles and water.