University of Minnesota researchers tested a trailer-drying technique known as thermal-assisted drying and decontamination. (Note the gold-colored unit at the back of the trailer.)

Researchers continue to move closer to finding all of the pieces to solve the porcine reproductive and respiratory syndrome virus-transmission puzzle.

Minnesota PRRS researcher, Scott Dee, says during the last several years, researchers have thoroughly studied suspected routes of PRRS virus transmission and either proved or disproved them.

There are more practical methods that can be implemented on farms to help prevent or minimize the risk of virus spread that have been scientifically proven to date, Dee says.

“As far as PRRS virus transmission and biosecurity, we’ve gone step by step in our research,” says Dee, a swine veterinarian with the Swine Disease Eradication Center at the University of Minnesota College of Veterinary Medicine.

Pig transportation is one recent risk area where researchers have provided answers and resulting protocols have been implemented. “The science has provided many good options for producers that basically can take the transport vehicle out of the question,” he says.

“If PRRS virus moves from site to site and you know you are handling your trailers correctly, and you have validation that procedures were done correctly, then you can cross that off when trying to figure out how the virus was transmitted.”

Dee’s research has included several specific projects that looked at transportation as a risk factor for spreading the virus to PRRS-negative or naive pigs, and how to clean up contaminated trucks and trailers to prevent its spread. Much of the research was funded by the National Pork Board, with some additional support provided by PIC, DuPont and Preserve International.

Take-home messages

Dee says the transport research has provided two important take-home messages:

1. Contaminated pig-transportation vehicles can be a source of PRRS virus spread.

2.  Practical options are available to producers to eliminate or greatly reduce the risk of PRRS virus spread by such vehicles.

Once researchers determined that transport vehicles can play a definite role in PRRS virus spread, the next goal was to find options to reduce this risk, Dee says.

"'Options' is an important word because we realized that not every program works for everyone, so we wanted to come up with some different alternatives for producers,” he explains.

Dee and his group previously studied different disinfectants and drying techniques in small, scale-model trailers. “We showed under those conditions that certain disinfectants, such as Synergize (Preserve International) or Virkon (DuPont), were very effective against PRRS virus in contaminated trailer models, and that drying was very effective at reducing pigs’ risk of picking up the virus.

“We realized the models were limited, so we wanted to see if the results would be similar in real-world conditions. So we conducted a new round of research in a full-sized, double-deck livestock trailer.”

Virkon and Synergize were tested in the full-sized trailer. Researchers also tested a trailer-drying technique known as thermal-assisted drying and decontamination (TADD). PIC actually developed the TADD system as a response to the previous research involving the scale-model trailers. That research showed if you let the trailers air dry for eight hours overnight, PRRS virus could be eliminated.

Dee says, “Everybody told us eight hours was too long for a real trailer to sit. So PIC came up with 1-million-btu-per-hour heaters. These are portable heaters on wheels that have big pipes that run the length of the trailers (see photos). They blow warm air at high velocities down the length of the trailer to expedite the drying process, rather than just let it air dry.”

The researchers used a modified live vaccine virus to simulate real PRRS virus at 15 different sites in the trailer. They then treated the trailer with one of the two disinfection programs or the TADD system.

“We came back after two hours, which is the timeline the industry told us, and we swabbed the sites again to see if there was any virus. We tested it before treatment and we tested it after treatment.”

The two disinfectants and the TADD system all worked about the same in terms of eliminating PRRS virus from the trailers, according to Dee. “They were all highly successful.”

A key point about the disinfectants was that they were applied with equipment that bubbles the disinfectant into a foam.

 “The foamers are a fairly common item on farms, but no one had looked at them in this situation,” he explains. “By foaming the disinfectant, you not only can see where you’ve been, but the foam also acts as a surfactant and helps adhere the disinfectant to the surface for a longer time.”

Researchers also tested control trailers that had been inoculated with PRRS virus. They let those trailers sit for two hooours without any treatment.  “As we expected, those trailers still contained live vaccine virus,” Dee says.

He now believes that the transportation part of the PRRS virus-transmission puzzle has been solved. Bob Thompson, health-assurance veterinarian with PIC, agrees and points out that TADD technology has been increasingly implemented throughout the pork industry.

“We’ve seen trucks and trailers as a route of transmission and we’ve eliminated that,” Dee says. “We’ve seen fomites as a route and we’ve figured out ways to clean them up. We know that the personnel risk for transmission is boots, coveralls and hands. Basic things can be done there to clean that up. We know insects can carry it short distances in the summer, and we know how to block them with screens and insect-control programs.”

One aspect of PRRS virus transmission where the jury is still out is airborne transmission. Dee and others are concentrating much of their current research efforts in that area and are hoping to provide producers some additional answers in the near future.


In the past, there has been disagreement in scientific circles about whether aerosol transmission was a significant factor in spreading the porcine reproductive and respiratory syndrome virus. While the answers still elude researchers, there is increasing agreement that it does play a role.

“We’ve been doing a lot of work on aerosol transmission and we have identified a risk factor, which is what we call isolate pathogenicity,” says Scott Dee, a University of Minnesota swine veterinarian and PRRS researcher.

“Some PRRS virus strains are very virulent; some are not,” he explains. “We’ve studied some virulent viruses that cause more disease and cause higher levels of virus in tissues and blood. Those strains are shed through the animal's breath at a higher frequency, and they are more contagious via aerosols than some strains of lower virulence. This has been an awakening for me on the aerosol issue.”

Partly as a result of this discovery, Dee’s research group has been focusing much of its current efforts on evaluating air filtration. They are considering whether filtering air coming into swine barns can reduce the risk of virus-contaminated aerosols.

“It’s still somewhat preliminary but I think we understand now that isolate pathogenicity is a risk factor for aerosol spread and that air filtration is probably going to be the last important biosecurity intervention that producers will need.”

Dee says he hopes to have some concrete answers for the pork industry later this year.