USDA researchers have confirmed that the porcine reproductive and respiratory syndrome (PRRS) virus moves by aerosol. Now the challenge is to develop an air filtration system to slow its spread.

No one quite knows for sure why or how the PRRS virus infects one farm and not another.

Work by researchers Scott Dee, DVM (University of Minnesota) and Jeff Zimmerman, DVM (Iowa State University), members of the USDA's National Research Institute (NRI) PRRS Integrated Program Project, suggest that the ability of the virus to spread by aerosol may be related to its replication rate and virulence in pigs.

In their studies, the researchers compared highly virulent (MN-184) and mild or avirulent (MN-30100) strains of PRRS virus and found significant differences in their ability to shed and transmit virus via aerosol.

For example, aerosol spread of the highly pathogenic MN-184 strain was thought to be involved in PRRS outbreaks affecting a number of boar studs in Minnesota and surrounding states a few years ago.

Research suggests that the more virulent a PRRS virus is, the more frequently it is shed in aerosols of infected pigs, and may be a good candidate for aerosol transmission.

Boar studs are looking for evidence that an air filtration system would protect against exposure to the PRRS virus and provide health insurance for commercial farms downstream, says Dee.

French Air Filtration Systems

Pursuing the issue further, Dee is conducting a series of trials on a French air filtration system used to ward off PRRS spread.

Last year, Dee toured Brittany, France, where hog farms are smaller, but the region is similar in density to southern Minnesota and Iowa. French farms stocked with PRRS-negative animals repeatedly experienced disease breaks, he says.

“These breaks were happening over and over again, and a number of these producers couldn't afford to keep depopulating and repopulating their herds. So they began working with two companies, IMV, Inc. and Fancom, that set up air filtration systems,” says Dee.

Operations were breaking with the PRRS virus every three to five months. But after installing the air filtration systems, many remained PRRS negative three to five years later.

The French model is a three-stage, positive-pressure ventilation system with three sets of screens that set up against a hog barn. The first stage, or pre-filter screen, prevents bugs and mechanical debris from entering. It's similar to a porch screen. The second screen is a bag filter. The third is a High Efficiency Particulate Air (HEPA) filter, which filters out particles small enough to exclude the PRRS virus, explains Dee.

A large turbine fan between the second and third filters pushes air through the HEPA filter. The air is pushed through holes into the barn ceiling, and down onto the pigs, before being exhausted through the outlets.

This process not only eliminates the PRRS virus, it also reduces dust and creates a very nice environment for people and pigs, says Dee.

Scale Model Tests

“I wanted to test this air filtration concept because there really hasn't been any scientific assessment of whether this French system works,” observes Dee.

The concept hinges on the fact that the PRRS virus is 0.5-0.6 microns in size and the HEPA filter screens out particles 0.3 microns in size and larger.

The French provided Dee with a small scale model of the air filtration system. He built two plastic pig boxes, connecting them with ductwork and the air filtration scale model.

In the first trial (Figure 1), five experimentally infected pigs exposed to a highly pathogenic strain of PRRS known to shed by aerosol were placed in the first chamber. One negative pig was placed in the second chamber. A control model used the same setup but without the air filtration system.

Infected pigs had been exposed to the PRRS virus for six hours. The air filtration system protected the negative pig from infection in all 20 attempts. In six of 20 replications in the control group without air filtration, exposure infected the negative pig (Figure 2).

To provide a stiffer virus challenge, in the second trial Dee took 2 pints of solution containing PRRS virus and loaded it into a mister to create a “virus fog” that was pulled through the air filtration system (Figure 3). The trial was repeated 10 times with zero rate of infection. In the control group, nine out of 10 pigs were infected when exposed to the fog.

A low-cost filtration trial was also run. Three screens included a pre-filter similar to a porch screen, a fiberglass furnace filter screen and a higher-quality, electrostatic filter. The same challenge as Trial 2 was run and the filters failed four out of 10 times, resulting in infection of the negative pig.

As a result, in the next test Dee plans to double the three filters used in the low-cost trial, to see if results improve.

Primary sources of funding for Dee's air filtration trials include Boehringer Ingelheim Vetmedica Inc. and the USDA-NRI PRRS Integrated Program Project.

Future Studies

Dee and Zimmerman will be submitting a proposal to the PRRS Integrated Program Project for a one-year study to evaluate air filtration technology on a larger scale at the Swine Disease Eradication Center's research farm in western Minnesota.

Equipment for collecting air samples and assessing them for the presence of PRRS virus, the impact of different weather conditions that may play a role in aerosol transmission, and the efficacy of air filtration are objectives to be tested, says Dee.

Iowa Boar Stud Builds Air Filtration Unit

Iowa Select Farms based in Iowa Falls, IA, has installed a HEPA filter system with a series of two prefilters at its north boar stud.

Staff veterinarian Andy Holtcamp reports that stud, which is located less than a mile from busy Interstate 35, has had numerous porcine reproductive and respiratory syndrome (PRRS) breaks. Iowa Select's southern boar stud has never had a PRRS break in its eight-year history.

The 350-head north boar stud has broken with PRRS every two years between November and February, even though there have been no animal introductions around the PRRS breaks, reports boar stud supervisor Brian Qualley. The stud has been depopulated several times.

“We couldn't really point our finger at any breaches in biosecurity or animal movements. No animals had been removed or entered for six months prior to one of the PRRS breaks, and there hadn't been any new personnel hired that would have been unaware of the biosecurity protocols in place,” stresses Holtcamp.

The air filtration system was installed in November-December 2004. There have been no PRRS breaks or other major health issues at the stud since that time, says Qualley.

Qualley says Iowa Select installed a three-bank filtration system that includes 115 pre-filters, 115 intermediate filters and 115 final filters. The filters are all 24 × 24 in.

“The final filters are rated 99.99% efficient and will filter particles down to 0.3 microns,” Qualley explains.

To accommodate the filter bank, Iowa Select workers added 30 ft. to the north boar stud, explains Qualley. One end of the barn has a bank of fans that pushes the air through the filters. On the opposite end are three chimney exhaust fans.

Also, all existing fans were removed and sealed to create a positive-pressure ventilation system.

Qualley adds that since the new system was installed, the building environment has improved. “The environment was better this past winter since we moved more air, keeping the slats drier,” he says.