Air filtration systems are used successfully in hospitals and nursing homes to safeguard health, and now are on the verge of becoming a vital tool in the fight against porcine reproductive and respiratory syndrome (PRRS) virus.

Four years of research at the University of Minnesota’s Swine Disease Eradication Center (SDEC) farm at remote Holloway, MN, has made it clear to veteran PRRS researcher Scott Dee, DVM, and his team of Andrea Pitkin and Satoshi Otake, DVM, that air filtration systems hold the best chance of defeating the area spread of this decades-old disease nemesis.

Dee estimates he has devoted 40% of his professional veterinary career to stopping PRRS. He is so confident of the capabilities of air filtration in blocking the pesky virus that he offers this blunt assessment:

“With the information this technology has generated in our experimental research farm design, in conjunction with our growing body of evidence from the field, I believe it has the potential to change the industry as it pertains to the control of respiratory diseases of pigs.

“For example, if we can prove sustainability over a sufficient period of time, along with a healthy return on investment as current buildings are retrofitted or built new, they will be equipped with filtration. As our research progresses and new technologies come forward, the systems will become cheaper to install and operate. I believe lenders will see this intervention as an effective means to reduce risk and promote it as a good investment in disease prevention that will enhance profitability.”

Airborne Spread

In 1999, Dee left private practice at Morris, MN, to study PRRS virus transmission and biosecurity at the University of Minnesota. By 2005, he had become very frustrated. Despite research on flies, mosquitoes and transport, proving that a variety of fomites and vectors could spread the PRRS virus, hog farms were still becoming infected with the virus on a regular basis.
Meanwhile, the concept of winds carrying the tiny virus from farm to farm was being hotly debated. There were skeptics who didn’t believe in aerosol spread of the virus — Dee included.

“I didn’t believe it because I couldn’t reproduce it, and it wasn’t for a lack of trying,” he recalls.

Then Dee and former graduate student Jenny Cho, DVM, embarked on a project to determine whether the virus’ ability to infect pigs via airborne spread was strain-dependent. At the time, a recently emerged, highly prevalent strain of PRRS virus, MN-184, was seemingly wafting its way across the countryside and infecting hog farms at will.

In a project comparing the ability of aerosol spread of MN-184 and a less virulent strain, the research team recorded a significant difference in the frequency of virus shed and transmissibility via the airborne route in pigs infected with the MN-184 virus.

“We showed that pigs would get infected via the air with MN-184, and we couldn’t accomplish that with the milder strains,” Dee adds.

Research Model

To build on those findings, Dee hatched the idea of building a production region model to resemble the southern Minnesota hog-dense counties that have been plagued by PRRS virus reinfections.

Using small, modular buildings from Double LL Corp., paid for by PRRS CAP 1 funds provided by the U.S. Department of Agriculture, the goal was to evaluate various routes of PRRS virus transmission and biosecurity in Year 1; Pitkin led this effort. By Year 2, Mycoplasmal pneumonia was added to the mix as industry interest and funding for the project grew. Otake then joined the team to carry on Pitkin’s efforts.

In the model, Building 1 was established as the 300-head source population, to raise pigs from 50 lb. to market weight in a continuous-flow, mechanically ventilated barn where pigs were experimentally infected with PRRS and mycoplasma.

Building 2 served as the non-filtered control unit, matched in every way to the other test barns to represent “industry standards” for biosecurity — except it lacked air filtration.

Building 3 tested mechanical filtration and electrostatic filtration. Mechanical and electrostatic filters capture particles in their fibers, preventing them from moving forward.

Building 4 tested antimicrobial filtration. Antimicrobial filters coat particles that pass through the fabric of the filter with antimicrobial compounds to kill pathogens.

Buildings 2-4 housed pigs starting at 25-30 lb., 10 pigs/building.

The filtered barns were sited about 360 ft. downwind, southeast from Building 1 (source barn), in order to maximize exposure to the northwest prevailing winds, Dee explains.

Last year, two other PRRS strains (1-18-2 and 1-26-2), common to southern Minnesota and northern Iowa, were added to the challenge.

“We wanted to see what a mixture of viruses would do as far as airborne transmission goes; it’s common for farms to have a mixture of several PRRS strains within the finishing population,” he observes. “Using this model, we have demonstrated the ability of PRRS virus to travel out to 5.5 miles in the air.”

The test buildings were outfitted with filters to reduce the risk of airborne spread of PRRS and mycoplasma, including prefilters to screen out visible objects, including dirt, dust, bugs and other debris. A double-door entry chamber was installed to prevent introduction of outside “dirty” air.

Other Biosecurity Efforts

A biosecurity program is more than just installing filters. It includes taking other precautions, training and auditing personnel, and making sure other potential routes of infection are blocked, Dee says.

“Clearly, the question is not whether the filters work or whether the data behind their validation is accurate. It’s the implementation at the farm level that will make this procedure successful,” he adds.

When workers enter the research buildings, their skin, coveralls and boots are swabbed “to prove that people aren’t ‘walking’ the virus into the building,” he explains. All incoming supplies are fumigated, stored during downtime, and then swabbed prior to use. Personnel shower and work in the filtered, non-infected pig rooms first before moving into the infected control barn. The small buildings are run all-in, all-out.

A single truck picks up PRRS- and mycoplasma-negative weaner/grower pigs from an isolated, off-site area in northern Minnesota to be used in the trials. After each four-week replicate, buildings are emptied, washed and disinfected, and pigs are moved to the continuous-flow, source population finishing facility to maintain virus circulation in order to produce contaminated bioaerosols that will challenge the filtered buildings.

Air inlets are also screened to block insects. An exterminator visits monthly to keep out rodents. Although rodents don’t carry the PRRS virus, they could damage expensive filters and compromise filtration effectiveness. The feed truck only delivers hog feed to this research farm and otherwise delivers cattle feed. An on-farm incineration enclosure is used to dispose of mortalities. Manure pits are pumped out by a septic system service.

Co-investigators Pitkin and Otake, who both earned advanced degrees under Dee, have been very dedicated to developing and testing PRRS virus control and biosecurity protocols. Banks of security cameras on the farm encourage compliance.

The research farm is located 10 miles from the nearest hog farm. The owner of that farm monitors his pigs monthly, and they have remained negative for PRRS virus and mycoplasma during the four-year project.

Impact of Weather

After the first year of air filtration research, a weather station was installed to determine optimum climactic conditions for PRRS or mycoplasma transmission via the airborne route, he says. The station measures direction and speed of wind, levels of sunlight, barometric pressure, temperature and humidity.

“When there is an air-transmission event occurring in the non-filtered control facility, we can access data from the weather station and determine the meteorological events associated with this event,” Dee explains.

Conclusive data shows that spread occurs optimally on cool, cloudy, humid days when there are light, directional winds but with slight gusts, he says. The PRRS virus moves best when temperates range 30-45°F and humidity averages 80%, with 3-4 mph winds and gusts of 6-8 mph.

Dee says the common theory is that aerosols from pigs exit a building in a plume that moves slowly across the landscape. The virus survives well in low-light, cloudy and foggy conditions, but becomes dispersed on sunny, hot, windy days.

Climactic risk factors are described in Table 1.

Filtration Results
The last pigs in the air filtration trials left the research farm in early November, so the final results are being tabulated, Dee reports.

In four years of research at the site, Dee estimates there have been 1,450 days of data collection on over 4,800 pigs with 36,000-plus PCR (polymerase chain reaction) samples collected, including air, personnel, fomites, transport and insects, which were all tested for PRRS and mycoplasma.

But the evidence is clear that with all filter types used, not a single case of PRRS virus or mycoplasma infection resulted from any of the filtered facilities, he affirms.

Sow Farm Research

Further support for air filtration is provided by work with southern Minnesota veterinarians Paul Ruen at Fairmont, Gordon Spronk at Pipestone and Darwin Reicks at St. Peter. The project is funded by the USDA PRRS CAP 2, the National Pork Board and the Minnesota Pork Board.

“These clinics have been the national leaders in the application of filtration, first to boar studs and now to sow farms in southern Minnesota and northern Iowa,” Dee says. These boar studs have remained PRRS-free for almost five years and now the focus has turned to the sow farm.

In the third year of a four-year study, 10 sow farms are filtered and 26 farms are non-filtered control herds, matched in the frequency of outbreaks, the size and the density with the surrounding area.

Interestingly, during the study period, 92% of the non-filtered sow farms have broken with PRRS, including half that broke with the virus several times, he notes.

“We have learned that while very effective at aiding in the prevention of airborne infection, air filtration is not magic. It cannot stop PRRS virus infection due to other routes, such as contaminated transport, nor can it prevent the non-compliance of personnel,” Dee adds.

Cost Factors

The knock on air filtration systems for hog barns has been the big cost, Dee admits. When amortized over five years, the cost averages about $2/pig. Well-publicized data shows PRRS breaks cost in the range of $5-15/pig, not to mention the emotional toll of dealing with high mortality rates.

“As we accumulate more data, there is a growing interest in adapting technology of air filtration to farms, especially those in swine-dense regions with a history of annual breaks and the high-level frustration that goes with it,” he points out. “We have been making regular visits to the filtered farms in the study, asking them how it is going, and if the new level of biosecurity has caused them any problems. Basically, they are so happy not to have PRRS and are willing to work very hard to keep a break from happening again.”

Filters cost $125 apiece, prefilters $3-4 or more each.

The disease-free cull gilts or barrows used in the trials were provided at no charge by Genetiporc.

Results are Promising

A commitment to shutting down PRRS transmission can succeed using air filtration, provided biosecurity is solid, according to results at the Minnesota swine research farm that are being validated in the field.

“Filtration works. There is no doubt about this. We have proven that here at the farm and in the field under very challenging conditions,” Dee says. “While we still need more time to truly assess its value, the preliminary results are very exciting.

“It’s the implementation that is the current challenge; however, we feel that with proper training and oversight of highly motivated farm personnel, this tool will enhance the long-term success of area/regional control and elimination programs for this economically significant disease, as well as others that travel between farms via the airborne route.”