Air filtration systems get high marks for keeping porcine reproductive and respiratory syndrome (PRRS) virus out of farms.

Controlling aerosol spread of PRRS virus seemed nearly impossible a few years back, before the advent of air filtration systems for hog barns.

Boar stud expert Darwin Reicks, DVM, Swine Vet Center at St. Peter, MN, addressing a packed crowd at the Leman Swine Conference in St. Paul, MN, in mid-September, provided evidence that the use of air filtration has been a resounding success.

In 2½ years of experience with various types of air filtration systems on 15 client farms in hog-dense areas of south central Minnesota, just three farms have had PRRS breaks, he reports. Those three breaks occurred during the summer when the farms were not filtering outside air at all, relying instead on cool cell systems for ventilation. About 75% of the filtered farms had a prior history of PRRS breaks.

“We have not had any PRRS breaks on the sites during 100% filtration, nor have there been any breaks on the two bailout sites (discussed below). Of the 15 farms that are filtered, 13 are at boar studs, one is at a research farm and one is on a sow farm. Two more sow farms and a boar stud are installing filters this fall.

“Air filtration seems to be the technology that will be necessary for regional eradication,” stresses Reicks.

However, only a handful of farms have adopted the technology because of its high cost, he says. He estimates a PRRS outbreak costs an operation about $200/sow, virtually the same initial cost to install an air filtration system on a hog farm using 95 DOP filters.

Those filters provide 95% efficiency at screening out virus particles that are 0.3 microns in size. The smaller PRRS virus adheres to the particles of the filter as it attempts to pass through the openings in a process called interception.

Replacing DOP filters every two years adds an ongoing cost of $30-$40/sow/year. DOP filters are preferred because they are about half the cost of HEPA filters, and there is much less air restriction.

The cost of filtration can be balanced against the threat of a PRRS break, which increases when hog units are built in pig-dense areas, says Reicks.

There have been two breaks of swine influenza virus (SIV) in 100% filtered farms in the last three years. One of those breaks occurred last fall when the virus apparently survived the boar isolation period, according to diagnostic test results. The source of the other flu break was unknown.

Filtration Challenges

One of the main challenges with air filtration has been the restriction of the filter on airflow at the low static pressures typically operated in hog barns, says Reicks.

“The typical fans used to generate negative pressure are not very effective at pulling against the large static pressure created by the restriction of the filter,” he adds.

Implementation of a partial filtration system is simple and very low cost, says Reicks. “Many sites simply put a filter above each of the ceiling inlets (pictured on page 13). If one filter was placed above the inlet, the airflow is adequate until the outside temperature reaches 60-65° F.” For farms that initially just put filters on the ceiling inlets, they need to allow air to come into the barns unfiltered as the temperature heats up in the summer.” The problem is that the virus could easily travel 1-2 miles at that temperature, and without air filtration, the farm could become infected.

At higher temperatures, air filtration simply produces too much restriction, and producers must either remove the filters or utilize air coming through a cool cell in tunnel ventilation mode (summertime ventilation).

“One of the challenges with this system is that during the late spring and early fall in the Midwest, the temperature often gets hot enough during the day where tunnel mode is necessary. The problem is at night when the temperature drops down to 50-60°F when survivability of the PRRS virus is favorable,” observes Reicks. If the curtains don't go up and seal tight you have a big opportunity for virus to enter.

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