Preliminary research at South Dakota State University (SDSU) demonstrates that a biofilter can be an effective means of reducing airborne transfer of porcine reproductive and respiratory syndrome (PRRS) virus on production sites that contain more than one barn.

A growing number of pork producers are installing air filtration systems to block the passage of the PRRS virus into their hog barns.

But the cost of such systems has raised questions about whether alternative methods of filtration are available.

Since biofilters are considerably less expensive than air filtration systems and are being installed on swine barns to reduce odor emissions, the SDSU researcher set out to learn if biofilters can stop PRRS virus airborne transmission coming from the barn exhaust.

The swine barn at the Southeast Ag Experiment Station near Beresford, SD, features a vertical biofilter treating the exhaust air. A wood frame that supports a plastic tarp was built around the biofilter to funnel the air leaving the biofilter to a hole near the top (Figure 1).

The source of PRRS virus aerosol consisted of a cold fog mister filled with 6.3 pints of a PRRS modified-live virus vaccine (Ingelvac PRRS MLV supplied by Boehringer Ingelheim Vetmedica, Inc.). The fogger nozzle was set to apply the content directly into the ventilation exhaust fan.

A liquid cyclonic collector was used to collect the virus from the air and convert it into a liquid. Samples were analyzed for virus determination at South Dakota State University Animal Disease Research and Diagnostic Laboratory by polymerase chain reaction.

The air sampler was located in the biofilter inlet duct, about 5 ft. from the fogger, to determine the virus concentration entering the biofilter and also in the biofilter outlet. Another sampler was located at the biofilter outlet.

At the start of the trial, pigs were PRRS-negative and no virus was passing through the biofilter.

Three trials were conducted. The first trial involved a standard dose of a PRRS modified-live virus vaccine without any dilution that would be used routinely for intramuscular vaccination of pigs.

The second and third trials involved a PRRS virus dose from a PRRS modified-live-virus vaccine that simulated a more approximate level of virus that might be obtained from aerosolization of PRRS virus from infected pigs.

Five samples were obtained in each of the trials using the following sampling methods:

  • The first sample consisted of a background sample of air that had passed through the biofilter before introducing the virus. This sample was used to verify the barn and equipment were negative for the PRRS virus.

  • For the second and third samples, the virus was introduced through the fogger for 5 minutes and then the cyclonic collector ran for an additional 10 minutes.

  • The fourth sample collected virus from the air entering the biofilter. This sample was used to verify that the vaccine became aerosolized and that there was virus entering the biofilter.

  • A fifth sample was taken for 15 minutes at the biofilter outlet, one hour after the virus was added to the exhaust airstream. This sample was taken to pinpoint if any virus remained in the biofilter.

During the third trial, as the PRRS virus was sent through the biofilter, sampling was conducted simultaneously at the biofilter air inlet and outlet.

In this trial, five runs were conducted. No PRRS virus was introduced during the first run. During the second through the fifth runs, PRRS virus was introduced for 7.7 minutes with a fogger at the wall ventilation fan and two sets of samples were taken for 15 minutes at the inlet and outlet.

The results summarized in Table 1 demonstrate that when the biofilter was challenged with a PRRS virus concentration equal to a dose of PRRS modified-live-virus vaccine, some PRRS-positive virus passed through the biofilter.

However, when the concentration of virus was reduced to a level that more closely corresponds to actual concentrations in the exhaust air that have been measured from swine barns, the biofilter exhaust air tested negative for PRRS virus. But in both cases, virus isolation results were PRRS-virus negative, indicating that only viral nucleic acid was detectable and not live virus.

The results demonstrate that a biofilter reduces the PRRS virus concentration. But further studies are warranted to find out if microbial activity plays a role in this reduction or whether the virus is being filtered by a mechanical barrier.

Researchers: Dick Nicolai, Eric Nelson, Jane Christopher-Hennings, Bob Thaler and Steve Pohl, all of South Dakota State University. Contact Nicolai by phone (605) 688-5663, fax (605) 688-6764 or e-mail