Strengths and limitations of air filtration

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Headlines this summer carried news of porcine epidemic diarrhea virus, but porcine reproductive and respiratory syndrome virus (PRRSV) continues to plague the industry. Extensive efforts have been made to eliminate PRRSV from infected populations, but re-infection due to airborne spread of the virus is a frequent occurrence.

click image to zoomPipestone Veterinary ClinicCourtesy Pipestone Veterinary Clinic Evidence of long-distance airborne transport of PRRSV has been documented, as far out as 5.65 miles. Dr. Scott Dee, director of research at the Pipestone Veterinary Clinic in Pipestone, Minn., headed up a four-year study period to test this theory under experimental conditions. In this model of a swine production region, airborne transmission of PRRSV to susceptible populations housed in filtered facilities was prevented 100 percent of the time.

As a result, air filtration systems have been used in pig facilities as a means of reducing this risk in commercial pork production systems. Although pilot studies had previously generated promising results, they were limited by both sample size and duration of the study period. Therefore, Dee’s long-term sustainability study was intended to measure the effect of air filtration on reducing the risk of PRRSV infection in a large number of herds over an extended period of time.

Large herds studied
Herds in the study were required to have 2,400 sows or more. In addition, they needed to be surrounded by at least four growing-pig sites within a radius of three miles, and candidate herds couldn’t supply pigs to any of the surrounding sties. Finally, herds selected for the study had to have experienced a minimum of three external PRRSV infections over the previous four years despite using standard biosecurity practices.

Treatment herds used validated air filtration technologies (Camfill-Farr, Clarcor) known to be comparable with negative-pressure ventilation systems. Following installation of the filters, PRRSV status was monitored across all the herds on a monthly basis. During the visits, herds were assessed for clinical evidence of PRRS and production data were reviewed. In addition, blood samples were collected.

Analysis
The effect of air filtration was analyzed at three levels, including:

  1. The likelihood of infection in contemporary filtered and non-filtered herds.
  2. Likelihood of infection in equivalent periods of time before and after the implementation of air filtration.
  3. Time to failure (new PRRSV infection) in filtered and non-filtered herds.

Results
Throughout the study period (September 2008 to mid-January 2012), eight new PRRSV introductions occurred in the filtered herds, while 89 new introductions occurred in the non-filtered herds. According to the study, all infected herds exhibited clinical signs of PRRSV. Regarding the three levels researched in the study, results were:

  1. New PRRSV infections in filtered breeding herds were significantly lower than in contemporary non-filtered control herds. The odds for a new PRRSV infection in a non-filtered breeding herd were 8.03 times higher than the odds in a filtered breeding herd.
  2. The implementation of air filtration significantly reduced the occurrence of new PRRSV infections in breeding herds. The odds for a new PRRSV infection in breeding herds before air filtration was 7.97 times higher than the odds after air filtration was initiated.
  3. The proportion of air filtered breeding herds detecting new PRRSV infections was significantly lower than in non-filtered breeding herds during the study.

“Control and eradication strategies for PRRS are crippled by our inability to prevent the airborne spread of the virus between susceptible populations,” writes Dee. “It is critical to properly evaluate available strategies to reduce risk. The results obtained from the current analysis at three different levels are consistent and clearly indicate that under the conditions employed, air filtration significantly reduced the occurrence of new PRRSV infections in large breeding herds located in high-density areas.”


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