Standards of filter testing for PRRS virus

Air filtration is a newly adopted biosecurity methodology in swine farms. Although air filtration has been shown to decrease the number of porcine reproductive and respiratory syndrome virus (PRRSV) breaks in sow farms, questions still remain to when to change filters. However, it is not clear what methodology producers should follow to evaluate filter efficiency. 

The objective of a recent University of Minnesota (U of M) study was to develop a standardized method to evaluate filter performance against PRRS following ASHRAE Standard 52.2 2012 (a standard established by the American Society of Heating, Refrigerating and Air-Conditioning Engineers). Filters are designed to entrap particles of specific sizes with specific efficiencies for each particle size range.

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In the U of M study, researchers conducted testing using a modified ASHRAE 52.2 standard method. MERV 14 filters were challenged with an aerosolized solution of potassium chloride (KCL) particles tagged with a fluorescent dye and PRRSV. Three concentrations of KCL were chosen, 1%, 10% and 20%, and concentrations run in triplicate. PRRSV solution contained 10^7 PRRSV particles of a PRRSV modified live vaccine. The system operated at 650 cfm and aerosols were generated using a high-volume aerosol generator.

Air samples were collected upstream and downstream from the filter. Air samples were collected and tested with an optical particle counter to count total particles. Air samples were also collected with an Anderson Cascade Impactor able to separate particles by size (approximately 0.3 to 10 microns), and then samples were tested by quantitative PRRS reverse transcription polymerase chain reaction (RT-PCR) and a fluorometer to estimate mass. For each particle size, the particle size removal efficiency was calculated.

Results
The results showed that under the conditions of the U of M study, filter efficiency for PRRSV was similar to filter efficiency using KCL particles (ASHRAE 52.2 standards). In addition, the efficiency of particle removal was similar across the methods used (optical particle counter, fluorometer or PCR).

Testing used filters
As a follow-up from this study, the researchers used these methods to evaluate filter efficiency of used filters. Twenty filters ranging in age between 6 and 24 months were used. The research results indicated that the methods validated here were also applicable to used filters. However, there was more variation observed in the efficiencies reported with the RT-PCR and fluorometer results compared to the optical particle counter. Thus, filter efficiency measured as total particles or as PRRSV particles was similar; however, the reported efficiency was more variable with PRRSV than with total particles, according to the researchers. The major variation and discrepancy was observed at particle sizes of < 2 microns.

Furthermore, according to the research team, the results from the optical particle counter, which measures total particles, were the most conservative measurements for each of the filters. Overall, results of the research indicate that filter efficiency testing for PRRSV can follow ASHRAE 52.2 standards and that the standards can be applied to new and used filters.

Researchers: Montserrat Torremorell, Carmen Alonso, Bernard Olson, Darrick Zarling, Thomas Khuen, University of Minnesota. Contact Torremorell at (612) 625-1233 or email [email protected]

 

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