Non-thermal plasma reactors can inactivate PRRSV

Hog-farm biosecurity measures have largely focused on minimizing the transmission of infectious agents on various surfaces. However, it’s been shown that porcine reproductive and respiratory syndrome virus (PRRSV) — and possibly other respiratory diseases — can be transmitted via air. In fact, PRRSV has been detected in air more than 5.5 miles downwind of infected swine.¹

Air-filtration systems have helped to reduce disease transmission. According to a previous National Pork Board study, only 20% of filtered hog barns recorded a new PRRSV outbreak versus 92% of farms without such filters in the control group.²

Despite these results, the adoption of air filtration for hog barns, typically MERV-14 or MERV-16 filters, has been mostly limited to breeding-swine units for several practical reasons.

“Applying air filtration to hog barns involves structural retrofits to buildings that can be costly, in addition to the periodic replacement of used filters,” noted Herek Clack, PhD, researcher and associate professor of engineering at the University of Michigan. The ongoing operational costs, and management and labor demands can be additional deterrents.

Other options

Clack, therefore, wanted to investigate other options such as non-thermal plasma (NTP) reactors, which involve electrical discharges comprised of reactive radicals and excited species that inactivate viruses and bacteria. His findings were summarized in the National Pork Board’s Research Review newsletter.

“Of the two defining characteristics of infectious aerosols — transport and infectivity — particulate filters only address transport,” he noted. “However, NTPs address both characteristics by electrostatic removal of larger particles (approximately > 1 mm) and sterilization of the remaining smaller particles by direct plasma exposure.”

NTPs also have been proven for surface disinfection and pathogen inactivation in other areas such as food production.

In a Pork Checkoff-funded research project, Clack set out to measure the inactivation of aerosolized PRRSV by a prototype packed-bed NTP reactor.³ The three biggest questions were:

1. To what degree does the prototype packed-bed NTP reactor inactivate airborne PRRSV?
2. Under the same operating conditions, does the prototype reactor inactivate PRRSV at a higher or lower efficiency than the viral surrogate MS2?
3. And if yes, should PRRSV inactivation be considered a conservative benchmark as compared to MS2 or vice versa?

Inactivating PRRSV

For the study, Clack suspended PRRSV in the air at the entrance to a small wind tunnel using a mister supplied with a liquid containing active PRRSV. The virus aerosols were then exposed to a region of NTP within a prototype NTP reactor installed in the wind tunnel test section.

Lab analyses of the PRRSV collected from the upstream and downstream air space of the NTP reactor provided pre- and post-treatment TCID50, which measures the amount of an infectious viable agent. The reduction in TCID50 following NTP treatment represents the inactivation efficiency achieved by the NTP reactor based on experimental conditions.

The results showed that PRRSV was inactivated to a similar degree (e.g., 1.3-log; > 90%) as the surrogate virus, MS2 phage, at the same conditions. PRRSV inactivation was achieved at an applied voltage of 20 kV and an air-flow rate of 12 cfm. Differential pressure across the reactor was minimal compared to that imposed by HEPA filters. A high-porosity consumer-grade ozone filter positioned downstream of the reactor effectively reduced residual ozone concentrations down to levels similar to the ambient laboratory environment.

“The study showed the potential of properly optimized NTP reactors to prevent airborne PRRSV transmission into hog barns via ventilation air,” Clack noted.

Additionally, the results showed that an optimized NTP airstream-sterilization system would:

• Reduce or eliminate the need to retrofit hog facilities to establish an air-tight building;
• Eliminate the expense and disposal of replacing used air filters;
• Offer protection from airborne viruses regardless of mutations;
• Offer on-demand or tunable protection as meteorological or climatic conditions warrant.

Editor’s note: More details about this study are available in the complete report.

¹Otake S, Dee S,  Corzo C,  Oliveira S, Deen J. Long-distance airborne transport of infectious PRRSV and Mycoplasma hyopneumoniae from a swine population infected with multiple viral variants. Vet Microbiol. 2010;145:198-208.
²Dee S. University of Minnesota. An assessment of air filtration for reducing the risk of PRRSV infection in large breeding herds in swine dense regions. National Pork Board research #09-209.
³Clack H. University of Michigan. Demonstration of Airborne PRRSV Inactivation by a Non-Thermal Plasma. National Pork Board research #16-198.