Project Title: Modeling PRRS virus aerosol transmission within and between farms

Institution: Iowa State University

Outcome/Results: No PRRS virus was detected in the air exhaled by pigs infected under experimental conditions and sampled for two weeks afterwards. Although the sampling equipment is very sensitive, it is theoretically possible that pigs shed virus below the level of detection. The PRRS virus is most stable at lower temperatures and lower humidity with temperatures below freezing begin optimal for survival.

Summary: The objective of this research was to try to understand two major stages of aerosol transmission: 1) how much PRRSV is aerosolized by pigs and 2) how long does airborne PRRSV remain infectious.

How much PRRSV is aerosolized by pigs? We found that, even in acutely infected pigs, PRRSV is aerosolized at extremely low levels, if at all. That is, although we had no difficulty in detecting PRRSV in the mouths of acutely infected pigs, we were unable to detect any virus in the respiratory exhalations of 26 acutely infected pigs sampled repeatedly for 2 weeks after they were inoculated. In a related study funded by the USDA PRRS CAP project, we determined that the system we used to collect virus was relatively sensitive. Thus, we did not prove that zero PRRSV was aerosolized, but the level of excretion must be below the threshold of detection of the system. Importantly, this does not eliminate the possibility that a large number of pigs excreting low levels of PRRSV may still be sufficient to cause transmission between farms.

How long does airborne PRRSV remain infectious? Our half-life (T1/2) study of aerosolized PRRSV showed that the virus was most stable at lower temperatures and lower relative humidity. Temperatures below freezing are optimal for PRRSV “survival” in aerosols. The virus is rapidly inactivated at warmer temperatures and higher humidity.

What is needed to fully evaluate aerosol transmission? Better estimates of PRRSV shedding are needed to fully evaluate the risk of farm-to-farm aerosol transmission.