#04-192

Complete

Date Full Report Received

10/01/2006

Date Abstract Report Received

10/01/2006

Investigation

Institution:
Primary Investigator:

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. That is, it could detect PRRSV if the source produced 1 x 101.38 TCID50 over a 5 minute sampling period (Hermann and Zimmerman, 2006). 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. Our T1/2 estimates probably represent the maximum T1/2 because we did not account for any effects of ultraviolet inactivation. Alternatively, our T1/2 estimates could be thought of as most representative of night-time virus “survival.” 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.