Even when meteorological conditions are considered ideal for airborne transfer of PRRS (porcine reproductive and respiratory syndrome) virus, other factors are just as likely to produce lateral transmission.

“This study indicates that the mere occurrence of wind blowing from a known PRRS-infected site to a recently infected site, even under appropriate environmental conditions for PRRS virus survival, is insufficient to conclude that the mechanism of virus spread was airborne,” says lead researcher Robert Morrison, DVM, University of Minnesota.

Researchers investigated eight cases where airborne transmission of PRRS virus was assumed to have occurred. Genetic similarity of the virus on the positive and newly infected sites was very similar. Distance between sites ranged from 0.7 to 7.45 miles apart. In all eight cases, wind speed and direction, temperature, humidity and lack of sunlight were consistent with conditions considered ideal for airborne transmission.

However, in eight other control (noninfected) cases, airborne transmission did not occur, yet the wind speed and direction from a known, infected site, temperature, humidity and lack of sunlight were again all considered appropriate for aerosol spread from a nearby infected site. Distance between sites ranged from 0.9 to 4.34 miles.

Thus, researchers concluded that the mere occurrence of wind blowing from a PRRS-infected site to a PRRS-negative site under ideal environmental conditions for virus survival is not sufficient for PRRS transmission to occur.

“While airborne transmission of PRRS virus can occur, these findings suggest that other factors such as strain of the virus, topography between sites, population sizes and stocking density may play an important role,” Morrison summarizes.

Researchers: Robert B. Morrison, DVM and Enrique Mondaca-Fernandez, DVM, both of the University of Minnesota and Jeffrey Zimmerman, DVM, Iowa State University. Contact Morrison by phone (612) 625-9276, fax (612) 625-1210 or e-mail bobm@umn.edu .