Safety of Porcilis PRRS - Reversion to virulence

The Porcilis PRRS parent strain had to be passaged 5 times in order to test reversion to virulence. Direct passaging, achieved by transferring blood directly from pig to pig, only yielded a maximum of 3 passages. This demonstrates the poor survival rates of the vaccinal virus in the pig, making it difficult for the PRRS virus to spread. Virus amplification by culturing re-isolated virus in the lab between pig passages, was necessary to achieve 5 passages.

Reversion to virulence studies

The direct 3 passage strain and the amplified 5 passage strain were given to 2-day old piglets. Controls included pigs inoculated with a wild-type PRRSV strain, the Porcilis PRRS parent strain and an uninoculated negative control group. From the results, the following conclusions could be drawn:

1. Growth rates were unaffected in the groups vaccinated with passaged virus and unpassaged virus or in the negative controls. However, the performance of the positive control (wild-type PRRS virus) group was significantly worse.
2. No clinical signs were observed in the negative controls or the groups inoculated with the parent strain. Minor clinical signs, during the first week only, were seen in the groups inoculated with the passaged viruses, but severe signs were demonstrated by the wild-type infected group (positive controls).
3. Extensive lung lesions were seen in the wild-type infected groups, moderate lesions, during the first week only, in the groups inoculated with the passaged viruses, and none at all in the controls or the piglets inoculated with the parent strain.

These results demonstrated that with forced reverse passages with the Porcilis PRRS parent strain there was no significant reversion to virulence.