The production of interferon (IFN)- by an animal in response to a viral infection is known to be a principal determinant of the animal’s ability to fight the infection. In addition, this substance is also known to play a major role in promoting the development of vaccine-induced adaptive immunity, thus acting akin to a vaccine adjuvant. Accordingly, the ability of a vaccine to stimulate an IFN- response would be expected to have an impact on the strength of the protective immunity elicited by the vaccine. The goal of this project was to determine the strength of cross-protective immunity provided by two different PRRS live virus vaccines that have either a high (vaccine strain G16X) or low (vaccine strain Ingelvac PRRS MLV) capacity to provoke an IFN- response in swine following their administration. The level of protective immunity elicited in grower pigs by either of these two vaccines was determined by challenging vaccinated animals with a genetically divergent (heterologous) and highly virulent PRRS virus, termed LTX1. While both PRRS live virus vaccines used for this project belong to the Type 2 North American (NA) lineage 5, the LTX1 strain, belongs to the “Canada-like” lineage 1. Type 2 PRRS viruses belonging to lineage 1 are highly virulent and were introduced within the last 10 years into in the U.S. from Canada. The LTX1 virus was selected as the challenge strain because it was responsible for a 2012 PRRS outbreak in a sow farm of the highest severity observed in the field by a group of veterinarians in Illinois. Two additional groups of pigs were not vaccinated and served as controls. Four weeks after vaccination, one of the unvaccinated groups and both of the vaccinated groups were challenged with the PRRS virus strain LTX1 and monitored for 14 days. The intensity and duration of the viremia following challenge, the amount of virus in the bronchoalveolar lavage fluid at 14 days post challenge, as well as body weight change were measured in all groups and used as parameters to evaluate cross-protective immunity. Pigs inoculated with the G16X vaccine exhibited a relatively high systemic IFN- response within 4 days after vaccination. Although both vaccines were equally able to minimize the negative effect of the virus challenge on body weight gain, the G16X vaccine was more effective in providing protection as evidenced by a significant reduction in the peak level of viremia resulting from the virulent virus challenge as well as a faster elimination of the wild-type virus from both the blood stream and the lung. Similar results were obtained in a second experiment using another contemporary PRRS virus isolate also belonging to lineage 1. The PRRS virus vaccine G16X, elicits a sizable IFN- response upon vaccination and provides effective cross-protective immunity as against virulent and genetically divergent (heterologous) type 2 (North American-like) PRRS viruses belonging to lineage 1, which is now a predominant lineage in pig farms in the American Midwest.

Contact information: Federico A. Zuckermann, University of Illinois, College of Veterinary Medicine. Email: