Bacterin vaccination provides insufficient protection against Streptococcus equi subspecies zooepidemicus infection in pigs

Authors: Hau, Samantha; Buckley, Alexandra; Brockmeier, Susan

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2022
Publication Date: 2/28/2022
Citation: Hau, S.J., Devries, A.C., Brockmeier, S. 2022. Bacterin vaccination provides insufficient protection against Streptococcus equi subspecies zooepidemicus infection in pigs. Frontiers in Veterinary Science. 9. Article 827082. https://doi.org/10.3389/fvets.2022.827082.
DOI: https://doi.org/10.3389/fvets.2022.827082

Interpretive Summary: Streptococcus equi subspecies zooepidemicus (SEZ) causes severe disease in pigs. Pigs quickly become very sick, and losses reach 30-50% of animals. The losses due to SEZ are costly to producers. There are no vaccines available to prevent SEZ infection. For many swine diseases, vaccinating with killed bacteria can prevent infection. This has not been tested for SEZ strains in North America. This study tested whether a killed vaccine could prevent SEZ in pigs. Pigs were vaccinated with a killed vaccine or mock vaccinated. After boost vaccination, pigs were challenged with a North American SEZ isolate. Pigs were monitored for clinical signs and survival was compared between groups. We found that the killed vaccine did not prevent disease with SEZ. The immune response was not strong enough and antibody titers were too low to prevent disease. We believe the vaccine may need more bacteria or a different adjuvant to fully stimulate the immune response. This study also tested SEZ spread between naïve animals and previously infected animals. Two pigs that survived SEZ infection were mixed with three contact animals. One contact animal became sick with SEZ. The others remained healthy but showed signs of SEZ exposure. This proved animals surviving SEZ infection can act as a source of SEZ for naïve animals. Continued research should test novel vaccines to prevent SEZ.

Technical Abstract: Streptococcus equi subspecies zooepidemicus (SEZ) is a zoonotic pathogen capable of causing severe disease in many mammalian species. Historically, SEZ has not been a common cause of disease in pigs in North America; however, in 2019 SEZ caused mortality events leading to severe illness and 30-50% mortality in exposed animal groups. Because of the rapid progression of disease, it is important to investigate intervention strategies to prevent disease development. In this study, pigs were divided into four groups: (1) vaccinated with an inactivated SEZ vaccine generated from a highly mucoid 2019 mortality event isolate, (2) vaccinated with an inactivated SEZ vaccine generated from a genetically similar, non-mucoid isolate from a guinea pig, (3) and (4) sham vaccinated. Following boost vaccination, groups 1-3 were challenged with a 2019 mortality event isolate and group 4 were non-challenged controls. Antibody titers were higher for SEZ vaccinated animals than sham vaccinated animals; however, no anamnestic response was observed, and titers were lower than typically seen following the use of inactivated vaccines. Vaccination did not provide protection from disease development or mortality following challenge, which could be associated with the comparatively low antibody titers generated by vaccination. Surviving pigs also remained colonized and transmitted SEZ to naïve contact pigs three weeks following challenge, indicating healthy animals can act as a source of SEZ exposure. Future investigation should evaluate different vaccine formulations, such as increased antigen load or an alternative adjuvant, that could induce a more robust adaptive immune response.