Prevention and control of Streptococcus equi subspecies zooepidemicus infection in pigs

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

Submitted to: American Association of Swine Veterinarians Annual Meeting
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2021
Publication Date: 2/27/2022
Citation: Hau, S.J., Devries, A.C., Brockmeier, S. 2022. Prevention and control of Streptococcus equi subspecies zooepidemicus infection in pigs. American Association of Swine Veterinarians Annual Meeting. Pg.51-52. https://doi.org/10.54846/am2022/14.
DOI: https://doi.org/10.54846/am2022/14

Interpretive Summary:

Technical Abstract: Background: Streptococcus equi subspecies zooepidemicus (SEZ) is a zoonotic bacterium causing infections in a wide range of mammalian species. SEZ has historically caused outbreaks of severe disease in swine in China and Indonesia; however, it was an infrequent cause of infection in pigs in North America. In 2019, high mortality events due to SEZ were reported in the U.S. and Canada. Pigs developed high fever, severe lethargy, and reluctance to rise, and mortality rates of 30-50% were observed in affected animals. Because SEZ has not been a frequent cause of disease in North American pigs, there are no approved vaccines or intervention strategies to prevent the losses caused by SEZ. To better understand how to limit the effects of SEZ introduction into a group of pigs, we evaluated three different SEZ control strategies: pre-exposure to an avirulent SEZ isolate (study 1), vaccination with an autogenous vaccine containing inactivated bacteria (study 2), and the use of long-acting ceftiofur after the development of clinical signs (study 3). Methods: Study 1: five sows previously inoculated with a non-virulent SEZ isolate from a horse were challenged 21 days post-inoculation with a 2019 mortality event isolate. Study 2: 47 six-week-old pigs were vaccinated twice with one of the following: an autogenous vaccine generated from a highly mucoid, 2019 high mortality event isolate (n=15); an autogenous vaccine generated from a genetically similar, non-mucoid SEZ isolate from a guinea pig (n=16); or sham vaccinated with adjuvant only (n=16). Autogenous vaccines contained 109 bacteria and EmulsigenD was utilized as the adjuvant. Pigs were challenged two weeks following boost vaccination with a 2019 mortality event isolate. Study 3: 32 ten-week-old pigs were challenged with a 2019 mortality event isolate. Following the development of clinical signs, 16 pigs were treated with ceftiofur crystalline free acid (Excede, Zoetis) and the other 16 were not treated. Following recovery, two non-antibiotic-treated pigs (one vaccinated, one sham vaccinated from study 2) were commingled with three naïve contact pigs and, in addition, two antibiotic-treated pigs (from study 3) were commingled with three naïve contact pigs 30 days post-challenge to evaluate SEZ transmission from previously exposed, healthy pigs. Pigs in all studies were evaluated for serum antibody titer and survival post-challenge. Results: Study 1: After 21 days, sows inoculated with an avirulent SEZ isolate had a small, non-significant rise in titer to SEZ. Prior exposure to an avirulent SEZ isolate resulted in a delayed onset of clinical signs and prolonged survival after exposure to a high mortality event isolate; however, there was no difference in survival between animals pre-exposed to SEZ and animals without prior SEZ exposure. Study 2: Both autogenous vaccine groups had a significant rise in mean SEZ titer compared with the sham vaccinated group. A rise in titer was observed after primary vaccination; however, no anamnestic response was noted after boost vaccination. Challenge resulted in severe disease and >90% mortality in all groups. No statistical differences were observed in disease onset, median survival time, or mortality rates between either bacterin vaccine group and the sham vaccine group. Study 3: All animals developed clinical signs by 24 hours post-challenge. Animals in the treatment group were given a weight appropriate dose of ceftiofur. Most (15/16) treated animals returned to normal behavior and normal body temperature by 24 hours post-treatment. One pig developed neurologic signs and was euthanized due to worsening condition. At 5 days post-challenge (4 days post-treatment), surviving pigs began exhibiting clinical signs (fever, lethargy) and all pigs were retreated with ceftiofur. Retreatment resolved clinical signs; however, 10-11 days post-challenge (5-6 da