The dual antigen iELISA uses two Streptococcus equi subsp equi surface protein antigens composed of N‐terminal portions of SEQ2190 (Antigen A) and SeM (Antigen C). It is currently used to identify animals exposed to S. equi, which have developed an immune response to the target antigens. This prospective case-control study investigated the usefulness of the ELISA in horses vaccinated with a live attenuated intranasal vaccine against S. equi.

The study consisted of 26 horses vaccinated annually with Pinnacle IN and 26 non-vaccinated horses. Blood samples were obtained at annual strangles vaccination, 5 weeks post-vaccination and 10 weeks post-vaccination in horses that received a booster. Seropositivity was defined as an OD450 nm value ≥0.5 for one or both antigens.

At the 5-week time point, 76% of vaccinated horses were seropositive compared to 4% of non-vaccinated horses. Vaccinated horses were 14 times more likely to be seropositive or suspect than non-vaccinated horses. The OD450 value was significantly larger for Antigen C (SeM) than Antigen A (SEQ2190) for vaccinated horses.

Bottom line: With a high rate of seroconversion to both antigens due to vaccination, the dual antigen ELISA is not recommended for the diagnosis of previous S. equi exposure in horses that have been vaccinated with Pinnacle IN.