Serological responses of Australian horses using a commercial duplex indirect ELISA following vaccination against strangles

OBJECTIVE: To determine the nature of serological responses in Australian horses using a commercial duplex indirect ELISA (iELISA) following vaccination against strangles. DESIGN: A group (n = 19) of client‐owned horses from five properties were recruited to receive a primary course of a Streptococcus equi subsp. equi (S. equi) extract vaccine. Serological responses were determined by duplex iELISA incorporating S. equi‐specific fragments of two cell wall proteins, SEQ2190 and SeM (antigens (Ag) A and C, respectively). METHODS: The horses were administered a primary strangles vaccination course. Blood was collected immediately prior to each of the three vaccinations at 2‐week intervals and additionally at 28 and 56 days following the 3rd vaccination (V3). RESULTS: Significant increases in mean antibody levels of horses following vaccination were limited only to AgC, which was significantly increased at T2/V3, 14 days following V2 (ratio of geometric means = 3.7; 95% confidence interval (CI): 1.6, 8.4; P = 0.003). There was no increase in mean antibody to Ag A (ratio of geometric means = 1.4; 95% CI: 0.6, 3.2; P = 0.39). Four horses (22%) exceeded the test cut‐off for AgC following vaccination. CONCLUSION: Vaccination of Australian horses is unlikely to interfere greatly with detection of strangles using the duplex iELISA. No responses would be anticipated to AgA following vaccination with Equivac© S/Equivac© 2in1 and only a minority are likely to respond to AgC. We conclude that the results of this study validate the usefulness of the duplex iELISA to assist control measures for strangles outbreaks in Australian horse populations.