Horses vaccinated with live attenuated intranasal strangles vaccine seroconvert to SEQ2190 and SeM.
Authors: Boyle Ashley G, Mitchell Catriona, Stefanovski Darko, Waller Andrew S
Journal: Equine veterinary journal
Summary
# Editorial Summary: Dual Antigen iELISA and Live Attenuated Strangles Vaccination The dual antigen iELISA detects antibodies against two key *Streptococcus equi* surface proteins (SEQ2190 and SeM) and is routinely used to identify horses with natural exposure to strangles; however, its utility becomes problematic when applied to vaccinated populations. Boyle and colleagues conducted a prospective case-control study comparing 26 horses receiving annual Pinnacle IN (live attenuated, non-encapsulated SeM-2 strain) vaccination against 26 unvaccinated controls, with serum samples collected at vaccination, 5 weeks post-vaccination, and 10 weeks post-booster, analysed using mixed-effects ordered logistic regression. Remarkably, 76% of vaccinated horses seroconverted by 5 weeks compared to just 4% of controls, with vaccinated animals 14 times more likely to test seropositive or suspect (P = 0.02); notably, antibody responses to Antigen C (SeM) were significantly stronger than responses to Antigen A (SEQ2190). These findings have important diagnostic implications: the dual antigen iELISA cannot reliably distinguish between vaccinated and naturally exposed horses, potentially rendering it unsuitable as a diagnostic tool in Pinnacle IN-vaccinated populations and risking false-positive identification of exposed animals. Practitioners managing vaccinated herds should be aware that seropositivity alone does not confirm field exposure, and alternative diagnostic approaches (such as guttural pouch endoscopy or culture) may be necessary to confirm active infection when clinical signs warrant investigation.
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Practical Takeaways
- •Do not use the dual antigen iELISA to test horses vaccinated with Pinnacle IN, as it will not distinguish between vaccination-induced seropositivity and exposure to natural infection
- •Pinnacle IN vaccination reliably induces antibody response to both SEQ2190 and SeM antigens, confirming vaccine take in 76% of vaccinated horses by 5 weeks
- •Consider alternative diagnostic methods or unvaccinated control populations when screening for strangles exposure in vaccinated populations
Key Findings
- •76% of horses vaccinated with Pinnacle IN seroconverted to dual antigen iELISA at 5 weeks post-vaccination compared to 4% of non-vaccinated controls
- •Vaccinated horses were 14 times more likely to be seropositive or suspect seropositive than non-vaccinated controls (OR 14; P = 0.02)
- •Antigen C (SeM) generated significantly higher OD450 values than Antigen A (SEQ2190) in vaccinated horses (P < 0.001)
- •High seroconversion rate to both antigens means the dual antigen iELISA cannot reliably distinguish vaccinated horses from naturally infected horses