Influence of penicillin treatment of horses with strangles on seropositivity to Streptococcus equi ssp. equi-specific antibodies.
Authors: Pringle John, Storm Emma, Waller Andrew, Riihimäki Miia
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Penicillin Treatment and Immunity in Equine Strangles When treating strangles with penicillin, veterinarians face a clinical paradox: whilst antibiotics resolve the acute infection, they may compromise the protective immunity that normally develops following natural disease. Pringle and colleagues studied this phenomenon prospectively during a natural outbreak affecting 41 unvaccinated Icelandic horses, dividing them into three groups based on penicillin timing—early treatment (within 11 days of fever onset), delayed treatment (16–22 days), and no treatment—and monitoring serological responses to *Streptococcus equi* antigens A and C over ten months using enhanced ELISA. All horses developed antibodies within two months; however, significantly fewer early-treated horses maintained seropositive status at four to six months post-infection compared to untreated controls (P = .04 and .02 respectively), whilst delayed treatment showed intermediate results. This finding provides the first robust evidence supporting long-standing clinical concerns that prompt penicillin therapy may attenuate long-term humoral immunity, potentially leaving horses vulnerable to reinfection and warranting reconsideration of antibiotic protocols in strangles cases where owners can manage uncomplicated disease, or alternatively suggesting a role for post-infection vaccination strategies to restore protective immunity.
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Practical Takeaways
- •Early penicillin treatment of strangles may compromise long-term immunity, potentially increasing re-infection risk; consider withholding antibiotics in mild cases or delaying treatment until after day 11 of fever if clinical signs permit
- •Horses treated early with penicillin may not develop adequate protective antibody persistence and could require re-vaccination or booster protocols
- •This finding supports the traditional practice of allowing strangles to run its course without antibiotics in uncomplicated cases to maximize natural immunity development
Key Findings
- •All 41 horses developed seropositivity to S. equi within 2 months of the index case
- •Horses treated with penicillin within 11 days of fever onset (Group 1) had significantly fewer seropositive animals by 4-6 months compared to untreated horses (P = 0.04 and 0.02)
- •Early penicillin treatment during acute strangles interferes with persistence of humoral immunity to S. equi
- •Delayed penicillin treatment (16-22 days post-fever) did not significantly reduce seropositivity persistence compared to untreated controls