Influence of penicillin treatment of horses with strangles on seropositivity to Streptococcus equi ssp. equi‐specific antibodies

BACKGROUND: Antibiotic treatment of horses with strangles is reported to impair the development of immunity to subsequent exposure to Streptococcus equi ssp equi (S. equi). However, apart from a single clinical report, evidence‐based studies for this hypothesis are lacking. HYPOTHESIS/OBJECTIVE: To determine whether penicillin treatment during clinical strangles influences the development or persistence of seropositivity to S. equi‐specific antibodies. ANIMALS: A natural outbreak of strangles with 100% morbidity in 41 unvaccinated mature Icelandic horses. METHODS: A prospective longitudinal study of acute clinical strangles from onset through full recovery approximately 10 months after the index case. Horses were monitored clinically 6 times for S. equi, as well as serologically for antibodies to antigens A and C of S. equi using an enhanced indirect ELISA. Seven horses received penicillin within 11 days of onset of fever (Group 1), 5 between 16 and 22 days after onset of fever (Group 2), and the remainder (Group 3, n = 29) received no antibiotics during clinical disease. The proportions of seropositive horses in each group were compared using an extension of Fisher's exact test with P < .05 as the level of significance. RESULTS: Although all horses were seropositive to S. equi within 2 months of the index case, significantly fewer horses treated early (Group 1) remained seropositive by 4 to 6 months (P = .04 and .02, respectively). CONCLUSIONS AND CLINICAL IMPORTANCE: Findings support earlier suggestions that penicillin administered during acute strangles can interfere with persistence of humoral immunity to S. equi.