Markers of long term silent carriers of Streptococcus equi ssp. equi in horses

BACKGROUND: Difficulty in detection of silent carriers of Streptococcus equi is a key reason for its continued spread to immunologically naïve groups of horses. OBJECTIVE: To determine whether clinical examination, markers of inflammation, or serology differentiate silent carriers of S. equi in recovered comingled horses. ANIMALS: Ninety‐eight warmblood yearlings and 72 unaffected mares on a large breeding farm (outbreak A), 38 mature Icelandic horses at a riding stable (outbreak B), and 27 mixed breed horses at a boarding stable (outbreak C). METHODS: Prospective observational study 6 months to 2 years after strangles outbreaks. Carriers were defined as any animal positive on culture or qPCR to S. equi from nasopharyngeal lavage or guttural pouch endoscopy and lavage. Most horses had complete physical exams and 1 group included evaluation of white blood cell counts and serum amyloid A. Sera from all horses was tested for antibodies to antigens A and C of S. equi using an enhanced indirect ELISA. Descriptive statistics were calculated. Data were compared using paired t tests, Wilcoxon ranked test, chi square, or the Fishers exact test. Significance was set at P < .05. RESULTS: Apart from weanlings at 6 months in outbreak A, there was no significant association between any clinical markers or serology with carrier state (P = .06‐1). Moreover, 3/12 culture positive carriers were seronegative to S. equi. CONCLUSIONS AND CLINICAL IMPORTANCE: Silent carriers of S. equi do not differ clinically or on markers of inflammation to their noncarrier herd‐mates. Moreover, serology alone will not distinguish carriers in comingled horses.