Comparison of Sampling Sites and Laboratory Diagnostic Tests for S. equi subsp. equi in Horses from Confirmed Strangles Outbreaks

Background Strangles is a contagious equine‐specific disease caused by Streptococcus equi subsp. equi . Unfortunately, detection of S. equi can fail in up to 40% of horses with strangles. Whereas recent molecular biologic methods and sampling techniques have improved recovery of S. equi optimal sampling methods and laboratory analyses remain ill‐defined. Objectives To determine the yield of S. equi from horses with acute strangles in confirmed outbreaks by field‐sampling methods subjected to culture and biochemical identification, and real‐time PCR directly and after culture. Animals Fifty‐seven horses of varying breeds and ages from 8 strangles outbreaks. Methods Prospective study. Culture with biochemical identification and real‐time PCR directly, and from culture, were performed on nasal swabs, nasopharyngeal swabs, and nasopharyngeal lavages. Results Real‐time PCR directly from samples identified the highest number of infected horses, with 45/57 nasal swabs, 41/57 nasopharyngeal swabs, and 48/57 nasopharyngeal lavages S. equi positive. Biochemical identification (highest positives 22/57) was inferior to real‐time PCR for S. equi recovery regardless of sampling method. Real‐time PCR of nasopharyngeal lavage directly and after culture yielded 52/57 positives whereas direct real‐time PCR of nasopharyngeal lavage combined with either nasopharyngeal swabs or nasal swabs yielded 53/57 positives. Three horses were negative on all samples. Conclusions and Clinical Importance Nasopharyngeal lavage analyzed by a combination of real‐time PCR directly and after culture or, alternatively, real‐time PCR directly on a nasopharyngeal lavage and a nasal/nasopharyngeal swab can identify S. equi in over 90% of acute strangles cases.