Comparison of sampling sites and laboratory diagnostic tests for S. equi subsp. equi in horses from confirmed strangles outbreaks

Abstract

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. © 2013 by the American College of Veterinary Internal Medicine.