Streptococcus equi Detection Polymerase Chain Reaction Assay for Equine Nasopharyngeal and Guttural Pouch Wash Samples

BACKGROUND Bacterial culture and polymerase chain reaction (PCR) assays for the detection of Streptococcus equi in nasopharyngeal washes (NPW) and guttural pouch lavage (GPL) samples have low sensitivity. In human diagnostics, processing of samples with flocked swabs has improved recovery rates of bacterial agents because of improved surface area and elution factors. HYPOTHESIS For S. equi subsp. equi (S. equi) detection in NPW and GPL samples we hypothesized that: direct-PCR would be more reliable than flocked swab culture (FS culture); flocked swab PCR (FS-PCR) would be equivalent to direct-PCR; and FS culture would be more reliable than traditional culture. SAMPLES A total of 193 samples (134 NPW and 59 GPL) from 113 horses with either suspected S. equi infection, convalescing from a known S. equi infection, or asymptomatic horses screened for S. equi. METHODS Prospective study. Samples were submitted for S. equi direct-PCR. Using logistic regression, direct-PCR (gold standard) was compared to FS culture, traditional culture, and FS-PCR also performed. RESULTS Direct-PCR was statistically more sensitive than FS-PCR, FS culture, and traditional culture (P < .001). All methods had sensitivities <70% relative to the direct-PCR. FS culture had a similar sensitivity relative to traditional culture. The odds of GPL samples being positive on direct-PCR (P = .030) and FS-PCR were greater than those for NPW samples (P = .021). CONCLUSIONS AND CLINICAL IMPORTANCE Use of flocked swabs during laboratory preprocessing did not improve detection of S. equi via either PCR or bacterial culture from samples. Direct-PCR is the preferred method of detection of S. equi.