Detection and differentiation of wild-type and a vaccine strain of Streptococcus equi ssp. equi using pyrosequencing

Streptococcus equi subspecies equi (S. equi), the causative agent of strangles, is an important equine pathogen. Strangles is a highly contagious disease and a commercial modified live vaccine (MLV) is used for protection, which although effective, may also result in clinical signs of the disease. A rapid means to differentiate between the MLV and wild-type infection is crucial for quarantine release and limiting the disease spread. This study describes the use of a pyrosequencing assay targeting a single nucleotide deletion upstream of the SzPSe gene to distinguish between the wild-type and vaccine strains. A set of 96 characterized clinical specimens and isolates were tested using the assay. The assay was successful in differentiating between wild-type S. equi and the vaccine strains and in discriminating S. equi from other Streptococci. The vaccine strain was identified in 61.7% (29/47) of the strangles cases in horses with a history of MLV vaccination.