Localised mitogenic activity in horses following infection with Streptococcus equi

•This study aimed to determine the sites of sAg production during S. equi infection.•Abscess material, chondroids and serum were tested in mitogenic assays.•Abscess material and chondroid samples were mitogenic in vitro.•Serum samples were not mitogenic in vitro.•Results support localised in vivo production of sAg during S. equi infection.

Streptococcus equi subspecies equi (S. equi) is the causative agent of strangles, a highly contagious upper respiratory disease of equids. Streptococcus equi produces superantigens (sAgs), which are thought to contribute to strangles pathogenicity through non-specific T-cell activation and pro-inflammatory response. Streptococcus equi infection induces abscesses in the lymph nodes of the head and neck. In some individuals, some abscess material remains into the guttural pouch and inspissates over time to form chondroids which can harbour live S. equi. The aim of this study was to determine the sites of sAg production during infection and therefore improve our understanding of their role. Abscess material, chondroids and serum collected from Equidae with signs of strangles were tested in mitogenic assays. Mitogenic sAg activity was only detected in abscess material and chondroids. Our data support the localised in vivo activity of sAg during both acute and carrier phases of S. equi infection.