Concurrent Streptococcus equi subsp. equi infection, purpura haemorrhagica and immune-mediated myositis in a Quarter Horse filly

This report describes a 2-year-old Quarter Horse filly that was diagnosed with concurrent Streptococcus equi subsp. equi infection, purpura haemorrhagica, and immune-mediated myositis. The filly was presented for evaluation of fever of unknown origin, rapid weight loss and inappetence after exposure to strangles that was initially unknown. Over the course of hospitalisation the filly developed mandibular lymphadenopathy, guttural pouch empyema, and mucosal petechiation and ecchymoses. The presence of Streptococcus equi subsp. equi was confirmed via PCR and culture of a lymph node aspirate and biopsies of the skin confirmed leukocytoclastic vasculitis, consistent with purpura haemorrhagica. Genetic testing confirmed that the filly had one copy of the MYH1 mutation (N/My). Following therapy with systemic corticosteroids, plasma transfusion and antimicrobial therapy the filly improved and was discharged. Communication with the owner 11 months later confirmed that filly appeared healthy but remained poorly muscled. This report is the first to the authors' knowledge describing concurrent Streptococcus equi subsp. equi infection, purpura haemorrhagica, and immune-mediated myositis.