Strangles, caused by the Gram-positive bacteria Streptococcus equi subspecies equi (S. equi), is a highly contagious upper respiratory infection in horses. Control of outbreaks requires strict isolation protocols and hygiene measures.
Source: Equine Veterinary Education, March 2017, (abstract access only).
Also see: Lessons learned from a strangles outbreak on a large Standardbred farm