Bacteria called Streptococcus equi cause strangles. The bacteria mainly affects the upper respiratory tract but can spread to other lymph nodes in the body. Symptoms include high fever, depression, thick yellow nasal discharge, enlarged submandibular lymph nodes (under the jaw), and a productive cough.

This disease is highly contagious but mortality rates are usually very low. Younger horses are more susceptible than older horses that may have developed full or partial immunity to the bacteria. The bacteria can be spread by direct contact with secretions horse-to-horse or human-to-horse on hands, clothes, shavings or buckets.