Equine Strangles Study Guide

Overview and Clinical Importance

Strangles is the most frequently diagnosed infectious disease of horses worldwide, caused by the gram-positive, beta-hemolytic bacterium Streptococcus equi subspecies equi (S. equi). This highly contagious upper respiratory tract infection is characterized by lymphadenopathy and abscessation of the submandibular and retropharyngeal lymph nodes. The disease derives its name from the potential for enlarged lymph nodes to compress the pharynx and upper airway, causing respiratory distress. Strangles became a reportable disease in the United States in 2017.

Etiology and Microbiology

Streptococcus equi subspecies equi is a gram-positive, capsulated, beta-hemolytic, Lancefield group C coccoid bacterium. Unlike S. equi subspecies zooepidemicus, which is a commensal organism that can cause opportunistic infections, S. equi is considered an obligate parasite and primary pathogen in equids. The organism is highly host-adapted and produces clinical disease only in horses, donkeys, and mules.

Key Microbiological Characteristics

Transmission and Pathogenesis

Routes of Transmission

Direct transmission: Nose-to-nose contact with infected horses or subclinical shedders