Which organism causes strangles in horses?

• A. Staphylococcus aureus
• B. Streptococcus equi
• C. Corynebacterium equi
• D. Strongylus vulgaris

Answer: (B)

Streptococcus equi is the organism that causes strangles in horses. This highly contagious respiratory infection leads to fever, nasal discharge, and swelling or abscesses of the head and neck lymph nodes, especially the mandibular and retropharyngeal groups. The bacterium spreads through nasal secretions and contaminated equipment, and it typically appears as gram-positive cocci in chains that are beta-hemolytic on blood agar; it’s a Lancefield group C streptococcus. While older texts sometimes used the name Corynebacterium equi, the current, accepted causative agent is Streptococcus equi subsp. equi. The other organisms listed do not cause this condition—Staphylococcus aureus is associated with skin and wound infections rather than strangles, and Strongylus vulgaris is a parasite that affects the horse’s vasculature, not the lymph nodes of the upper airway.