Clostridial myositis is a rare but serious bacterial infection, which causes inflammation and death of muscle and release of bacterial toxins into the bloodstream. This condition is also referred to as myonecrosis, malignant edema, and gas gangrene. It occurs most often in horses that have recently received an intramuscular injection.

Clinical Signs

Clinical signs appear 6-72 hours following the injection, and horses initially exhibit acute swelling, heat, and pain of the affected area. The disease progresses rapidly and the horse’s condition may decline within hours. The affected animal exhibits signs of systemic toxemia; death can supervene rapidly in severe cases. Clostridial bacteria produce gas that results in a characteristic emphysematous (bubbly) feel or crepitation of the region. Clostridial myositis is a true medical emergency, with survival linked to prompt intervention through aggressive antibiotic treatment and wound debridement.

A Closer Look

The Clostridium genus consists of more than 150 known species of Gram-positive, anaerobic, spore-forming bacteria. The spore-forming ability of these bacteria allows survival for long periods of time in the environment. When spores encounter a location without oxygen, such as damaged muscle, they are triggered to proliferate and produce exotoxins, which cause extensive tissue and vascular damage. The clostridial species that commonly cause myositis include C. perfringens, C. septicum, and C. chauvoei.

Clostridial myositis has been reported following intramuscular inoculations of vaccines, ivermectin, antihistamines, phenylbutazone, vitamins, prostaglandins, and most commonly, flunixin meglumine. Infrequently, cases occur following inadequate perivascular administration of compounds, foaling trauma, or puncture wounds. In a study by Peek et al in 2003, stallions and Quarter horses were overrepresented, and the authors hypothesized this might be due to the heavy muscling of these groups