Clostridial myonecrosis is a rare, but massive and life-threatening infection of muscle and connective tissue. It is caused by one of the types of anaerobic (lives without oxygen) Clostridium bacteria.

The infection usually results from injection of an irritating substance into the muscle. It has been more frequently associated with the intra-muscular injection of flunixin meglumine (Banamine). Similar infections can also be caused by puncture wounds, and can be a rare complication after castration or other surgery.

It has been thought historically that the organism was carried into the tissues on the needle or was within the medication. A study has shown that dormant Clostridia (spores) actually can be found within healthy muscle tissue of normal horses. Irritation to this tissue creates a good environment for activation of the dormant spores and subsequent rapid bacterial growth.

The Clostridium bacteria produces severe toxins that damages and kills the tissue, creating an oxygen-free environment of dead tissue that allows increasingly rapid bacterial growth. The bacteria have specific toxins that destroy connective tissue, allowing the process to migrate through the tissues. The bacterial toxins also get into the bloodstream and cause signs of severe illness, shock and death.

Signs of this condition are a rapidly enlarging area of painful swelling. Often, gas can be felt crackling under the surface of the skin. Affected horses usually are depressed and lack appetite. Most have a fever, and have high heart and respiratory rates.

Diagnosis is usually assumed by the rapidly worsening signs of disease. i.e. a severe, painful swelling, fever and illness in a horse that was recently injected. Definitive diagnosis requires confirmation of gas and fluid accumulation in the tissues and detection (and culture or other identification) of Clostridium bacteria in the living horse.

Treatment requires aggressive use of appropriate intravenous antibiotics, surgical opening of the infected areas and in many cases, intensive care with IV fluids and other medications.