Clostridial myositis in horses is a potentially fatal disease that requires immediate attention once it is suspected. The disease arises at an injection site in the muscle. An injection of the drug flunixin will almost invariably result in an injection site myositis and myonecrosis, but any injectable is capable of causing the disease. Flunixin should always be given in the vein, never in the muscle! The bacteria that cause the disease are not inoculated via the injection but are already present in the muscle. Normally, they do not cause disease. But when the muscle is damaged and anaerobic conditions arise, the bacteria grow and release toxins. The bacteria belong to the genus Clostridium. There are various species that cause the disease, but C. perfringens is the most common and usually results in the best outcome if treated early.
The most common early signs of the disease are colic, pain, fever, and crepitus over the injection site. Crepitus, also called subcutaneous emphysema, is caused by air in the subcutaneous tissue. This can be easily felt, and once you feel it you will remember what it feels like because it is so distinctive. While it is scientifically cool, it is one feeling that I hope to never find on a horse because this disease is not easy to treat. If you diagnose this disease, you have to cut the muscles open and expose them to oxygen. This alteration of the environment from anaerobic to aerobic will kill the bacteria. This, coupled with antibiotics and anti-inflammatory drugs, is the preferred treatment.
I have seen some horses get intramuscular injections of this medication and remain totally fine, but it is not worth the risk. If the situation turns bad, you will regret it for both your animal and your bank account. This disease can occur due to other injections as well, so it is important to know the clinical signs to look for. Fever, colic, pain, and crepitus at the injection site are all signs that this disease could be present and veterinary help should be sought.