Clostridial Myositis: A Danger for Horses Receiving Injections

Visiting veterinarian ready to vaccinate horse in a pasture.
Visiting veterinarian ready to vaccinate horse in a pasture. OIE Bulletin-John Grewar

Newsdate: Wednesday, May 15, 2024, 11:00 am
Location: LEXINGTON, Kentucky

Clostridial myositis is a rare but serious bacterial infection, which causes inflammation and death of muscle and release of bacterial toxins into the bloodstream of horses.

Horse receiving an injection.

Horse receiving an injection

Clostridial myositis is a true medical emergency in horses, with survival linked to prompt intervention through aggressive antibiotic treatment and wound debridement.
© 2016 by Kondrashov MIkhail Evgenevich

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 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.

The Clostridium genus consists of over 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 exo-toxins, which cause extensive tissue and vascular damage.

The clostridial species that commonly cause myositis include C. perfringens, C. septicumand 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.

The mechanism by which the bacterial spores arrive in the muscle of the horse is unknown. It is possible spores are introduced at the time of injection.

Another theory is that bacteria are translocated from their normal environment in the intestine, in times of inflammation or colic and arrive in the muscle via the bloodstream.

No association linking whether or not the injection site is cleaned prior to injection and the development of myositis has been established.

A higher incidence of myositis with irritating substances such as nonsteroidal drugs and vitamins is reported, potentially due to increased tissue damage and creation of an oxygen-free environment.

Diagnosis involves aspirating a small amount of fluid for anaerobic culture and Gram-staining to look for the presence of gram-positive rods. To treat the infection, large incisions are made into the muscle and fascia to expose the bacteria to oxygen and debride dead tissue.

General supportive care is critical because these bacteria produce toxins that have secondary effects on the horse, including the potential to reduce the contractility of the heart.

Clostridial toxins may also result in anemia, thrombocytopenia, and leukopenia. Horses are commonly treated with high doses of intravenous penicillin, intravenous fluids, cardiovascular sup-port, and wound care.

Hyperbaric therapy, where available, is suggested as an adjunct to routine treatment. Survival has been reported to range from 31% to 73% and appears to be better for infections with C. perfringens compared with C. septicum or C. chauvoei.

Horses that survive the initial toxemic stages of disease have an improved prognosis. The wounds created by a combination of infection and treatment are usually large, and may take weeks to months to heal entirely.

Horses which do not survive show signs of intravascular coagulation and multi-organ failure. There is no definitive prevention for clostridial myositis.

When giving intramuscular injections, use large and well vascularized muscle groups, and when possible avoid giving irritating substances in the muscle if there is an alternative route such as oral or intravenous administration.

Note: This still-timely article was first posted on EquiMed in 2019.

Information provided by Diane Furry - University of Kentucky - Equine Disease Quarterly Article: Rebecca Ruby, BVSc (dist), MSc, DACVP

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