Wounds major cause of malignant edema

Many readers may be familiar with blackleg, the disease that affects cattle, sheep and goats in this part of the world. Caused by a resilient, spore-forming bacteria, blackleg can be prevented through vaccination.

The clostridial group of bacteria can also cause significant illness in horses. The first, which I will discuss below, affects the muscles, while the second, which I will cover in my next column, is an intestinal disease.

The equivalent disease in horses to blackleg is called malignant edema. It is typically caused by Clostridium septicum, which is a different species from that which causes blackleg, Clostridium chauvoei, although other types of Clostridium can be involved. Horses acquire the disease in a different manner than cattle. Rather than external muscle trauma from mixing or chute handling, horses get the disease most often through penetrating wounds.

Usually this is caused by people injecting material purposefully into muscle bellies such as vaccinations, pain medication and antibiotics. Many cases occur following treatment for a different condition such as colic. Intravenous injections that accidentally leak outside the vein may also create enough damaged tissue to start the germination of clostridial spores. Puncture and laceration wounds from sticks, wire and other sharp objects may also cause the disease.

With any injection, it is inevitable that there is some degree of tissue damage. This is why we tend to selectively inject our food-producing animals in the least desirable cuts of meat.

In horses, an injection or puncture wound creates the ideal oxygen-free environment for the clostridial bacteria to germinate.

Once started, it sets off a vicious cycle. The activated bacteria release nasty toxins, creating more tissue and blood vessel damage. This creates an even larger area of oxygen-free tissue for the bacteria to invade. These bacteria also produce a significant amount of gas, so the tissue actually has small air bubbles. Left untreated, the muscles swell and die, and can ultimately lead to death of the horse.

Horses affected by malignant edema may be extremely lethargic, run a fever and may have swelling over the site of infection. The affected muscle turns dark red to brown and the area is extremely painful. Death can occur in less than 48 hours if untreated.

Diagnosis is based on the history of injection or wound to the area, presence of clinical signs suggestive of the disease and identification of Clostridial bacteria in the affected muscle tissue.

It is critical to use good hygiene whenever administering injections to horses to minimize introduced bacteria and tissue damage. Never share needles and avoid injecting into obviously dirty skin. It is also important to avoid injecting medications into muscles that are strictly meant for intravenous injections.

Another important point for prevention is to inject horses in locations that are easily drained and treated should malignant edema occur. This means preferentially injecting in the large muscles of the neck, pectorals and semitendinosis (the “butt cheek” muscles of the hind end) rather than up on the loins. Clostridial infection of those upper hip muscles are incredibly difficult to treat because the draining infection travels by gravity down into the deeper hip muscles and the legs.

If a horse develops this condition, treatment is among one of the most barbarous looking solutions in veterinary medicine. Under sedation and pain control, the skin and affected muscles are cut into with a sharp scalpel blade. These wounds are left open to expose the infected muscle to oxygen in the air and allow for draining. Antibiotics are also used in treatment. Complications include scar tissue formation from the fillet muscle healing, systemic toxemia and antibiotic-associated diarrhea. Horses affected are hospitalized, which can be prohibitively expensive.

Euthanasia is a consideration in many cases. But the disease isn’t necessarily a death sentence — one study found that more than 80 percent of horses survived with prompt and aggressive treatment.

Dr. Jamie Rothenburger, DVM, MVetSc,PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. Twitter: @JRothenburger

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