Gas gangrene is a rare but life-threatening bacterial infection and one of those diseases where prompt veterinary help can mean the difference between life and death for a horse.
Its official name is Clostridial Myonecrosis, but it is commonly referred to as gas gangrene or myonecrosis. Clostridium is the type of bacteria that causes the infection, myo means muscle and necrosis is the death of living muscle tissue.
The Clostridium bacteria
There are many types of Clostridium bacteria. Characteristic is that they are all gram-positive (important to know when the vet examines which bacteria it is and what medication he can use), and spore-forming. These spores are basically the survival mechanism of the cell.
The endospore can exist for years in a dormant state, surviving huge differences in temperature and conditions. They occur in nature, in the soil, but also in the body of some horses. Here they do no damage as long as there is an oxygen-rich environment,
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so in healthy tissue that has good blood flow. If this situation changes in an anaerobic environment (low oxygen), the spore can develop into a cell and multiply very quickly and produce harmful toxins.
How does gas gangrene develop in horses?
Gas gangrene can develop after giving intramuscular (into the muscle) inoculations (vaccinations), or injections with ivermectin, antihistamines, bute, vitamins, or flunixin (the latter drug is often used for mild colic). However, a (stab) wound,
or trauma during or after a difficult delivery, can also be the cause. A study in America (Peek et al. 2003) showed that a proportionally higher number of stallions and Quarter Horses were treated with gas gangrene, and it is thought that this group of horses is at greater risk because of the heavy muscling.
It is unknown in 2020 how Clostridium ends up in a muscle. Perhaps the spores of the bacteria enter with the injection, or if the bacteria were already present in the horse’s body, it arrives in the muscle through the inflammatory process through the blood.
The muscles most commonly affected are those where the most intramuscular injections are generally given: the neck and glutes. However, the infection can also occur in the head or in other places, for example, if a horse has major damage in the mouth (such as mouth ulcers)
What are the symptoms and why is clostridial myonecrosis also called gas gangrene
If myonecrosis develops after an injection, swelling will usually develop within six to 72 hours. This swelling can quickly expand in area and size. The horse will show pain, and gas can often be heard under the skin when touching the swollen spot. This is because the bacterium produces toxins that are accompanied by a build-up of gas in the tissues, which explains the name gas gangrene.
Because of the pain and infection, many horses will be depressed, and will not want to eat and/or drink. There may be a fever, and there may be tachycardia (increased heart rate). If the bacteria originated from a wound in the mouth,
the horse will have difficulty swallowing, one or both eyes may close if the head starts to swell, and the horse may have difficulty breathing. The risk of laminitis is also high with gas gangrene.
Preliminary diagnosis is often based first on the clinical signs, such as swelling of the affected body part, especially if the animal has just had an intramuscular injection or received a deep wound. If gas bubbles are present, they can be heard cracking just under the skin (this can also be tested by ultrasound). An anaerobic culture can also be made with a small amount of liquid and a gram stain to confirm whether this is indeed gas gangrene.
Treatment for gas gangrene will involve a combination of appropriate antibiotics (e.g. penicillin), administering anti-inflammatory drugs and pain relievers, cutting open the muscle and connective tissue where the infection is located to allow gas and fluid to escape (the bacteria cannot survive in an oxygen-rich environment), and the removal of dead tissue.
Typically, the patient will need to be admitted to a veterinary clinic where the animal can receive intensive care (such as an IV). Aside from causing tissue damage, the bacteria produce toxins that can cause secondary problems such as cardiac arrhythmias, anemia, and sepsis.
Provided that treatment is started on time and the owner is willing to accept a long (and expensive!) healing process, the chances of survival are between 31% and 73%. Wound care is extremely important because the wounds caused by opening and removing damaged muscle tissue are large, and healing can take weeks to many months.
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If the bacteria has entered the head, the prognosis is very poor. Cutting open tissue in this area is nearly impossible, and most horses develop major swallowing and breathing problems within 3 or 4 days due to swelling of the head and throat. Euthanasia is then the only option.
Can gas gangrene be prevented?
There is no vaccination against clostridial myonecrosis. In countries where owners are allowed to inject their horses themselves, it is recommended that an alternative route be chosen, if possible, intravenously or orally.
After injection or incurring a wound, the horse should be closely monitored and in case of swelling and/or fever the owner should immediately contact the vet to have this examined. It may be harmless, but if it turns out to be gas gangrene, prompt medical attention may still save the horse.