Gas gangrene is a potentially serious, necrotizing disease, caused by an infection of the subcutaneous tissue and muscle. It is caused by a group of bacteria called Clostridia. There are approximately 100 different species of Clostridia, but the most commonly implicated is Clostridium perfringens. Gas gangrene is sometimes referred to as myonecrosis or myositis, or malignant oedema, although very similar, there are slight differences between clostridial myonecrosis and malignant oedema, with malignant oedema generally confined to subcutaneous tissue alone, rather than the muscle. 

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What causes it?

Clostridial myositis develops when spores of bacteria known as Clostridia are introduced to a site in the body, without oxygen, such as an area of damaged muscle. Some theories exist as to the source of the spores, some suggest they are from the soil, others suggest they may come from the gut. These bacteria grow in damaged tissue and produce toxins which cause further damage. These toxins are then released into the body causing severe and potentially fatal effects such as circulatory collapse and organ failure. Animals suffering from this condition may be found dead, such is the speed of progression. The fatality rate can be between 30 and 50%.

The most common cause of this condition is infection following an intramuscular injection with a non-antibiotic, irritant substance, or an intravenous injection if some of the substance being injected leaks out into the surrounding tissue. It can also be associated with puncture wounds or wounds involving significant tissue trauma. Clinical signs are usually seen 6-72 hrs following an injection. In the early stages, swelling with heat and pain are seen around the site of the injection or wound. 

Occasionally horses will have mild, transient swelling with heat and pain around an injection site, usually following a vaccine. This usually responds well to medication with non-steroidal anti-inflammatories, and supportive care, like hot packing the region and rarely develops into gas gangrene but should be carefully monitored just in case. 

The symptoms of gas gangrene

In cases with Clostridial infection, signs will rapidly progress. Bubbles of air start to form under the skin, which can feel crunchy or bubbly when palpated (a bit like rice crispies under the skin – Ed.). If this subcutaneous crepitus is noticed, you should call your vet immediately. Affected animals are often stiff and reluctant to move. Your vet may diagnose the condition based on history and clinical signs, in combination with changes seen on bloodwork. Ultrasonography is also used to aid diagnosis; but the most definitive answer is usually found by culture of fluid from the site. 

Once the disease is recognised, prompt action is needed to try to treat it

As the bacteria like living in conditions with no oxygen, one of the first steps taken is to open the site of swelling to expose the bacteria to oxygen. Any dead, necrotic tissue should be removed, a process which unfortunately does sometimes involve the removal of substantial portions of muscle and tissue. Surgical resection may need to be repeated over a period of days as the disease progresses. Antibiotics are warranted, and penicillin is considered the antibiotic of choice. Advanced cases may suffer from head swelling, if they cannot lift their head due to muscle weakness. This swelling may become severe enough to restrict breathing, in which case a temporary tracheotomy tube may be needed.

The prognosis for survival in these cases is poor

Some species of Clostridia lead to a poorer prognosis than others, for example, infection with Clostridium septicum is associated with a 50% survival rate, while up to 70% of cases with Clostridium perfringens will survive. Other factors linked to survival include the speed of diagnosis and treatment, with those undergoing prompt intervention with antibiotic therapy and wound debridement most likely to survive. The wounds need to be lavaged daily and may take weeks to months to resolve. During treatment, horses will also need pain relief, usually in the form of non-steroidal anti-inflammatories or opioids and fluid therapy in combination with intensive nursing to support and encourage the horse to eat and drink. 

It should be emphasised that this is an exceptionally rare condition

Reports in the literature are sporadic, evidencing the very low incidence of the disease. Other, far more common, but still infrequent adverse events following an injection include a transient muscle swelling, stiffness or abscessation. The risk is so small that it would not preclude the administration of a drug by intramuscular or intravenous injection. One study investigated the risk of infection in cases where skin was disinfected before injection and those where disinfection was not used. There was no significant statistical difference, likewise, clipping or not clipping the hair is unlikely to change the risk of gas gangrene. 

In all cases, following injection or trauma, it is very good practice to observe your horse’s behaviour and demeanour as well as monitoring any injection site or injured area for heat, pain or swelling and in particular subcutaneous oedema or crepitus.  If you are at all concerned, please speak to your veterinary surgeon as soon as possible and they will be able to offer advice or an examination to rule out any potential problems. 

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