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Intramuscular Injection Complications: Clostridial Myonecrosis

Many horse owners are proficient at giving intramuscular injections, but few are aware of a life-threatening complication that can develop. The doctors at Mid-Rivers Equine Centre would like to make you aware of a serious, but rare, infection, clostridial myonecrosis (muscle death), which can occur after a horse has received an intramuscular injection. This infection can be gruesome and life-threatening. It attacks the muscle tissue voraciously as it spreads through living tissue at a rapid pace. Clostridial bacteria are everywhere in our environment; they live in the soil and thrive in areas with little to no oxygen. Diseases caused by clostridial bacteria include botulism, tetanus, and black leg (cattle). These bacteria produce spores that remain dormant, sometimes for years, until they arrive in a warm, moist, low-oxygen environment. This is the case with a deep puncture wound that allows tetanus to develop. This also explains why tetanus is rarely seen with large wounds which are exposed to the air. With deep puncture wounds (for example, when a horse steps on a nail), the wound quickly seals and effectively blocks the oxygen out. That is when the bacteria begin to grow.

Also known as "gas gangrene," clostridial myonecrosis can occur when a medication is given deep into the muscle. It is most commonly associated with the administration of large (over 5cc) volumes of medications which are irritating to the tissues. However, cases of clostridial myonecrosis have also been seen following vaccinations. The drugs most commonly associated with the disease in horses include ivermectin (given intramuscularly), antihistamines, and flunixin meglumine (the most well-known brand of this is Banamine). It is unknown if the clostridial spores are introduced with the injection or if they are already present in the muscle (studies have shown that clostridial spores can be found in the muscle of some horses). After the injection, though, the tissue irritation causes areas of decreased oxygen levels, and the clostridial bacteria begin to grow.

Beau, A Clostridial Myonecrosis Patient

Photos of Clostridial Myonecrosis

We are sharing photos of our patient, Beau (with his owner's permission), because we believe it is the best way to demonstrate how devastating clostridial myonecrosis is. Beau spent eight days at Mid-Rivers Equine Centre, followed by receiving another nine months of at-home care before his wounds were completely healed. Although he has now recovered, the process came at a great cost emotionally, financially, as well as physically.

The road to recovery under this massive amount of trauma is long and arduous. It was not without its ups and downs. Beau lost a vast amount of muscle tissue and skin from his neck and had great difficulty holding his head up for any length of time. Additionally, he suffered from occasional bouts of depression and colic.

His at-home care regimen included cleaning of this enormous wound twice daily. "It was so hard for me to look at the wound that I had to ask my husband to clean it," said Beau's mom, Carla Emert. Cleaning consisted of scrubbing the wound; flushing it; checking carefully for fly strikes; applying silver sulfadiazine cream to the wound; applying petroleum jelly beneath the wound to keep drainage from sticking; and applying a fly ointment around the area. Carla stated that her husband, "would spend forty-five minutes twice a day cleaning and dressing the wound."

While Carla and her husband do not at all regret taking the hard road to save Beau, there was a time or two when she wondered if whe was doing the right thing. It was painful to see him through the recovery. Today he is back to his "mischievous and prankster" self, said Carla.

Today, Beau deals well with his previous challenges. His owners built a ledge around the inside of his stall for Beau to rest his head when his neck gets tired. When his veterinarian, Dr. Hoover, talks about him, she always has a smile on her face. The same cannot be said for Beau. "One thing Beau learned from all of this is that he does not like seeing Dr. Hoover," Carla laughs.

Warning Signs

A sign of impending trouble following an intramuscular injection is rapid, painful swelling of the muscle at or around the injection site, which may appear as quickly as a few hours after the injection or may take as long as two or three days. In Beau's case, his neck became sore and swollen within a couple of days. Additionally, you may notice a crackling sensation that you can hear or feel when you push on the infected area. That is gas moving beneath the tissue. If you suspect your horse may have clostridial myonecrosis, you should contact your veterinarian immediately. Time is of the essence. Your veterinarian can use an ultrasound to look for pockets of gas or fluid around the swelling. Many horses develop swelling after vaccination or other intramuscular injections, but the ultrasound appearance of this disease is a bit different than it would be for a more routine swelling.


The clostridial bacteria thrive in an oxygen-free environment. Therefore, in order to stop the infection quickly, a veterinarian must fenestrate (cut open) the infected area, to allow oxygen to contact the affected tissue. Ultrasound evaluation helps in making the decision of where to cut. The veterinarian will err on the side caution by cutting too much rather than not enough, as this infection can spread amazingly quickly when not treated. Dead tissue must be removed, and the area around the wound must be closely monitored for any sign of spread of the infection. Often, oxygen is infused into the tissue in order to kill the bacteria. Because the wound must be exposed to air, the wound cannot be closed with sutures or bandages. In some cases, massive amounts of tissue may be lost. The resulting wound must be thoroughly cleaned daily. The horse will also be receiving antibiotics and pain medications in the initial phase of the long recovery process. The overall survival rate of this procedure has been recorded to be as high as seventy-three percent, much higher than had been documented before 2000.

Below is an example of a horse that had an injection site reaction that resulted in clostridial myonecrosis. A needle has been inserted into the muscle allowing the gas, which is produced by the infection, to be released.

Do Not Wait To Get Help

It is important to understand that this infection spreads amazingly rapidly, destroying tissue as it goes. Sometimes delaying aggressive treatment by hours can be the difference between the horse surviving or not. When in doubt, call your veterinarian. If Beau's owners had waited hours longer, he may have needed to be euthanized.

If you do administer an injection intramuscularly, be aware that any swelling or pain at the injection site in the following days may be a red flag and could represent a true emergency. Contact your veterinarian immediately.


Clostridial myonecrosis is difficult to prevent. Horse owners often ask if wiping the skin with alcohol before administering an injection will help prevent the infection. While it is always better to inject through a cleaner site, alcohol will not kill the spores, and as research has shown, the spores may already be in the muscle prior to the injection.

It is better to minimize the number of intramuscular injections. If giving flunixin (Banamine) orally is an option, that would be better than giving it intramuscularly. It is usually well-absorbed orally and takes effect in about ninety minutes (versus forty-five to sixty for intramuscular injection). It does frequently have a bad taste, but there are many creative ways to try to disguise that.

January 05, 2014

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