Eosinophilic Myositis in Dogs

Eosinophilic myositis, also called masticatory muscle myositis, is an immune-mediated disease that impacts the masticatory muscles (the big muscles on the top and each side of the head that help open and close the jaw). In eosinophilic myositis, the immune system of the dog will attack the special muscle protein that is present in these muscles. This reaction of the immune system will lead to pain and swollen muscles and as the disease continues the muscles will shrink. Should your dog experience this condition, he will likely feel significant pain when opening his mouth.

Also called masticatory muscle myositis, eosinophilic myositis is an immune-mediated disease where the dog’s immune system will attack the muscle protein of the masticatory muscles, leading to inflammation and pain.

In the acute phase of this condition your dog may demonstrate jaw pain and difficulty opening his mouth. In addition, you may also notice swelling of the muscles on top of your dog’s head; a week or two later your dog may show and continue to experience a progressive loss of muscles. The pain he is experiencing will lead to difficulty eating and drinking.

• Pain
• Swelling of muscles on head
• Difficulty opening mouth
• Muscle atrophy
• Trouble eating
• Trouble drinking

In the chronic phase of the disease, you will observe significant muscle degeneration. After some time, the masticatory muscles will become fixed and the dog won’t be able to open his mouth.

Types 

Dogs can experience several forms of myositis. Aside from eosinophilic myositis, other forms include:

Extra-ocular Myositis - Involves the muscles that are responsible for movement of the eyeball.

Polymyositis - A general myositis, this is thought to be the result of an infectious organism; lesions are often found on the dog’s esophagus, as well as the muscles of his front limbs.

Dermatomyositis - Thought to be hereditary and more likely in Shetland Sheepdogs, Australian Cattle dogs and others with rough coats; in this disease type, skin lesions occur along with muscle atrophy.

Eosinophilic myositis may occur due to the response of your dog’s body to a parasite or virus within his muscle. It can also be the result of an abnormal reaction of his immune system that is directed against his muscles. If your dog is experiencing eosinophilic myositis, his body will produce antibodies that will only impact the masticatory muscles. It is not clear what causes the immune system to attack its own muscles.

In some dogs, eosinophilic myositis will be linked to cancer. The inflammation may occur first and then over time turn into cancer, or cancer somewhere else in your dog’s body can lead to an immune reaction that targets the muscles.

Upon noticing your dog struggling to open his mouth or appearing to be experiencing pain when doing so, it is important that you schedule an appointment with your veterinarian. Your dog will undergo a full physical examination. Should your dog have swollen masticatory muscles and appear to experience pain when opening his jaw, your veterinarian will consider that he may have eosinophilic myositis.

A muscle biopsy will be conducted where your veterinarian can take a closer look at cells through a microscope in order to confirm the diagnosis. Your veterinarian may also use a basic blood test that will show the level of antibodies that are focused against the masticatory muscle (called 2M antibody titer test). Other tests may be requested in order to eliminate other causes of the symptoms that your dog is experiencing.

Your veterinarian will look to suppress your dog’s immune system so that it will stop attacking his muscles. Medications called immunosuppressives will likely be prescribed. High doses of prednisone are usually prescribed first. Other drugs that suppress the immune system, (like azathioprine, cytarabine and cyclosporine), may also be used in conjunction with the steroids. The goal is for the high doses of medication to help your dog return to his usual function. Once the disease is under control, the amount of the medication that your dog takes will be slowly reduced. The slow reduction is to attempt to avoid his relapsing. Ideally your dog will be able to stop taking the medication, though some dogs need to continue on a low dose of medication to keep the disease under control.

Immunosuppressive medications have side effects, one of which is that the immune system may be shut down too much which will make your dog less resistant to infection. Should it be found that the disease is a result of infection, your dog’s treatment will be focused on resolving the infection.

While it may take a few weeks for improvement to be seen, the prognosis for dogs with eosinophilic myositis is fair. Medications like prednisone can cause muscle wasting which may make it seem like your dog’s condition is worsening. You will want to work closely with your veterinarian to ensure that your dog’s condition is improving as well as make sure that the medications he is taking are not causing other issues.

Early recognition of the disease is important so that treatment can begin right away. Without immediate treatment your dog may experience an irreversible loss of function in his jaw and significant atrophy of his jaw muscles.