If your dog suddenly finds it difficult or painful to take exercise they may have myositis. Myositis is an inflammation of the muscle. It can be a serious and painful condition and may be an early indicator that your pet is ill in some other way. A veterinary examination is important to try to identify a cause of the problem so that appropriate treatment can be given.

Literally, myositis means muscle (myo-) inflammation (-sitis). This type of muscle disease (myopathy) represents a group of different diseases which all share the feature of inflammatory cells within the muscle. Myositis can affect:

  • Just one muscle
  • Groups of muscles, e.g. muscles used in chewing found on the top and side of the head (masticatory muscle myositis), or muscles moving the eyeball (extra-ocular muscle myositis)
  • All muscles in the body (polymyositis, dermatomyositis and necrotising myopathy).

The inflammation in the muscle can be due to:

  • Response of the body to an infectious agent (parasite or virus) within the muscle (infectious myositis).
  • Abnormal immune reaction of the body directed against the muscle (immune-mediated myositis). No-one knows why the immune system suddenly becomes over-excited and attacks the muscles in this way in some animals.
  • Myositis may also be associated with cancer. Inflammation may develop first and turn into cancer with time (pre-cancerous change), or cancer elsewhere in the body may trigger an immune reaction to the muscle (paracancerous effect).

The signs caused by myositis can vary considerably depending on the muscles affected. An animal with a generalised myositis (polymyositis) has a stiff stilted gait, muscle pain, weakness and cannot exercise normally. In the early stages of disease generalised muscle swelling occurs and later the muscles are wasted.

Other signs associated with polymyositis include regurgitation of food and water, difficulty swallowing and sometimes breathing problems. Some breeds of dog (rough coated collies, Shetland sheepdogs, Australian cattle dogs) have a condition called dermatomyositis in which skin lesions develop associated with muscle atrophy, pain or abnormal gait.

Masticatory muscle myositis initially causes swelling of the muscles on the top of the head followed, a week or two later, by progressive muscle wasting (loss of muscles). The dog usually has difficulty moving its jaw, which causes pain and problems when picking up food or drinking. Eventually, masticatory muscle myositis causes the muscles to become fixed so that the dog cannot open its mouth.

The eyes may also appear sunken as muscles behind the eye shrink. Myositis in the muscles around the eye eventually cause protrusion of the eyeball. There may be swelling around the eye, eventually associated with deviation of the eyeball, and impaired vision.

Myositis can be confirmed by taking a sample of muscle tissue for examination. This will show the inflammatory cells within the muscle. Masticatory muscle myositis can be diagnosed by a simple blood test which measures the level of antibody directed against the masticatory muscle (2M antibody titre test). Other tests may be necessary to eliminate a potential infection (blood test) or to rule out the presence of a cancer in the body (chest and abdominal X-rays and ultrasound).

Treatment of myositis is usually aimed at trying to counteract the over-excitation of the immune system by giving drugs to suppress the immune system (immunosuppressives). The main treatment is usually high doses of steroids (prednisolone). Other immunosuppressive drugs (such as azathioprine or cytarabine, mycophenolate, cyclosporin, and cyclophosphamide) can also be used in combination with steroids.

The short-term aim of the treatment is to return the animal to normal using high doses of medication. When the disease is controlled the quantity of drugs is slowly reduced (hopefully without the animal relapsing). The long-term aim is to take the animal off any drugs, but usually this is not possible and a continued low dose of medication is needed to keep the signs at bay.

All drugs can have side-effects and immunosuppressives are particularly powerful drugs. The main risk of using these drugs is that the immune system will be shutdown too much, making your dog more prone to infection.

In the rare cases where cancer or an infection is found to be the underlying cause of the myositis, treatment should be directed against this. Unfortunately, the outlook in these cases is rather guarded.

The outlook for animals with myositis is usually fair – although improvement may not be seen for several weeks. Corticosteroids can cause muscle wastage and this may give the impression that the animal is getting worse, even though the disease is well controlled. Your vet will need to monitor your pet closely whilst it is receiving treatment, both to ensure it is getting better and also to check that the drugs are not causing any serious unwanted effects.

If an underlying cause of the myositis can be identified and successfully treated it may be possible to withdraw medication altogether without the condition relapsing.