Myositis is an inflammatory disease of the muscles. It can be a serious and painful condition and probably an early indicator of a possible underlying disease.
This type of muscle pathology (myopathy) represents a group of different pathologies characterized by the presence of inflammatory cells in the muscles.
Myositis can affect:
- Muscle groups. For example: the chewing muscles located in the upper and lateral part of the head (myositis of the chewing muscles) or the muscles responsible for the movement of the eyeball (myositis of the extra-ocular muscles) or all the body muscles (polymyositis, dermatomyositis and necrotizing myopathy)
Muscle inflammation (inflammation) can be due to:
- Body response to an infectious agent (parasite or virus) present inside the muscle (infectious myositis).
- Abnormal reaction of the immune system directed against the muscle (immune-mediated myositis). In chewing myositis, the body produces antibodies that attack muscle portions present only in the chewing muscles. The reason for this reaction by the immune system is unknown.
- Myositis can also be associated with cancer. The inflammation that has developed can transform into neoplasia over time (precancerous alteration) or a tumor present in other body sites can trigger an immune reaction to the muscle (paracancerous effect).
Signs and symptoms
The signs caused by myositis can vary significantly depending on the muscles affected.
There myositis of the chewing muscles initially it causes swelling of the muscles located in the upper part of the head. Subsequently, a week or two later, progressive muscle atrophy occurs. The dog usually has difficulty moving his jaw, experiencing pain and problems with taking food or water.
Over time, chewing myositis completely inhibits mandibular mobility, preventing the dog from opening its mouth. The animal’s eyes may appear sunken, due to the contraction of the muscles located behind the eye. Myositis of the muscles around the eye initially causes the eyeball to protrude. It is possible the presence of swelling in the area surrounding the eye associated with deviation of the eyeball and changes in vision.
The dog affected by generalized myositis (polymyositis) has a stiff gait, muscle pain, weakness and is unable to perform physical activity normally. The initial stages of the disease are characterized by generalized muscle swelling and subsequently by muscle atrophy. Other signs associated with polymyositis include regurgitation of water and food, difficulty swallowing and sometimes breathing problems.
Some dog breeds (Shetland Sheepdog, Australian Cattle Dog, Rough Collie) may experience a condition called dermatomyositis, in which skin lesions associated with muscle atrophy, pain or abnormal gait develop.
The diagnosis of myositis can be confirmed by taking and then examining a sample of muscle tissue. The results will show the presence of inflammatory cells in the muscle. Myositis of masticatory muscles can be diagnosed with a simple blood test that measures the level of antibodies directed against the masticatory muscles (test for the identification of 2M antibodies). Other tests may be necessary to eliminate an infectious potential (blood test) or to exclude the presence of a neoplasm in the dog’s body (x-rays / thoracic and abdominal ultrasounds).
In general, the treatment of myositis aims to counteract the hyperactivity of the immune system through the administration of drugs called immunosuppressants. The main treatment usually involves high steroid dosages (prednisolone). In combination with steroids, other immunosuppressive drugs (such as azathioprine, cytarabine, mycophenolate, cyclosporine and cyclophosphamide) can be used. The short-term therapeutic goal is to return the dog to normal by using high dosages of the chosen drug. When the disease is under control, the dosage is gradually reduced (in the hope that no recurrence will occur). The long-term therapeutic goal is to permanently stop the administration of drugs, although this is generally not possible, making it necessary to continuously administer a low dose in order to avoid recurrence of symptoms.
All drugs have side effects and immunosuppressants are particularly potent drugs. The main risk related to the use of these drugs is that the immune system is excessively suppressed, making the dog more susceptible to infections.
In the rare cases where the underlying cause of myositis is a neoplasm or an infection, therapy should aim to treat these conditions. Unfortunately, in these cases the prognosis is somewhat reserved.
The outlook for dogs with myositis is generally good, although several weeks are required to observe improvement. Corticosteroids can cause loss of muscle mass and this can make one think of an aggravation of the condition, even if in reality the disease is under control. The veterinarian will carefully monitor the dog during the entire duration of the treatment, to ensure that the animal’s health improves and verify that the drugs do not cause serious side effects.