Masticatory muscle myositis is an inflammation of the muscles used for chewing: masseter, temporal and pterygoid muscles. It particularly affects dogs of the German Shepherd and Cavalier King Charles breed. Other breeds can affect, including large dogs such as Boxers, Goldens retriever or Weimaraner for example. Relatively infrequent, this disease is however to be known because it can be very serious for the animal and engage its vital prognosis. Here, we tell you about Myositis in dogs.
Causes of chewing muscle myositis in dogs
This disease is of autoimmune origin, that is, it is the body’s own antibodies that turn against it. Masticatory muscles consists of special muscle fibers, called 2M, which are present only in these muscles. This is why they are the only ones affected.
In predisposed breeds, a genetic origin probably involves, but the responsible gene did not yet identified. Thus, it is currently impossible to detect carrier animals.
Symptoms of chewing muscle myositis in dogs
Symptoms usually appear in young adults. However, juvenile forms exist, especially in Cavalier King Charles, then appearing from the age of 10 to 12 weeks.
Two forms can distinguish: an acute form and a chronic form.
- In the acute form, the onset of symptoms is sudden. The dog exhibits swelling of the chewing muscles and pain on palpation. The mouth remains closed completely or partially, opening is impossible and causes severe pain. This leads to difficulty in eating, or even total anorexia. The animal may complain when it tries to eat or grab a toy with the mouth. He depresses and may have a fever. He may also drool because he is unable to swallow his saliva. The local lymph nodes usually enlarge, and if the muscles are very swollen, it can lead to exophthalmos, which means the eyes protrude slightly from their sockets. The eyelids cannot then close completely and ophthalmological symptoms appear: tearing,
- In the chronic form, the symptoms are less clear. This form can succeed the acute form if no treatment has put in place. The masticatory muscles are then atrophied because the attacked muscle fibers die and become fibrotic. This state is then irreversible. The opening of the mouth will restrict but the pain is less severe. The masticatory muscles then taking up less space, the opposite phenomenon can be observed at eye level, that is to say enophthalmos. The eyes indeed seem a little more retracted in their sockets, unlike the exophthalmos.
Diagnosis of masticatory muscle myositis in dogs
The diagnosis is based on a detailed clinical and neurological examination of the animal. The symptoms are often quite suggestive, although they are more progressive in the chronic form. Additional examinations will then be necessary.
A blood test makes it possible in particular to highlight signs of myositis. A specific dosage of anti-2M fiber antibodies exists, but it may happen that this is normal if the animal has already received treatments.
Biopsies of the masticatory muscles with histological analysis in the laboratory usually confirm the diagnosis.
A CT scan or MRI of the head may recommend. This helps identify the affected muscles. It also helps rule out other causes of symptoms, such as a broken jaw, or the presence of a tumor.
Treatment and prognosis
Treatment is based on the administration of immunomodulatory therapy to prevent antibodies from destroying muscle fibers. In the first line, corticosteroids generally use. They can be given in injectable form, especially at the beginning if taking oral therapy is not possible.
If the animal cannot open its mouth at all, it may be necessary to feed and hydrate it with a syringe or through a probe. Hospitalization may be recommended if the condition of the animal warrants it and if infusions are necessary.
The earlier the treatment is started, the better the prognosis.
Indeed, if the animal can no longer drink and eat, its condition can quickly deteriorate and its vital prognosis be engaged. In addition, as long as the muscle fibers are alive, they can be conserved and remain functional. On the other hand, once fibrosis, they can never be recovered. Treatment should therefore start as soon as possible. It can usually give over several months, gradually reducing the dose, until eventually considering stopping it. However, relapses can then occur and treatment can give for life, seeking the minimum effective dose. In some cases, other immunomodulatory treatments may use.
Unfortunately, there is currently no means of prevention or genetic screening. The best thing is to contact your veterinarian immediately as soon as any abnormal symptoms appear. He can then advise you and tell you what to do next.