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CANINE MASTICATORY MYOPATHY

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Eosinophilic Myositis
The term "Eosinophilic" means the presence of eosinophils, a type of white blood cell within the affected area.  "Myositis" means inflammation of muscle fibres.  In its acute form, it is characterised by the rapid accumulation of large numbers of these cells within the muscles, causing them to be swollen and painful.  If the muscles associated with chewing are affected it makes it very painful for the dog to eat.  Sometimes the swelling can be so severe that the eyes bulge or the mouth is prevented from opening more than several centimetres.  Once the muscle fibres have been damaged, there is little oportunity for them to repair themselves so they are replaced by scar tissue which is inelastic and makes it even more difficult for the dog to open its mouth.

It is unknown what starts this process, but it is generally accepted that the immune system is dysfuntional and starts to attack the muscles with antibodies.  The limb muscles are generally not affected at the same time as the facial muscles because they have a different structure.  Sufferers of this condition can be generally quite weak as they are undernourished due to the inabilty to open the mouth to eat.

As soon as possible it is important to limit the damage to the muscle fibres.  This is achieved by using steroids which are given for short periods of time, during attacks only, or on a lowest dose, semi continuous basis to control symptoms.
source:Vin McNally BVSC MRCVS


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What is masticatory muscle myositis?

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related term: eosinophilic myositis
This disease affects the masticatory muscles - the large muscles on the top and sides of the head which act to close the jaw. These muscles have a unique muscle protein composition. Masticatory muscle myositis is an immune-mediated disease in which the body’s immune system attacks the unique muscle protein found in the masticatory muscles. This causes the muscles to become painful and swollen initially, and the muscles will atrophy as the disease progresses. Opening the mouth is particularly painful.

How is masticatory muscle myositis inherited?
Unknown.

What breeds are affected by masticatory muscle myositis?
The German shepherd dog is most frequently affected, but this disease is also reported in other large breed dogs, including the English pointer and springer spaniel.

For many breeds and many disorders, the studies to determine the mode of inheritance or the frequency in the breed have not been carried out, or are inconclusive. We have listed breeds for which there is a consensus among those investigating in this field and among veterinary practitioners, that the condition is significant in this breed.

What does masticatory muscle myositis mean to your dog & you?
Many dogs affected with masticatory muscle myositis will improve with drug therapy.

How is masticatory muscle myositis diagnosed?
Your veterinarian will suspect this disease if your dog has painful, swollen masticatory muscles, with pain on opening the jaw. The diagnosis can be confirmed with a muscle biopsy to examine the cells with a microscope. Special techniques may be used to test for specific antibodies.

How is masticatory muscle myositis treated?
Affected dogs are usually treated medically, with glucocorticoids such as prednisone. Most dogs improve with drug therapy, although the treatment may be long-term.

Breeding advice
Affected animals should not be bred.

FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.

Resources
Kornegay JN. 1995. Disorders of the skeletal muscles. In EJ Ettinger and EC Feldman (eds) Textbook of Veterinary Internal Medicine, p.727-736. WB Saunders Co., Toronto.
Ackerman L. 1999. The Genetic Connection: A Guide to Health Problems in Purebred Dogs, p. 121. AAHA Press,Lakewood, Colorado.
Copyright © 1998 Canine Inherited Disorders Database. All rights reserved.Revised: May 16, 2002.
This database is a joint initiative of the Sir James Dunn Animal Welfare Centre at the Atlantic Veterinary College, University of Prince Edward Island, and the Canadian Veterinary Medical Association.
reprinted with kind permission from:-
 Alice Crook, DVM,Coordinator, Sir James Dunn Animal Welfare Centre, Atlantic Veterinary College,University of Prince Edward Island, 550 University Ave.Charlottetown, PEI C1A 4P3
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Masticatory Muscle Myositis
Masicatory Muscle Myositis


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The above information is simply informational. It's intent is not to replace the advice of a veterinarian nor to assist you in making a diagnosis of your pet. Please consult with your own veterinarian for confirmation of any diagnosis. Your pets life may depend on it.