and Susan E. Aiello, DVM, ELS
Myasthenia gravis is a disease of the nervous system that occurs in dogs and (rarely) cats.
Myasthenia gravis in dogs can be present at birth (ie, congenital), but is usually acquired in adult dogs. The most commonly affected breeds include German Shepherd dogs and retrievers. The most common cause of myasthenia gravis in dogs is an autoimmune condition in which antibodies in the blood attack receptors for a chemical called acetylcholine, which transmits nervous impulses to the muscles.
Dogs affected with myasthenia gravis typically show up with stiffness, shaking, and weakness after exercise.
The signs usually go away with rest. The muscles in the head and throat may also be affected, leading to a slack appearance in the face and difficulty swallowing. Some dogs will spit up their food and may even aspirate some into the wind pipe, which can cause pneumonia.
Diagnosis is based on signs of illness and several clinical tests. Your veterinarian may give your dog a shot of a chemical similar to acetylcholine, to see if signs resolve. A positive response to this test is suggestive of myasthenia gravis, but a definite diagnosis of acquired disease involves checking for specific antibodies in the blood. Diagnosis of congenital cases requires a muscle biopsy.
Treatment involves daily administration of drugs that replace the missing acetylcholine.
Your vet may also prescribe high doses of corticosteroids to suppress the autoimmune response. Prognosis is generally good, with many cases clearing up on their own. Dogs that develop pneumonia do not tend to do as well.