Characterization of inherited canine myasthenia gravis

Twenty-three smooth fox terriers with an inherited form of myasthenia gravis were studied. All dogs developed muscle weakness between 4 and 8 weeks of age. This weakness increased in severity with exercise and age. Muscle weakness was alleviated by the administration of a short acting cholinesterase inhibitor (edrophonium chloride). Electrophysiological examination revealed a decremental response to repetitive nerve stimulation, which decreased with anticholinesterase administration. Acetylcholine receptors (AChR) were assayed in nonaffected and affected dogs by extracting the AChR from intercostal muscle and labelling with an excess of 125I- alpha bungarotoxin and quantitating with immunoprecipitation. Affected dogs had only 23% AChR of the normal littermate controls. Anti-AChR antibodies were not detected in the serum of any of the dogs. Retrospective and prospective studies suggest an autosomal recessive mode of inheritance for this trait. Histologically the disease was characterized by atrophy of both type I and II muscle fibres. Ultrastructural abnormalities of the neuromuscular junction were not apparent..