A hereditary neuromuscular disorder in dogs.

A disorder characterized by reduced tolerance to exercise has been observed in the breed "Gammel Dansk Honsehund." Results of genealogical studies and test matings suggest an autosomal recessive mode of inheritance. Eighteen interrelated dogs were examined electrophysiologically. Recording of electrical and mechanical responses at low rates of nerve stimulation revealed a normal positive staircase phenomenon. At fast rates of stimulation for 5 minutes there was a facilitation of the electrical and mechanical responses during the first minute, followed by a gradual decrease of both responses in affected dogs but not in controls. After induced fatigue, stimulation at 3 Hz/3 seconds revealed an average decrement of 22%, which resolved within 3 to 4 minutes. Only guanidine had a favorable clinical effect and reduced or normalized the early facilitation. Examination of dog serum against human acetylcholine-receptor antigen showed negative titers. It is most likely that the disorder described here is similar to the congenital human type of myasthenia gravis.