Clinical, genetic and morphological characterization of congenital myasthenia gravis in the smooth fox terrier

Clinical muscular and neurological signs are described for 13 Smooth Fox Terriers affected with a congenital form of myasthenia gravis. Treatment with a long-acting cholinesterase inhibitor in six cases resulted in temporary improvement in muscle strength. From examination of the phenotypes of 132 Smooth Fox Terrier dogs from 25 matings, it was concluded that congenital myasthenia gravis in this breed is inherited in an autosomal recessive manner with complete penetrance. The symbol mg for the gene for myasthenia gravis is proposed. It is a lethal trait. Electron microscopy of the neuromuscular junction of affected dogs revealed fewer synaptic vesicles, shorter postsynaptic membrane, smaller postsynaptic membrane area and a lower ratio of post- to presynaptic membrane length than in normal littermates. Affected dogs also had a significant decrease in mean diameter of type I fibres of the biceps femoris and type I and II fibres of the biceps brachii muscles. [See also VB 54, 6559]..