A muscle disorder of Labrador retrievers characterized by deficiency of type II muscle fibers.

A muscle disorder was detected in 5 Labrador Retrievers. The age of onset was less than 6 months. The clinical signs, which were exaggerated with exercise, cold, or excitement, included abnormal head and neck posture, and a stiff, forced, hopping gait. A marked deficiency of skeletal muscle mass resulted in poor conformation. Both black and yellow Labrador Retrievers were affected, and pedigree studies indicated the disorder may have been inherited as an autosomal recessive trait. Some relief of the clinical signs was afforded by treatment with diazepam. Marked creatinuria and low serum creatine phosphokinase activity were the only consistent, abnormal laboratory findings. Histologic and cytochemical examinations of muscle biopsy specimens revealed a relative predominance of type I and a deficiency of type II skeletal muscle fibers. Electromyographic studies of the affected dogs indicated a myotonic defect.