Postmortem findings in four litters of dogs with familial canine dermatomyositis.

Postmortem evaluations were performed on 20 juvenile to young adult collie and collie-Labrador retriever crossbred dogs with dermatomyositis and 10 neonatal collies. Cutaneous, muscular, and vascular lesions were present in the juvenile and adult dogs and were most severe in areas of the head and distal extremities. In more severely affected dogs, lesions were more generalized, including myositis of esophageal muscle and arteritis of skin, muscle, bladder, and spermatic cord. Although viruses were not isolated from muscle, crystalline viral-like structures were present in cytoplasm of endothelial cells within skeletal muscle. The dogs with dermatitis and myositis consistently had lymphoid hyperplasia, especially of peripheral lymph nodes. More severely affected dogs were smaller than less severely affected littermates, and the more severely affected males had reduced weight of testicles and prostate glands, compared with body weight. The reduced weight of genital organs correlated positively with reduced fertility. A few lymphoid aggregates were present in or around thyroid glands of 6 of the 20 dogs. There was no histologic evidence of glomerular disease in any of the dogs. The neonatal collies had no evidence of dermatomyositis.