Canine Familial Dermatomyositis

Canine familial dermatomyositis (DM), has been seen primarily in the Collie, Shetland Sheepdog and their related croosbreeds. DM-like disease has been reported occasionally in the Chow Chow, Beauceron, Welsh Corgi, Lakeland Terrier, German Shepherd Dog, Miniature Schnauzer, Miniature Dachshund, and Kuvasz. Canine familial dermatomyositis is a heritable inflammatory disease affecting skin and muscle in a breed at known increased risk. The cause of this disorder is unknown. Autoimmune and viral factors in conjunction with hereditary predisposition have been suggested for DM in humans.

Signs of Dermatomyositis

Signs include hair loss with crusting and pigmentary skin changes. Animals first begin to show signs at about 12 weeks of age which may look like scrapes on the face, ears, elbows, hocks and other friction points. In German Shepherds and Welsh Corgis paw pads can also be affected which become swollen. In chronic cases, scarring of the skin occurs. Lesions occur over bony prominences due to susceptibility to trauma. Unless skin ulcers develop, the disease does not cause much pain or itchiness. Extensive exposure to sun may trigger aggravation of the disease. At the early stage of the disease, muscles are involved only lightly. Severely affected dogs show growth retardation, enlarged esophagus, lameness, and widespread muscle wasting (atrophy), and have difficulty in seizing or grasping or otherwise getting food into the mouth. Infertility is an additional feature of DM.

The condition is inherited as an autosomal dominant trait. This means that if one parent is affected, most of the puppies will be affected. This is an important consideration, since parents may be mildly affected themselves but should be closely scrutinized if they produce affected young. There is also speculation that a virus may be involved in the condition, since virus-like particles are occasionally observed in biopsies. It is thought that a virus could induce clinical signs typical of the disease in genetically predisposed dogs.

There is no blood test to identify carriers, and not enough data have been collected to permit pedigree analysis to select good breeding stock.

Diagnosis of Dermatomyositis

Because clinical signs of DM are often similar to those of demodicosis, dermatophytosis, facial pyoderma, and discoid lupus erythematosus, the disease is difficult to diagnose. Skin scrapings are usually performed to rule out demodicosis, and fungal culture is performed to rule out dermatophytosis. If small number of Demodex mites are found on the scrapings, skin biopsy is performed. Most cases are diagnosed on the basis of history, clinical signs, electromyography and biopsy. Well-chosen biopsies often reveal characteristic changes and electromyographic (EMG) studies may show abnormalities in the muscles as well.

Treatment of Dermatomyositis

There is no cure for dermatomyositis. Therapy is only symptomatic. The inability to diagnose dermatomyositis before clinical symptoms ensue has made control of the disease difficult. Both vitamin E and corticosteroids have been used to relieve scaling and scarring but neither will cure the condition. one study, dogs were treated with pentoxifylline. Four of the 10 dogs with familial canine dermatomyositis responded to treatment, and six dogs had a partial recovery.

Recently researchers have also tried to identify the gene responsible for this hereditary disease. Identification of a genetic marker that will indicate dermatomyositis would facilitate the development of a DNA-based test for the early detection of affected dogs.

Some animals may recover spontaneously. Recovered animals should definitely not be bred, since offspring will undoubtedly be affected, at least to some extent. In Collies and Shetland Sheepdogs the disease often presents as a severe autoimmune dermatitis of the ears, face, lips, tip of the tail, and over bony prominences of limbs with alopecia and masticatory myositis.

References

1. Peter J. Ihrke, Emily J. Walder, Verena K. Affolter, Thelma Lee Gross. Skin Diseases of the Dog and Cat
2. Linkage of dermatomyositis in the Shetland Sheepdog to chromosome 35.Clark LA, Credille KM, Murphy KE, Rees CA. Department of Pathobiology, College of Veterinary Medicine and Biomedical Sciences, Texas A+M University, USA
3. Rees CA, Boothe DM. Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, Texas A and M University, USA. Therapeutic response to pentoxifylline and its active metabolites in dogs with familial canine dermatomyositis