Dermatomyositis (and its related disorder, ulcerative dermatosis) occurs when a dog's skin and underlying muscles become inflamed. The first sign of the disease is a series of lesions that appear on the skin by the time the dog is six months of age. In many cases, muscle problems also develop later on. In dogs the condition is similar to that of humans, producing blisters and crusting that occur mainly in the groin and underarm regions.

Lesions caused by dermatomyositis can also appear on the dog's face, especially the muzzle, around the eyes and the tips of the ears. Other locations where they may be found are the tip of the tail, the toes and any bony prominences, such as the dog's elbows. The lesions usually appear in the form of blisters or small bumps, with reddening and hair loss at the affected sites. The blisters then form crusts, and over time the affected skin will scar and the hair loss become permanent.

In some cases the dog's muscles also may be affected. In these instances puppies will grow slowly and appear weak and lethargic. The muscles of their face and head also may appear smaller than normal, due to muscle deterioration. Dogs with the most severe cases may have difficulty chewing and swallowing, and their leg muscles also may deteriorate.

Dermatomyositis is seen primarily in young Collies, Shetland Sheepdogs and crosses of one or the other, and it's believed to be caused by an inherited defect in the immune system. Ulcerative dermatosis, meanwhile, is a rarer problem and usually occurs in middle-aged dogs of these same breeds. However, these conditions also have been diagnosed in breeds including the Australian Cattle Dog, Chow Chow, German Shepherd, Kuvasz and the Pembroke Welsh Corgi. For reasons not yet understood, Shelties experience fewer muscle inflammation complications than other affected breeds. Whatever the breed, even mildly affected animals should not be bred, as they often pass along a more deadly form of the disease to their offspring.

Dermatomyositis is inherited through a dominant gene, meaning that if either parent carries the gene, then all its puppies are susceptible, even though not all will develop the disease. Outside factors such as the strength of the animal's immune system, and possibly viral infections, also can influence its development.

The outcome of dermatomyositis varies widely. Animals with a mild case often outgrow the condition by the time they're a year old and are left with only a few scars on the face or legs. The condition is aggravated by sunlight, and many animals with a mild case benefit from being kept out of the sun and/or the regular use of sunscreen.

At the other extreme, animals with a severe case of dermatomyositis can sometimes be helped by corticosteroid medications. However, most eventually deteriorate to the point that the animal must be put to sleep.