Familial canine dermatomyositis is a hereditary, idiopathic inflammatory condition of the skin and musscles of young collies, Shetland sheepdogs, and their crosses.  It has also been reported in the Welsh corgi, Chow Chow, German shepherd dog, and Kuvasz and recognized in other purebred dogs.  The familial basis in other breeds is unproved.

CAUSE AND PATHOGENESIS

The cause of dermatomyositis in humans or dogs is unknown.  An immune-mediated pathogenesis is suspected because of detectable immunologic abnormalities, but it is unclear whether this immunologic reaction causes all of the changes seen or is in response to some pre-existing muscle or skin damage.  Although dermatomyositis could be induced by drugs, infections (especially viral ones), toxins, or internal malignancies, their causal relationship in dermatomyositis remains unproved.

A familial history is rare in humans, but common in collies and Shetland sheepdogs.  Breeding studies in collies support an autosomal dominant mode of inheritance with variable expressivity.  Limited studies in the Shetland sheepdog conducted by one author (WHM) suggest a similar mode of inheritance in this breed.

CLINICAL FEATURES

In collies and Shetland sheepdogs, there is no sex, coat color, or coat length association with dermatomyositis.  This may or may not be true for other breeds.  Because of the familial predisposition, lesions occur early in life, typically before 6 months of age.  Signs in some dogs appear as early as 7 to 11 weeks of age.  The progression of lesions is variable.  Some mildly affected dogs have few lesions, which heal rapidly without scarring.  Most dogs have new lesions after the first ones are recognized, but the rate of progression is variable.  The extent of the skin lesions is known by one year of age.  Unless management changes occur, lesions usually decrease in number and severity from that point on.

Skin lesions occur in areas of mechanical trauma and are commonly seen on the face (see Fig. 11:22C and D), especially around the eyes (Fig. 11:22E); on the tips of the ears; on caral and tarsal regions (Fig. 11:22F); on the digits; and on the tip of the tail (Fig. 11:22G).  Oral and footpad lesions can be seen but are rare.  Although intack vesicles can be seen in some dogs, primary lesions are usually absent.  Typical skin lesions are characterized by alopecia, erythema, scaling, and mild crusting.  Ulceration can be seen in severely affected dogs, large areas of normal skin remain, whereas severely affected dogs have involvement of the entire face, distal limbs, and tail.  Some dogs have long, soft claws.

The myositits typically occurs after the skin lesions are recognized and correlated with the severity of the skin lesions.  Mildly affected dogs have no clinical muscle disease and convincing evidence of this may not be found on EMG testing or muscle biopsy.  These cases may have epidermolyisis bullosa, dermatomyositis with focal but undetected myotitis, or dermatomyositis without the myositis.  In humans, the diagnosis of amyopathic dermatomyositis can be made only when no muscle changes are detected for 4 years or longer after the skin lesions have occurred.  To the authors’ knowledge, this type of follow-up testing has not been done in dogs.  The rare dog related to dogs with classic dermatomyositis has EMG changes of myositis with no skin lesions.  The significance of these findings is unknown.

Clinical signs of myositis are variable.  A common finding is a dirty water bowl that contains food particles.  These dogs do not have trouble chewing their food but do not swallowit completely, so residual pieces are washed from the mouth during drinking.  Some dogs have a peculiar high-stepping gait.  Severely affected dogs drink, chew, and swallow with difficulty; have a stiff gait; have megaesophagus; and often have secondary aspiration pneumonia.  The most common sign of the mypositis is asymptomatic atrophy, especially of the muscles of mastication and distal limbs.

The rare dog has skin lesions only in adulthood.  The lesions can be the classic superficial lesions of dermatomyositis or be more deeply ulcerated (see Idiopathic Ulcerative Dermatomyositis in Shetland Sheepdogs and Collies in this chapter).  These cases could represent an adult-onset variant of the diseases or be dogs who had mild, unrecognized disease as puppies.

DIAGNOSIS

The differential diagnostic considerations should include demodicosis, staphylococcal folliculitis, dermatophytosis, discoid lupus erythematosus, and epidermolyisis bullosa.  The latter might be consdered if there are no muscle signs or lesions and if vesicles are present.

Diagnosis is made by history, physical examination findings, biopsy of affected skin and muscle, EMG, and laboratory tests to rule out other conditions.  Biopsy of affected skin shows scattered vacuolar change of the surface and follicular basal cells (Fig. 11:23).  Occasional apoptotic basal cells (Civatt’s bodies) may be seen.  With confluent hydropic change, intrabasal or subepidermal clefting may be seen (Fig. 11:24).  Dermal inflammation can be absent.  Most cases show a mild perivascular to interstitial dermatitis in which lymphocytes, plasma cells and histiocytes predominate.  Mild pigmentary incontinence may be present in the superficial dermis.  Follicular atrophy and fibrosis are common findings (Figs 11:25 and 11:26).  Vasculitis may occasionally be seen in the skin.  Muscle biopsy may show mixed inflammatory exudates, accompanied by muscle fiber necrosis and atrophy.  In some cases, a vasculitis may be found.  Needle EMG abnormalities include positive sharp waves and fibrillation potentials in muscles of the head and of distal extremities.

Hemograms and serum chemistry profiles are usually unremarkable.  Creatine kinase levels are normal or slightly increased.  Neurologic examination and nerve conduction studies are usually normal.  Elevated concentrations of immunoglobulin G and circulating immune complexes may be found in active disease.  The magnitude of the elevations in immunoglobulin G and circulating immune complex levels correlates with the severity of the skin disease.

CLINICAL MANAGEMENT

The skin lesions of dermatomyositis are worsened by trauma and prolonged solar exposure.  Management changes to avoid these secondary insults should be instituted.  Mildly affected dogs usually require no additional treatment, as their skin lesions heal spontaneously.   Severely affected dogs are difficult to manage.  These dogs have widespread skin lesions and a generalized myopathy, which results in lameness and difficulty in drinking and eating.  These dogs often have aspiration pneumonia.  Although large doses of prednisolone (1 mg/kg q24h) improves the skin lesions, the disease is progressive and euthanasia should be encouraged.

Mildly to moderately affected dogs can usually be mantained as acceptable pets for extended periods.  Some skin lesions remain and muscle atrophy, especially of the muscles of mastication, is noted.  Oral does of vitamin E (200 to 800 IU/day) or marine lip supplements (e.g. DVM Derm Caps) appear to be beneficial for the skin but not the muscle lesions..  Occasional use of prednisolone (1 mg/kg q24h) is necessary in some dogs for traumatic flares.  Continued use of glucocorticoids should be discouraged because the muscle atrophy may be aggravated.

Treatment with pentoxifylline (Trental  [Hoecsht-Roussel]) has been recommended.  This drug increases tissue oxygenation by increasing microvascular blood flow.  It is a gastric irritant and must be given with food.  Dosages of 400 mg every 24 to 48 hours have been suggested.  Response is slow, and 2 to 3 months of treatment are necessary before efficacy can be determined.  No data are available on the efficacy of this treatment.

The above treatments usually minimize the development of new skin lesions, and those that do occur tend to be milder.  Muscle disease progresses and old dogs have prfound atrophy of the muscles of the head, the distal limbs, and sometimes the body.  With severe atrophy, the animal’s ability to eat and drink can be compromised and dietary manipulations are necessary.  The limb and body atrphy can cause an abnormal gait, but locomation is till possible.  Amyloidosis can occur in some chronically affected dogs.