Familial Canine Dermatomyositis

Familial canine dermatomyositis is a hereditary, idiopathic inflammatory condition of the skin and musclesof young collies, Shetland sheepdogs, and their crosses.  It has also been reported in the Welsh corgi, ChowChow, 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 issuspected because of detectable immunologic abnormalities, but it is unclear whether this immunologicreaction 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 internalmalignancies, their causal relationship in dermatomyositis remains unproved.

A familial history is rare in humans, but common in collies and Shetland sheepdogs.  Breeding studies incollies support an autosomal dominant mode of inheritance with variable expressivity.  Limited studies in theShetland 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 colour, or coat length association withdermatomyositis.  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 11weeks of age.  The progression of lesions is variable.  Some mildly affected dogs have few lesions, which healrapidly without scarring.  Most dogs have new lesions after the first ones are recognized, but the rate ofprogression is variable.  The extent of the skin lesions is known by one year of age.  Unless managementchanges 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); onthe 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 lesionsare characterized by alopecia, erythema, scaling, and mild crusting.  Ulceration can be seen in severelyaffected dogs, large areas of normal skin remain, whereas severely affected dogs have involvement of theentire 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 theskin lesions.  Mildly affected dogs have no clinical muscle disease and convincing evidence of this may not befound 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 ofamyopathic dermatomyositis can be made only when no muscle changes are detected for 4 years or longerafter the skin lesions have occurred.  To the authors’ knowledge, this type of follow-up testing has not beendone in dogs.  The rare dog related to dogs with classic dermatomyositis has EMG changes of myositis withno 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 swallow it completely; so residual pieces arewashed from the mouth during drinking.  Some dogs have a peculiar high-stepping gait.  Severely affecteddogs drink, chew, and swallow with difficulty; have a stiff gait; have megaoesophagus; and often havesecondary 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 ofdermatomyositis or be more deeply ulcerated (see Idiopathic Ulcerative Dermatomyositis in ShetlandSheepdogs and Collies in this chapter).  These cases could represent an adult-onset variant of the diseases orbe 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 considered ifthere 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, andlaboratory tests to rule out other conditions.  Biopsy of affected skin shows scattered vacuolar change of thesurface 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).  Dermalinflammation can be absent.  Most cases show a mild perivascular to interstitial dermatitis in whichlymphocytes, plasma cells and histiocytes predominate.  Mild pigmentary incontinence may be present in thesuperficial dermis.  Follicular atrophy and fibrosis are common findings (Figs 11:25 and 11:26).  Vasculitismay occasionally be seen in the skin.  Muscle biopsy may show mixed inflammatory exudates, accompaniedby muscle fiber necrosis and atrophy.  In some cases, a vasculitis may be found.  Needle EMG abnormalitiesinclude 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 orslightly increased.  Neurologic examination and nerve conduction studies are usually normal.  Elevatedconcentrations 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 theseverity 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 noadditional 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 lamenessand difficulty in drinking and eating.  These dogs often have aspiration pneumonia.  Although large doses ofPrednisolone (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 maintained as acceptable pets for extended periods.  Someskin lesions remain and muscle atrophy, especially of the muscles of mastication, is noted.  Oral does ofvitamin 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 somedogs for traumatic flares.  Continued use of glucocorticoids should be discouraged because the muscle atrophymay be aggravated.

Treatment with pentoxifylline (Trental  [Hoecsht-Roussel]) has been recommended.  This drug increasestissue oxygenation by increasing microvascular blood flow.  It is a gastric irritant and must be given withfood.  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 thistreatment.

The above treatments usually minimize the development of new skin lesions, and those that do occur tend tobe milder.  Muscle disease progresses and old dogs have profound atrophy of the muscles of the head, thedistal 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 atrophy can cause an abnormalgait, but locomotion is till possible.  Amyloidosis can occur in some chronically affected dogs.