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What is dermatomyositis?
This condition is one of inflammation (itis) of the skin (dermato)
and muscle (myo) that is seen in young collies and Shetland
sheepdogs. There appears to be a defect in the immune system
that predisposes dogs to this disorder. The skin lesions typically
develop first with variable muscle problems occurring later.
There are many similarities to dermatomyositis in people.
Ulcerative dermatosis may be a variant of this condition.
It is a rare disorder that occurs in middle-aged to older
dogs of the same breeds, and is manifest by skin lesions (blisters,
crusting) that are seen primarily in the groin and underarm
regions. Occasionally there are muscle abnormalities.
How is dermatomyositis inherited?
The trait is believed to be autosomal dominant with variable
expressivity. This means that if either parent is affected,
all puppies have a susceptibility to the disorder, but not
all will be affected equally. The variability suggests there
is more involved than simple inheritance, including internal
factors such as the individual's immune system (also affected
by heredity) and external factors (including possibly viral
infection). The most severely affected dogs may be homozygous
for the trait.
What breeds are affected by dermatomyositis?
This disorder is seen primarily in the collie, Shetland sheepdog,
and their crosses. It has been diagnosed in other breeds,
including the Australian cattle dog, German shepherd, chow
chow, Pembroke Welsh corgi, and Kuvasz. ...more here
Ulcerative dermatosis is seen in Shetland sheepdogs and
collies.
For many breeds and many disorders, the studies to determine
the mode of inheritance or the frequency in the breed have
not been carried out, or are inconclusive. We have listed
breeds for which there is a consensus among those investigating
in this field and among veterinary practitioners, that the
condition is significant in this breed.
What does dermatomyositis mean to your dog & you?
With this condition, the skin is almost always affected before,
and worse than, muscle. Typically, skin lesions occur by 6
months of age. There is reddening, hair loss, blisters or
small bumps, crusting and where severe, ulceration of the
skin. Most often affected are the face (especially the muzzle
and ear tips, and around the eyes), the tip of the tail, bony
prominences (over the elbows for instance) and the toes. Over
time, the affected skin becomes scarred.
The muscles are not always affected in dermatomyositis, or
the abnormalities may be so slight as to go unnoticed. When
there is muscle involvement, the puppies may be weak and lethargic
and have a slow rate of growth. Muscles (especially of the
face and head) may appear smaller due to muscle atrophy (shrinkage
and loss of use). The most severely affected dogs may have
difficulty in chewing or swallowing. The leg muscles may also
atrophy.
The degree to which pups are affected varies considerably.
Muscle inflammation is generally less severe in Shelties.
Generally the clinical signs fluctuate over time for no apparent
reason, and many mildly affected dogs will outgrow the condition
before a year of age, although some may have permanent scars
on their face or legs. In severely affected dogs, the condition
is progressive and these dogs may have to be euthanized due
to severe muscle atrophy and associated problems such as an
inability to eat and drink properly, which may be complicated
by pneumonia.
How is dermatomyositis diagnosed?
This disorder is usually suspected in a young collie or Sheltie
with crusting facial skin lesions, with or without muscle
weakness. There are other conditions which can cause these
types of lesions and your veterinarian will do tests such
as a skin biopsy to pinpoint the diagnosis. This is a simple
procedure done with local anesthetic, in which your veterinarian
removes a small sample of your dog's skin for examination
by a veterinary pathologist. The biopsy will show changes
in the skin consistent with this condition.
For the veterinarian: CBC, biochemical profile and urinalysis
are usually normal, and the results of standard tests for
autoimmunity are usually negative. In addition to history
and physical exam findings, diagnosis is made by biopsy (affected
skin and muscle), electromyography (EMG), and ruling out other
conditions. The main differential diagnosis, especially where
the muscle component is mild, is epidermolysis bullosa. The
skin lesions have a similar age of onset and clinical progression,
but with dermatomyositis, erythematous plaques or vesicles
can not be induced in normal skin by applying mild friction.
Dermatomyositis may be complicated by localized or generalized
demodicosis. Megaesophagus (+/- aspiration pneumonia) may
occur in dogs with severe muscle involvement.
How is dermatomyositis treated?
Skin lesions are exacerbated by trauma and by exposure to
ultraviolet light, so these should be avoided (by the use
of sunscreens for example). This may be all that is required
in mildly affected dogs, who are likely to outgrow the condition
with time.
Dermatomyositis can usually be managed fairly well in moderately
affected dogs, with the above precautions and the use of Vitamin
E and occasional use of corticosteroids for flare-ups. Your
veterinarian will work with you to determine how best to manage
the condition in your dog. Unfortunately, it is very difficult
to maintain the health and comfort of severely affected dogs,
and euthanasia is sometimes the best option.
For the veterinarian: Pentoxifylline may help by improving
microvascular blood flow. A response may take 2 or 3 months.
Short term use of glucocorticoids may be necessary for acute
flare-ups of skin or muscle inflammation, but long term use
should be avoided as it will exacerbate skin and muscle atrophy.
Breeding advice
Affected dogs should not be bred. Also, because it is difficult
to identify dogs that have only a mild form of this condition,
close relatives of affected dogs (siblings and parents) should
not be used for breeding. It is important to remember that
because of the variation in expressivity, offspring of only
mildly affected dogs may have much more serious clinical signs.
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