Dermatomyositis - DMS

 

Dermatomyositis in dogs is a hereditary, immune-mediated disease that affects the skin and, sometimes, the muscles and blood vessels. The disease is most common in Collies, Shetland Sheepdogs, and mixes of those breeds. It mostly affects young dogs, though it can develop in adult dogs, as well. Symptoms include inflammation and skin lesions that usually appear before dogs turn six months old, sometimes as early as seven weeks after birth. When the condition develops early in life, it’s usually more severe than when it appears during adulthood. It is inherited by puppies from their parents While there is no cure for dermatomyositis, dogs with this disease can be treated for their symptoms to avoid further progression. Recognition of the condition can aid in early management, and therefore keep a dog with dermatomyositis comfortable for a longer period of time.

Dermatomyositis (DMS), presents much the same as demodex, with skin lesions starting on the face any time after the age of 7 weeks. However, it is distinct from demodex as lesions can be found on all parts of the body and it attacks the muscles as well.

Dermatomyositis has been studied in both humans and dogs for decades, and it seems to be very similar in both species. This disease is not yet fully understood, but it is known that it is a hereditary condition passed from parents to puppies in their DNA:

In dogs, dermatomyositis seems to be mainly isolated to Collies and Shetland sheepdogs, but similar symptoms have been reported in other breeds. These potential breeds that may develop the disease include chow chows, Pembroke Welsh corgis, Lakeland terriers, German shepherds, Kuvasz, and any dogs mixed with these breeds.

A test for DMS has been developed. This test will not definitively indicate whether or not a dog will get the disease as DMS has both a genetic as well as an environmental component. Collies whose risk factor is determined to be high may never develop the disease as the genetic risk does not take into account environmental factors.The risk of developing DMS in UK Collies is rare, but not unknown.Dr Leigh Clark, Clemson University published a study as below'We had 6 collies from the UK and all were unaffected controls. 5 of them were Aabb and 1 was aabb. This equates to an allele frequency of 42% for A, one the risk alleles for DMS, and 0% for B, the other risk allele for DMS. A mating between two Aabb dogs would result in approximately 25% of puppies with the AAbb genotype, which carries a 40% chance of developing DMS.DMS looks a lot like other skin conditions and is very often misdiagnosed, especially if skin biopsies were not taken.'

A skin biopsy is the most commonly used method to diagnose dermatomyositis in dogs. Biopsies may also be taken from affected muscle tissue.1 Dermatomyositis. VCA Animal Hospitals. To perform a biopsy, a sample of a skin lesion is taken and evaluated in a laboratory and examined microscopically. To obtain this biopsy, sedation or local anesthesia will most likely be utilized by your veterinarian. Other skin diseases, including mange and ringworm, may also be ruled out by performing other tests prior to a skin biopsy. On rare occasions, a muscle biopsy and a test called an electromyogram may also be performed to diagnose dermatomyositis.

Symptomatic treatment is heavily relied on for dogs with dermatomyositis. Medications and vitamins used to manage dermatomyositis can be expensive, and at-home care can become labor-intensive, so dog owners should prepare for a commitment to keep their pet's symptoms under control as much as possible. At-home care is especially important in patients that develop megaesophagus to ensure the dog is able to consume a healthy diet. Pentoxifylline, vitamin E, prednisone, azathioprine, and cyclosporine are common options to manage dermatomyositis. Avoiding UV light exposure and activities that can further damage the skin are also important. Other care steps for owners may include offering assistance with feeding dogs who have difficulty swallowing and utilizing special shampoos at bath time.

Genetic Test
DMS is caused by a mutant allele in three loci, which are genes PAN2, MAP3K7CL, and DLA-DRBI. The genetic test for DMS gives us the information on whether the dog has a normal allele or the mutant risk allele for all three of those genes. To make it easier to underdstand it guves us the results in the form of 6 litters - two each of A, B and C, where some are capitals and some are lower case. These letters correspond to the aforementioned loci: A to PAN2, B to MAP3K7CL and C to DLA-DRBI.

Genotypes of DLA-DRB1, Locus A, and Locus B are considered together. For each possible genotype, the percentage of affected dogs in that group was determined. Based on the genotype, the likelihood of an individual dog developing DMS is classified as low (0% - 5%), moderate (33% – 50%), or high (90% – 100%). (See Table 3 (below) from the research article.)

Genotypes and risk interpretations:

LOW RISK GENOTYPES - In the study, these genotypes were rarely found in affected dogs [1].

aabb (homozygous or heterozygous DLA-DRB1*002:01)

Aabb (homozygous or heterozygous DLA-DRB1*002:01)

aaBb (homozygous or heterozygous DLA-DRB1*002:01)

AaBb (homozygous or heterozygous DLA-DRB1*002:01)

aaBB (heterozygous DLA-DRB1*002:01)

 

MODERATE RISK GENOTYPES - In the study, up to half of dogs with these genotypes developed DMS.

AAbb (homozygous or heterozygous DLA-DRB1*002:01)

aaBB (homozygous DLA-DRB1*002:01)

AaBB (heterozygous DLA-DRB1*002:01)

AABb (heterozygous DLA-DRB1*002:01)

 

HIGH RISK GENOTYPES - In the study, over 90% of dogs with these genotypes developed DMS.

AABb (homozygous DLA-DRB1*002:01)

AaBB (homozygous DLA-DRB1*002:01)

AABB (homozygous or heterozygous DLA-DRB1*002:01) - 100% of dogs having AABB were affected with DMS

Notice that homozygosity for DLA-DRB1*002:01 increases the risk of DMS from low to moderate (aaBB) and from moderate to high (AABb and AaBB).

 

It is recommended that breeding pairs be selected on their genotypes at locus A and B. Ideally, matings that could produce puppies with high risk genotypes (AABB, AABb, and AaBB) should be avoided. For example, if a bitch’s genotype is AAbb, it is recommended that she be bred to a dog with one of the following genotypes: aaBB, aaBb, or aabb. This mating will not produce puppies that are homozygous for A or B. Any aa-- genotype can be crossed with any --bb genotype without producing moderate or high risk genotypes, and aabb genotypes can likewise be crossed with any genotype. In general, a dog with a genotype of aabb can be bred to any other genotype to produce low risk puppies.

There is no cure for this inherited disease, so the goal is to simply keep the dog as comfortable as possible for as long as possible. In mild cases, many dogs can recover fully as they grow older while owners manage their symptoms until they resolve. Scarring may occur on the skin of dogs with moderate cases. Unfortunately, severe dermatomyositis can cause kidney disease and inflammation of the skin and muscles that continues to progress, so this disease can be fatal.1 Utilizing all available treatment options can help improve the dog's condition over time to determine the severity of the disease.