Dog Health by LowchensAustralia.com

More Dermatomyositis Information

Here's some more info from Kim Schive:

Dermatomyositis (DM), AKA sheltie skin syndrome, is a skin-and-muscle disorder that is known to affect collies (both varieties) and shelties, but is not generally seen in any other breed. Most of the research on canine DM (several varieties also occur in humans) has been done on collies, but it is likely that we are dealing with the same entity in both collies and shelties, so that research is relevant.

The mode of inheritance appears to be autosomal dominant with variable expression. "Autosomal" means it is non-sex-linked, and occurs with equal frequency in males and females. "Dominant" means that one parent must HAVE IT to pass it on to offspring (or, possibly, it can occur as a new mutation; however, while possible, I believe this is highly unlikely, because if new mutations occured in canines, DM would be a problem in almost every breed). "Variable expression" means that disease severity can vary from very mild--too mild to be diagnosed--to very severe.

Because the gene is dominant, there is no such thing as a "carrier." A dog either HAS it, or he doesn't--whether or not he gets diagnosed is the sticky part of the equation (see next paragraph). Affected dogs may have one gene for the disorder (D) and one normal gene (d). Such a dog would be referred to as heterozygous, and his genetic make-up would be Dd. Or, an affected dog may have two genes for the disorder (having inherited one from each parent). That dog would be referred to as homozygous, and his genetic make-up would be DD. It is probable that the more severely affected dogs are homozygous (DD), but nobody knows for sure.

The "variable expression" component is the scary part; it means that a dog who is heterozygous for the gene may be so mildly affected as to never be diagnosed. For example, as a young puppy, an affected dog may have show no signs other than a VERY small area of hair loss around the eyes or muzzle (or somewhere else, like the inside of a leg near the foot). That spot may clear up and the dog may show NO OTHER SIGNS EVER. Another dog, who also is heterozygous for the gene, may have more extensive hair loss and muscle atrophy, and probably will be diagnosed. The second dog will probably never be bred; but the first dog may be. The important point is that BOTH dogs have the same genetic make-up when it comes to DM and if bred, both are EQUALLY LIKELY to produce affected puppies.

Because of the way DM is transmitted, one affected dog can contaminate a pedigree for years to come. Breeders should be suspicious about ANY skin problem OR muscle-wasting condition that shows up at ANY age. Sure, it might just be demodectic mange, or an allergy... but you won't know for sure without a biopsy for DM (for accuracy, the biopsy must be done on an affected area of the body during a "flare-up"). Also, research with collies showed that some affected puppies had BOTH demodectic mange AND DM. So a positive diagnosis of demodectic mange DOES NOT rule out dermatomyositis. Breeders cannot affort to be complacent about this condition. Taking a wait-and-see attitude about even small amounts of hair loss, small scabs or small crusty areas--which may well clear up on their own--can result in a mildly affected dog entering (and contaminating) your breeding program.