Complex immune disease affecting the vasculature of skin and muscle in the Collie and Shetland Sheepdog (Sheltie). It produces scaling and erosive skin lesions in areas overlying bone (face, ears, limbs, tail tip) and, in a proportion of cases, myositis with muscle weakness. Onset usually occurs between 7-12 weeks and one year of age, often after a stressful trigger (vaccination, lactation, stress). Intensity is variable and some dogs improve with age.

Characteristic of the Collie and the Shetland Sheepdog, with sporadic cases in related breeds (Beauceron, Corgi). Most purebred Collies are homozygous for DLA-DRB1*002:01; in the Sheltie, ~78 % carry at least one copy. The frequency of PAN2 and MAP3K7CL risk alleles is high in both breeds, but the actual clinical incidence is lower than the prevalence of risk genotypes.

- Scaling and crusty skin lesions on the face, pinnae, distal limbs and tail\n- Erosions and ulcers on mucocutaneous junctions\n- Hair loss and atrophic scarring in old lesions\n- Myositis with muscle weakness and atrophy (more frequent in the Collie)\n- Worsening after stress, lactation or vaccination\n- Partial improvement in adults

Canine DMS was recognised in the Collie and Sheltie as an analogue of the human juvenile syndrome. Previous studies showed an association with the haplotype DLA-DRB1*002:01/-DQA1*009:01/-DQB1*001:01 in homozygosity, but its high frequency in healthy dogs indicated additional factors. Evans and colleagues (2017), using GWAS in the Collie and Sheltie, identified two additional risk loci: a missense in PAN2 (p.Arg492Cys, chromosome 10) and an indel in MAP3K7CL (c.383_392ACTCCACAAA>GACT, chromosome 31). The risk combines epistatic effects between PAN2, MAP3K7CL and the DLA haplotype: 9 of 27 combinations confer moderate or high risk and explain 93 % of cases. There is a genotype-phenotype correlation with earlier age of onset the more risk alleles are present.

- Test the three loci (PAN2, MAP3K7CL, DLA) before mating\n- Prioritise low-risk combinations; avoid matings of two individuals with PAN2/MAP3K7CL risk alleles, especially if both are homozygous for DLA-DRB1*002:01\n- Remember that the disease is complex: a 'high-risk' genotype does not equate to guaranteed disease, but it does mean a high probability\n- Progressively reduce PAN2 and MAP3K7CL risk alleles without removing DLA-DRB1*002:01 all at once (very widespread in the Collie) so as not to narrow the gene pool\n- In a dog with a high-risk genotype, monitor stressful triggers

Definitive diagnosis requires a skin biopsy. The differential diagnosis includes demodicosis, dermatophytosis, pemphigoid, cutaneous lupus and drug reactions. Muscle involvement may be mild in the Sheltie and marked in the Collie. Treatments described: pentoxifylline, corticosteroids, vitamin E and oclacitinib in those over 12 months. Extensive lesions heal with irreversible atrophic alopecia. The risk genetics is complex: two 'high-risk' dogs may not develop the disease, and vice versa.

1. Evans JM et al. (2017). Beyond the MHC: a canine model of dermatomyositis shows a complex pattern of genetic risk involving novel loci. PLoS Genet 13(2):e1006604. PMID: 28158183
2. OMIA:000270-9615. Dermatomyositis in Canis lupus familiaris (dog).