Prospective study of familial canine dermatomyositis. Correlation of the severity of dermatomyositis and circulating immune complex levels.

Familial canine dermatomyositis in collie dogs is a newly recognized spontaneous disease that resembles dermatomyositis in children. A litter of 9 collie dogs was studied from birth to 7.5 months of age. The onset and severity of dermatitis and myositis correlated with elevated serum levels of circulating immune complexes (CICs) and IgG. The immunoglobulin component of the CICs consisted principally of IgG. All dogs developed elevated levels of CIC before or concurrent with the onset of dermatitis. Myositis developed later. The CIC tended to peak between 14-18 weeks of age in all dogs except the most severely affected dog, in which the CICs continued to increase to 238 micrograms/ml (controls 30 micrograms/ml) at 7 months of age. In the moderately affected dogs the CICs tended to stabilize at the levels reached at 14-18 weeks, and in the mildly affected dogs the CICs tended to decrease to normal levels after 14-18 weeks. Although the dogs had electromyographic and repetitive nerve stimulation abnormalities, the abnormalities did not correlate with severity of dermatomyositis or degree of elevation of CICs or IgG. Necropsy at 7.5 months of age revealed that all dogs had myositis and 8 of 9 had dermatitis. Except for 1 dog, the severity of dermatomyositis correlated positively with higher levels of CICs. A strong positive association between elevated levels of CICs and IgG, but not IgM or IgA, was generally present. Elevated levels of CICs appear to be involved in the mechanisms that control the development, severity, and progression of dermatomyositis in collie dogs.