Erythema multiforme possibly triggered by food substances in a dog

A 5-year-old female border collie presented with erythematous skin lesions at the axillae, groin, mucocutaneous junctions, and pinnae. Biopsy revealed lymphocytic interface dermatitis with hydropic degeneration of basal cells and keratinocyte apoptosis. Based on gross and histological features, diagnosis of erythema multiforme was made. The disease was resolved by treatment with azathioprine, prednisolone, and a hypoallergenic diet. Finally, the skin lesion was controlled without drug therapy but recurred easily every time commercial foods except the hypoallergenic diet were used, suggesting that food substances triggered this outbreak.