Nasal solar dermatitis of the dog is a congenital, abnormal reaction of the skin to sunlight. it frequently occurs in Collies, Shetland Sheep Dogs, German Shepherds, and mixed breeds closely related to these breeds. It affects only the nose and eyes and the areas adjacent to them, and is commonly referred to as "collie nose." Although the exact pathogenesis is unknown, there appears to be little doubt that exposure to sunlight is one of the exciting factors. This is in contrast to feline solar dermatitis which appears not to be a photosensitive disorder.
Thomas, John and Hoskins, Johnny D.
"Nasal Solar Dermatitis: "Collie Nose","
Iowa State University Veterinarian: Vol. 37
, Article 6.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol37/iss1/6