Photosensitization is a clinical condition in which skin is oversensitive to sunlight; this condition is not sunburn. Certain molecules present in the skin are energized by light. When the molecules return to the less energized state, the released energy causes chemical reactions in the skin. Photosensitization can be difficult to distinguish from actual sunburn.

Photosensitization is often classified according to the source of the photodynamic pigment. The types known to occur in dogs include primary (type I) photosensitivity and secondary (or type III) photosensitivity. A wide range of chemicals in plants, fungi, and bacteria may act as photosensitization agents. Photosensitization can also occur in dogs that have liver damage caused by any of several types of poisonings.

The signs associated with photosensitivity are similar regardless of the cause. Photosensitive dogs squirm in apparent discomfort when exposed to light. They scratch or rub lightly pigmented, exposed areas of skin (for example, the ears, eyelids, or muzzle). Bright sunlight can cause typical skin changes, even in black-coated animals. Redness develops rapidly and is soon followed by swelling. If exposure to light stops at this stage, the abnormalities soon resolve. When exposure is prolonged, fluid discharge, scab formation, and skin death result.

Signs are easily recognized in cases of marked photosensitivity but are similar to the effects of sunburn in early or mild cases. When examining your pet for photosensitivity, your veterinarian will not only examine the skin but also look for signs of any of the diseases that may trigger this condition. Evaluation of liver enzymes and liver biopsies may be necessary to determine if your dog has liver disease. Laboratory tests may also be performed. Your veterinarian will also ask about your dog's access to poisons and whether or not your dog may have been exposed to rat poison or other poisonous chemicals.

Treatment involves mostly soothing the signs. While photosensitivity continues, dogs should be shaded fully or, preferably, kept indoors and allowed out only during darkness. The severe stress of photosensitization and extensive death of skin tissue can cause serious illness and even death. Depending on the individual case, injectable steroids may be helpful. Secondary skin infections and fluid discharge are treated with standard wound management techniques. Exposure to flies must be prevented because the skin damage caused by photosensitivity attracts flies and may lead to maggot infestations and secondary diseases. Skin abnormalities caused by photosensitivity heal remarkably well, even after extensive damage. The outcome for a dog is related to the site and severity of the primary lesion and/or liver disease, and to the degree of healing.

Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Patricia D. White, DVM, MS, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; William W. Hawkins, BS, DVM; Thomas R. Klei, PhD; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; David Stiller, MS, PhD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Stephen D. White, DVM, DACVD