PHOTOSENSITIVITY IN HORSES

The abnormal reactivity of the skin to light in response to photosensi tizing agents is referred to as photosensitivity. There are two basic types of photosensitizing agents: photoallergic and phototoxic. The ph ototoxic agents can be chemicals in the environment or primary plant p igments; or they can be hepatogenic, retained in the circulation as a result of failure of a liver excretory function. In all cases of photo sensitivity, a photodynamic agent is present in the skin. Usually, the agent is acted on by ultraviolet light; lack of melanin pigment in th e skin facilitates the interaction of the light with the photodynamic agent. Molecules of the agent enter into a high-energy state of excite ment. Subsequently, the skin becomes inflamed or even necrotic. Diagno sis of photosensitization is not difficult because of the characterist ic clinical signs. Treatment of affected horses consists of eliminatin g the photodynamic agent, removing the patient from sunlight, and trea ting the lesion as though it were a burn.