Photosensitivity is caused by the presence of a photo-activating substance in the skin which increases absorption of UV light resulting in inflammation and lesions very similar to those seen with mud fever.
Affected horses develop painful scaling/crusting lesions on all white areas or in some cases just the legs, pigmented areas will not be affected.
There are different forms depending on the origin of the photosensitising agent;
In primary photosensitisation the agent either enters the body directly or is produced within the body. This can be via ingestion of certain plants including clover, St Johns Wort and buckwheat, via injection of some drugs or via direct contact with some plants (eg. buttercups).
Secondary photosensitisation occurs when a horse has a degree of liver compromise which results in elevated blood levels of photo-activating compounds leading to accumulation in the skin. A number of things can cause liver disease but a key one is the ingestion of ragwort.
Diagnosis is made primarily by clinical signs and history and confirmed with a skin biopsy. Blood samples should be taken to identify any underlying liver disease.
Treatment involves identifying and removing the source of the photo-activating agent if possible. Management of the condition involves reducing exposure to UV light by stabling the horse during the day, using boots to cover any white areas/applying sunblock during turnout especially on sunny days. In acute flare ups topical steroid and/or antibiotic creams can be useful and in some cases antibiotics may be needed.