Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Vasculitis, Photoactive

Synonyms: White Leg Disease


Photoactive vasculitis is a type of photosensitization wherein inflamed blood vessels allow fluid to leak out into the surrounding tissues causing painful swelling (edema). It is caused by a variety of toxins, viral and other infections, as well as allergic skin reactions. Vasculitis may be caused by toxins in feeds, especially clovers (alsike), goatweed (St. John's wort, klamath weed), and some varieties of alfalfa, either in hay or grazed.

These toxins interact with sunlight to damage blood vessels in pink skin. This results in blockage of these vessels and lack of blood flow to areas of skin.

This usually appears as hot, painful swelling of pink skinned limbs, especially the hind limbs. The affected areas start out reddened, swollen and very painful to pressure. The skin may slough, leaving open wounds that must heal slowly and require careful wound care for the best cosmetic outcome.

Vasculitis results in patches of severely damaged unpigmented skin resulting from damage to larger vessels and loss of blood supply to skin. Sometimes, however, vasculitis is not confined to these white haired/pink skinned areas.

Diagnosis is usually through clinical exam. A vet usually recognizes this syndrome in unpigmented limbs. Conformation of the diagnosis would require skin biopsy.

Treatment requires protection from light, treatment of damaged skin, and change of management to avoid the photoactive toxins.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
more observations

Questions To Ask Your Vet:
  • Is there any way to determine what is causing this syndrome in my horse?
  • What management changes will I need to institute to keep this from happening again?

Use fly sheets, masks, and hoods intended to block UV rays. Choose a face mask with a muzzle extension, for horses with white markings on their muzzle. Apply either a physical or chemical sunscreen intended for horses to all areas susceptible to sunburn. Stable your horse or provide access to shade during the sunniest time of day.

Know your pasture. Research the plants that grow in and around all of the areas that your horses live. Remove plants such as alsike clover, that are associated with this disease. When in doubt, take a picture of the plant and send it to your vet for a discussion.

further reading & resources

Author: Doug Thal DVM Dipl. ABVP