Photo-sensitivity is a severe skin dermatitis that occurs in the non-pigmented (white skinned) areas of horses when exposed to sunlight, following ingestion or contact with certain plants or secondary products that are UV or light reactive.

Primary Photo-sensitivity

Primary or acute exposure is caused by ingestion of a photodynamic substance, such as certain plants and a number of different drugs and medications.

Primary Photo-sensitivity Causing Plants

  • Queen Anne’s Lace
  • St. John’s Wart
  • Giant Hogweed
  • Buckweat
  • Alfalfa

Secondary Photo-sensitivity

Secondary Photo-sensitivity is the most common type of photo-sensitivity seen in animals and occurs due to liver or bile duct damage, most often as a result of ingestion of poisonous weeds and mycotoxins over an extended period of time.

Secondary Photo-sensitivity Causing Plants

  • Tansy Ragwort
  • Pattern’s Curse
  • Hound’s Tongue
  • Fiddleneck
  • Rattlepod
  • Lantana


Symptoms can appear on the ears, muzzle, white skinned areas, and the coronary band. Symptoms include:

  • Redness
  • Swelling
  • Oozing
  • Scabs
  • Skin Necrosis
  • Itching
  • Licking
  • Skin Thickening
  • Crusting
  • Hairloss