Veterinary advice should be sought before applying any treatment or vaccine.


Sun Sensitivity

Photosensitivity is an extreme sensitivity, or immune system reaction resulting in a severe dermatitis. It may appear similar to a sunburn, and is similarly associated with sunlight exposure. However, while a sunburn is caused by extended exposure to UV light, photosensitivity occurs as a result of a reaction from the presence of a photodynamic substance within their tissues which is triggered by sunlight exposure. It is important to distinguish a sunburn from photosensitivity because the later is much more serious, and the cause needs to be determined.

Where Photosensitivity is Found on Horses

Photosensitivity occurs in the unpigmented areas (white skin areas) of the horse, predominately areas most exposed to the sun, such as the skin around the eyes, ears, face, muzzle, tail and coronary band.
Photosensitive areas on horses
Onset can occur within minutes of exposure to specific types of plants by direct contact, within hours after ingestion, or after several days of exposure.

Photosensitivity Causes

Photosensitivity can occur as a result of ingestion or contact with certain toxic plants while grazing in pastures, certain medications, or as a secondary result of liver damage. There are two general types of photosensitivity reactions--primary and secondary.




  • History
  • Clinical signs
  • Physical exam
  • Laboratory tests



Confirm source of exposure
Possible treatment for liver disease
Anti-inflammatory medications
Cover area with fly sheet or fly mask
Reduce exposure to sunlight
Change in turnout schedule


  • Make yourself aware of the weeds and plant species that can be invasive in pastures and/or poisonous to horses.
  • Take periodic walks around pastures to check for the presence of potentially poisonous plants
  • Check that hay does not contain dried up poisonous plants
  • If you borrow or hire farm machinery ensure it is clean prior to arriving on your property, the same goes for lending of your own equipment.
  • Quarantine new animals in a separate paddock the first 10 days to 2 weeks after arrival. Weed seeds can be passed through an animal's digestive tract.

Scientific Research

General Overviews

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Risk Factors

  • Horses receiving certain medications
  • Lighter-colored horses or horses with nonpigmented (white) areas are more at risk of developing photosensitivity; this is because darker skin contains melanin or pigment that helps provide protection from penetrating substances or light.
  • Horses are more likely to develop photosensitivity from plants during days or in geographic areas which have high levels of UVA and UVB irradiation.