Equis ISSN 2398-2977

Dermatitis: solar

Synonym(s): Photosensitization, Light sensitization

Contributor(s): Rosanna Marsella, Sue Paterson


  • Phototoxicity - a dose-related response to ultraviolet (UV) light   →   sunburn.
  • Photosensitivity - increased vulnerability to the effects of UV light due to sensitization by various agents, eg hepatotoxins/ingested compounds, contact chemicals.
  • Cause: exposure to sunlight +/- presence of photodynamic agents in the skin.
  • Signs: erythema, vesicle formation and sloughing of skin, especially depigmented and hairless areas   Skin: sunburn 01  .
  • Diagnosis: clinical signs and investigation of etiology.
  • Prognosis: dependent on cause, poor in chronic liver disease   Liver disease: overview  .
  • See also urticaria and angiedema   Urticaria / angiedema  .



  • Excessive exposure to UV light.
  • Unpigmented or light-colored skin.
  • Lack of hair cover.
  • Systemic or contact agents.
  • Ingestion of photosensitising plants:
    • Hypericum perforatum(St John's wort)   Dermatitis: photosensitization 04 - St Johns wort  .
    • Fagopyrum esculentum(buckwheat).
    • Ammi majus(bishop's weed).
    • Psorolea patens(native verbine, Bullamon lucerne).
    • Rape.
    • Clover.
  • Liver disease   Liver disease: overview  :
    • Ingestion of hepatotoxic plants (pyrrolizidine alkaloids, helianthron pigments, furocoumarins), eg ragwort,Seneciospp   Toxicity: pyrrolizidine alkaloid    Ragwort: Spring    Ragwort: flowering  ;Medicago polymorpha(burr medic, burr trefoil).
  • Chronic liver disease:
    • Reduced hepatic excretion of phylloerythrin (endogenous photodynamic agent).
    • Photosensitisation occurs in 25% of horses with hepatic dysfunction.
  • Oral, parenteral drugs, eg tetracylines   Oxytetracycline  , chlorothiazides   Etorphine hydrochloride  , sulfonamides .
  • Topical drugs.
    Drug-induced photosensitisation may be long-lived, even after discontinuation of the drug
  • Dermatophilus   Dermatophilosis  .

Predisposing factors



  • Drug therapy.
  • Access to photosensitizing or hepatotoxic plant species.
  • Underfeeding/poor grazing available   →   horses eat weeds.


  • Excessive exposure to UV   →   epithelial cells absorb light energy   →   release of cytokines from epithelial cells   →   dilation of peripheral vessels, plasma leakage + mast cell degranulation   →   increased tissue histamines, inflammatory agents, free radicals and vasoactive compounds   →   inflammation with erythema and swelling.
  • Photodynamic agent in epithelium + exposure to sufficient UV + lack of melanin and haircoat allowing cutaneous absorption of UV radiation   →   photosensitization.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Pilsworth R C & Knottenbelt D (2007) Photosensitisation and sunburn. Equine Vet Educ 19 (1), 32-33 VetMedResource.
  • Singh R P (1970) A case of photosensitization in a horse. Indian Vet J 47 (5), 450-451 PubMed.

Other sources of information

  • McKenzie R A (1994) Equine Opthalmology, Plant Toxicology and Maximizing Performance. Proc 16th Bain Fallon Memorial Lectures. pp 1-43 (good overview of plant toxicities plus reference list).
  • Stannard A A (1987) Photoactivated Vasculitis. In: Current Therapy in Equine Medicine. 2nd edn. Ed: Robinson N E. W B Saunders, USA. pp 646-647.