Horse Photosensitisation Management

Photosensitisation in horses causes painful sunburn-like reactions on unpigmented skin following ingestion of photodynamic plants or liver disease, and requires prompt identification and management.

Key Facts

Photosensitisation results from accumulation of photodynamic molecules in the skin that react with UV light
Primary photosensitisation follows ingestion of plants such as St John wort or buckwheat
Secondary photosensitisation occurs when liver disease impairs excretion of phylloerythrin from chlorophyll breakdown
Unpigmented or lightly haired skin areas are selectively affected, causing blistering and inflammation
Severely affected areas can develop necrosis requiring extensive wound management
Diagnosis of secondary photosensitisation requires investigation and treatment of the underlying liver condition

Welfare Considerations

Photosensitisation causes acute and severe welfare harm through intense pain and inflammation of affected skin areas. Horses experiencing acute photosensitisation show obvious distress from painful skin lesions, particularly around the muzzle, face, and unpigmented limb areas. Severe cases develop full-thickness skin necrosis requiring intensive wound management over weeks. Secondary photosensitisation is a clinical emergency because it reflects significant liver dysfunction that must be identified and treated. Management of affected horses requires immediate removal from sunlight and prompt veterinary assessment.

What You Can Do

Remove affected horses from sunlight immediately and stable during daylight hours until lesions resolve
Seek urgent veterinary assessment to determine whether photosensitisation is primary or secondary
Identify and remove any photodynamic plants from grazing areas if primary photosensitisation is confirmed
Support liver function assessment and targeted treatment if secondary photosensitisation is diagnosed
Apply fly repellent carefully around affected skin to prevent secondary fly strike on damaged areas