Photosensitisation in Horses: Welfare and Pasture Management

Photosensitisation in horses causes severe sunburn-like skin damage on unpigmented areas, arising from either primary plant toxin ingestion or secondary liver disease, with significant welfare implications.

Key Facts

Primary photosensitisation is caused by ingestion of plants containing photodynamic agents including St Johns Wort and buckwheat
Secondary photosensitisation results from liver failure to process phylloerythrin, a chlorophyll metabolite
Affected skin develops oedema, erythema, and vesicle formation, leading to serum crusting and sloughing
Unpigmented areas of skin including pink muzzle, white face markings, and pale pasterns are most affected
Management involves removing the causative plant or treating liver disease, and providing sun shelter

Welfare Considerations

Photosensitisation causes painful sunburn-like lesions that can be extensive, covering large areas of the face, muzzle, and limbs. The peeling and sloughing of affected skin causes acute pain comparable to severe sunburn in humans. Horses with facial lesions may resist handling and bridle contact due to local pain. Secondary photosensitisation indicates underlying liver disease that requires investigation in its own right. Provision of shade and removal of animals from causative pastures, combined with topical wound management, reduces welfare harm while diagnosis and treatment proceed.

What You Can Do

Survey pastures for photosensitising plants including St Johns Wort and remove or fence off affected areas
Provide adequate shade for horses with pink-skinned areas during high UV periods
Seek veterinary investigation of photosensitisation cases to exclude secondary liver disease
Apply sun protection cream to sensitive pink-skinned areas of horses prone to photosensitisation