Photosensitization and photodermatitis are not uncommon in the horse and are often confused with scratches when they affect white areas around the pastern. The underlying causes can be difficult to identify, but weeds including the wild parsnip (Pastinaca sativa) have been implicated.
The clinical presentation of five horses with photosensitization inspired this study, which included identification of the suspect plant as wild parsnip along with several toxic compounds involved, as well as feeding trials and the testing of topical exposures in horses and goats.
Horses exposed to the wild parsnip on pasture developed photosensitivity including painful, crusty lesions with epidermal necrosis and sloughing skin. The lesions were most common around the mouth and nose and more severe on white-skinned areas. Hay harvested from these same pastures reportedly caused similar lesions in other horses but did not affect cattle fed the same hay. Blood chemistry tests including liver enzymes remained normal. In the goat feeding trial, one female goat developed lesions on the white skin of the vulva. The contact dermatitis trials on horses and goats produced significant photodermatitis lesions on the skin where the plant extracts were applied.
The authors concluded that because there were no systemic abnormalities and the contact dermatitis trials produced comparable lesions, topical exposure seems to be the most important route of exposure with horses being more sensitive than cattle and other ruminates. They also noted a lot of variability between individual horses with some affected while other similarly exposed horses showed no clinical signs. They attribute this variability in part to factors such as hair distribution, pigmentation and differences in skin thickness, as well as differences in grazing preferences and behavior.