Plant-Associated Photosensitization in a Pasture-Raised Gypsy Mare Grazing Hypericum perforatum and Trifolium spp.: A Case Report

<jats:p>Photosensitization is a phototoxic skin disorder caused by interaction between ultraviolet (UV) light and photodynamic compounds present in or accumulated within the skin. This report describes a six-year-old Gypsy mare raised under extensive grazing conditions in northwestern Romania that developed erythematous and crusted lesions on the muzzle and perivulvar regions during early summer. The mare was observed grazing on Hypericum perforatum and Trifolium spp., both plants known to contain photodynamic substances capable of inducing either primary or hepatogenous photosensitization. No systemic abnormalities were noted. Hematology and serum biochemistry revealed only mild elevations in hepatic enzymes, consistent with a transient hepatocellular response. Based on clinical presentation, lesion distribution, and confirmed ingestion of photodynamic plants, a diagnosis of plant-associated photosensitization (probable mixed-type) was established. The mare was treated with topical zinc oxide cream (Sudocrem®) and a UV-protective lotion (Effol Skin Lotion Sun Protection®, Schwan-Stabilo, Germany), achieving full healing within seven days. Continued photoprotection prevented recurrence during the summer. This case underscores the risk of plant-induced photosensitization in horses with unpigmented skin grazing under intense sunlight and demonstrates the efficacy of early local management.</jats:p>