Primary and Hepatogenous Photosensitization in Livestock: A Review of Plant-Derived Phototoxins and Veterinary Implications

Photosensitisation is a clinically significant dermatological and systemic disorder affecting grazing livestock worldwide. The condition arises following the ingestion or dermal exposure to photodynamic compounds that, upon activation by ultraviolet (UV) or visible light, induce tissue injury. Plant‑associated photosensitisation remains one of the most important aetiological categories in veterinary toxicology and may occur via primary (direct phototoxic) or secondary (hepatogenous) mechanisms. This review synthesises current knowledge on the occurrence of photosensitising compounds in plants, their biochemical and toxicodynamic properties, and their clinical relevance in livestock species. Emphasis is placed on major primary photosensitising taxa, including Heracleum spp. and Hypericum perforatum, as well as hepatotoxic pyrrolizidine alkaloid‑containing plants such as Senecio spp. Mechanistic pathways, plant metabolite ecology, and toxicopathological outcomes are discussed alongside illustrative case material. The review aims to provide a consolidated veterinary toxicology framework for understanding plant‑associated photosensitisation in grazing systems.