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

Photosensitization 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 photosensitization remains one of the most important aetiological categories in veterinary toxicology and may occur via primary (direct phototoxic) or secondary (hepatogenous) mechanisms. This review synthesizes current knowledge on the occurrence of photosensitizing compounds in plants, their biochemical and toxicodynamic properties, and their clinical relevance in livestock species. Emphasis is placed on major primary photosensitizing taxa, including <i>Heracleum</i> spp. and <i>Hypericum perforatum</i>, as well as hepatotoxic pyrrolizidine alkaloid-containing plants such as <i>Senecio</i> spp. Mechanistic pathways, plant metabolite ecology, and toxicopathological outcomes are discussed alongside illustrative case material. Plant-associated photosensitization also carries substantial economic consequences through decreased productivity, increased treatment costs and culling losses in affected herds and flocks. This condition represents a significant animal welfare concern in pasture-based production systems, where prolonged sunlight exposure, limited shade availability, and variable botanical composition of grazing areas can exacerbate both the incidence and severity of clinical manifestations. This review may provide a consolidated veterinary toxicology framework for understanding plant-associated photosensitization in grazing systems.