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

## TL;DR

This review explains how certain plants cause photosensitization in livestock, a condition that harms their skin and liver, and highlights its impact on animal health and farming.

## Contribution

The paper provides a consolidated veterinary toxicology framework for understanding plant-induced photosensitization in grazing systems.

## Key findings

- Photosensitization in livestock can occur via primary or secondary (hepatogenous) mechanisms.
- Heracleum spp., Hypericum perforatum, and Senecio spp. are key causative plants.
- The condition has economic and animal welfare implications in pasture-based systems.

## Abstract

Plant-associated photosensitization is a significant health disorder in grazing livestock, arising after exposure to photodynamic compounds activated by ultraviolet or visible light. It may occur through primary phototoxic effects or secondary hepatogenous mechanisms. Key causative plants include Heracleum spp., Hypericum perforatum, and pyrrolizidine alkaloid-containing genera such as Senecio. This review outlines the biochemical basis, toxicodynamics, and veterinary relevance of these compounds, providing an integrated framework for understanding plant-induced photosensitization in grazing systems.

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 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. 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.