Phylloerythrin is a photosensitizing agent derived from microbial breakdown of chlorophyll in the GI tract. If the liver is damaged, then phylloerythrin builds up in the plasma. This phylloerythrin absorbs and releases light energy in the skin, causing SECONDARY photosensitization. This is the most common type of livestock photosensitization. So, when sheep eat hepatotoxic Tetradymia spp. and Artemisia nova, the compromised liver allows toxic photosensitizing phylloerythrin to build up. PRIMARY photosensitization occurs in the absence of hepatic disease when a photodynamic agent is ingested/injected/absorbed. Hypericum perforatum (St. John’s wart) is an example of a plant that causes primary photosensitization. Tetracycline is a medication that can cause primary photosensitization. Congenital inherited photosensitization is seen in some breeds of cattle and sheep. Ref: Blackwell’s 5-Minute Consult: Ruminant. Image courtesy of Lucien Mahin.