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SECONDARY (Hepatogenous) PHOTOSENSITIZATION
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Secondary or type III photosensitization is by far the most frequent type of photosensitivity observed in livestock.
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The photosensitizing agent, phylloerythrin (a porphyrin), accumulates in the plasma due to impaired hepatobiliary excretion.
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Phylloerythrin is derived from the breakdown of chlorophyll by microorganisms present in the GI tract.
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Phylloerythrin, but not chlorophyll, is normally absorbed into the circulation and is effectively excreted by the liver into the bile.
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Failure to excrete phylloerythrin due to hepatic dysfunction or bile duct lesions increases the amount in the circulation. Thus, when it reaches the skin, it can absorb and release light energy, initiating a phototoxic reaction.
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Phylloerythrin has been incriminated as the phototoxic agent in the following conditions: common bile duct occlusion; facial eczema, lupinosis, congenital photosensitivity and poisoning by numerous plants including Tribulis terrestris (puncture vine), Lippia rehmanni , Lantana camara , several Panicum spp (kleingrass, broomcorn millet, witch grass), Cynodon dactylon , Myoporum laetum (ngaio), and Narthecium ossifragum (bog asphodel).
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Photosensitization also has been reported in animals that have liver damage associated with various poisonings: pyrrolizidine alkaloid (eg, Senecio spp , Cynoglossum spp , Heliotropium spp , Echium spp ), cyanobacteria ( Microcystis spp , Oscillatoria spp ), Nolina spp (bunch grass), Agave lechuguilla (lechuguilla), Holocalyx glaziovii , Kochia scoparia, Tetradymia spp (horse brush or rabbit brush), Brachiaria brizantha, Brassica napus, Trifolium pratense and T hybridum (red and alsike clover), Medicago sativa, Ranunculus spp, phosphorus, and carbon tetrachloride.
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Phylloerythrin is likely the phototoxic agent in many of these poisonings.
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