Wild parsnip should be considered as a possible cause of skin problems in horses in the summer according to a recent report.
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Although the parsnip root has been used for food for centuries, handling the shoots and leaves of the plant requires caution as the sap is toxic. It contains chemicals known as furanocoumarins,
which can make skin more vulnerable to ultraviolet light. Brushing against or breaking the plant releases sap that when activated by exposure to sunlight can cause a severe burn, a condition known as phytophotodermatitis.
A recent report of an investigation of a natural occurrence of photosensitization of grazing horses identified wild parsnip as the likely cause.
Bryan L.Stegelmeier of the United States Department of Agriculture, Poisonous Plant Research Lab, Logan, UT is lead author of the report, which is published in the journal Toxicon.
Laboratory analysis of the plant identified five furanocoumarins: (xanthotoxin, bergapten, isopimpinellin, imperatorin and a putative methoxyimperatorin).
Ingestion of the plant caused minimal problems. Of four goats fed the plant, only one showed mild perivulvar irritation. However severe photodermatitis occurred in both goats and a horse when the plant was applied to the skin.
The authors conclude that wild parsnip-induced superficial necrotizing dermatitis was consistent with photodermatitis with no evidence of other allergic or inflammatory components.
For more details, see:
Wild parsnip (Pastinaca sativa)-induced photosensitization
Bryan L.Stegelmeier, Steven M.Colegate, Edward L.Knoppel, Kerry A.Rood, Mark G.Collett
Toxicon (2019) pii: S0041-0101(19)30180-1