Researchers in Utah have discovered that wild parsnip—an invasive weed found throughout the United States—can cause phototoxic reactions in horses even if they don’t eat it.
Many photosensitive skin reactions occur after a horse ingests a plant that contains photodynamic compounds. When ultraviolet rays from sunlight pass through the horse’s pink skin, they interact with the compounds in the skin and blood, resulting in painful burns with extensive blistering.
However, Utah State University and U.S. Department of Agriculture researchers recently determined that horses and goats may develop photosensitive skin reactions after simply coming in contact with the sap of wild parsnips.
A member of the carrot family, wild parsnip (Pastinaca sativa) can grow up to four feet tall, and looks and smells similar to the cultivated root vegetable found in grocery stores. The sap of wild parsnip contains furanocoumarins, chemicals known to cause intense localized skin reactions in people who come in contact with them on sunny days. Affected areas of skin can remain sensitive to sunlight for up to two years after initial contact.
The researchers report that only one of four goats that ingested wild parsnip developed skin irritation, but when a horse and the goats came in contact with the plant, they developed severe photodermatitis. The researchers determined that exposure to sap on the surface of the wild parsnip’s leaves was the cause of the reactions.
Reference: “Wild parsnip (Pastinaca sativa)-induced photo-sensitization,” Toxicon, September 2019