Common Name
St. John's wort, Klamath weed, goat weed, tipton weed
Botanic Name
Hypericum perforatum L.
Plant Family
Introduced from Europe, it has become a weed of roadsides, meadows, prairies.
Animals Affected
Cattle, sheep, horses.
St-Johns-wort Hypericum perforatum
Toxic Principle
Hypericin, a naphthodianthrone, is a red pigment produced in the glands found in the leaves and flowers. Highest concentrations are found in the black glands, while the translucent glands contain the precursor protohypericin. There is considerable variation in the amount of hypericin produced in the plants. Hypericin fluoresces in the presence of sunlight (ultra violet), and causes acute inflammation and necrosis of the endothelial cells of the capillaries of the skin. Initially the white, non-pigmented skin becomes intensely pruritic, reddened and swollen, and then sloughs off. Hypericin has very similar photodynamic effects as fagopyrin, the pigment found in buckwheat (Fagopyrum esculentum). Cattle are poisoned when consuming about 1% of their body weight, while sheep must eat about 4% of their body in green plant to develop photosensitization.
An erect branching perennial 1-3 feet tall with a rhizomatous root system. The leaves are eliptical to triangular that contain many glands that appear as translucent or black dots on the leaves and flower parts. The showy 5 petal, 5 sepal yellow flowers are produced terminally on the stems.The seeds are produced in a capsule.
Affected animals should be immediately moved out of the sun and preferably kept in a dark stall or barn. St. John's wort should be removed entirley from the diet of the animal. Antihistamines and antiinflammorty drugs may help in early cases.
Integumentary System
Animals with white skin grazing St John's wort initially develop reddening and edema of the skin, pruritis, and eventually sloughing of the skin. The pigmented or dark skinned areas adjacent to the white skin are not affected. In white breeds of sheep, the photosenstization may only be evident on the nose and ears as the fleece protects the animals's skin.
Serum liver enzymes are usually normal, helping to differentiate primary photosensitization from secondary photosensitization resulting from severe liver failure.
Special Notes
St. John's wort has become popular as a herbal medicine for its antidepressant effects. Some individuals taking the medication develop intense photophobia ands burning sensation in the skin when they are exposed to sunlight. References 1. Kako MDN, Al-Sultan II, Saleem AN. Studies of sheep experimentally poisoned with Hypericum perforatum. Vet Hum Toxicol 1993, 35:298-300.
Back-lit leaf to show translucent glands