An escaped weed in many areas, growing in distrubed soils, roadsides, but rarely persisting. It is also grown as a cover crop, and as a source of buckwheat flour.
Througout North America.
Cattle, horses, sheep, goats
Fagopyrin, a dianthroquinone is present in both the green and dried plant; but not the ripe seeds. Once absorbed from the gastrointestinal tract, the fagopyrin in the blood vessels of the non pigmented skin, reacts with ultra violet waves from sunlight. Radiant energy in the form of fluorescence produced by the fagopyrin when exposed to sunlight causes damage to the blood vessels in non pigmented skin. This primary photosensilization can be severe to the point of causing cell death and sloughing of the skin.
Fast growing annual herb with erect stem and a tap root. Stems are hairless accept at the nodes. Leaves have long petioles, the leaf shape varing from arrow to heart-shaped. Small white flowers are produced in the leaf axils. Seeds are angular, 3-sided and turn brown when ripe.
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 entirely from the diet of the animal. Antihistamines and antiinflammorty drugs may help in the early case. It may take up to 2 -3 months for the skin to heal and the hair regrow.
Animals with white skin are most severely affected. Black skinned animals are not affected except for showing photophobia if the eyes are not pigmented. Initially the non pigmented skin becomes reddened, severely pruritic, swollen and painful. Severe photosensitization results in serum exidation and necrosis of the skin, causing the skin to become dry, parchment-like and eventually sloughing. Affected animals become very agitated when exposed to sunlight often desperately seeking shade under vehicles, buildings, trees etc. to avoid exposure to sunlight light.
Photophobia and tearing may be evident
Photosensitization. Serum liver enzymes are usually normal, helping to differentiate primary photosensitization from secondary photosensitization resulting from severe liver failure.
Buck wheat is often grown as a cover crop for soil improvement, and as such poses a risk to animals that gain access to the field.
It is important for a veterinarian to differentaite between primary and secondary (liver induced)photosentization as the prognosis for primary photosensitization is much better. The main plant primary photosensitizers are Buckwheat,
St. John's wort, bishops weed (Ammi majus), and spring parsely (Cymopterus species)
Buckwheat leaves and flowers
Horse with photosenstization