St. John's wort (Hypericum perforatum
) is an upright perennial herb that is native t Europe. It has been introduced to most other regions worldwide, and is well known for its medicinal value. H. perforatum
has yellow-orange, five-petaled flowers which have black dots along the margins. Its leaves are oblong to linear and are arranged opposite one other along the stems with small, pinhole-like dots. The stems are often reddish and have two ridges that run along their length. H. perforatum
produces reddish-brown sticky fruit capsules which contain many dark brown or black seeds.
Typically, horses only consume H. perforatum
when no alternative forage is available. H. perforatum
contains quinones, which can cause photosensitization
in horses. The extent of skin damage is typically limited to the unpigmented (white-colored areas) skin that were in contact with the plant. The toxin is a quinone called hypericin. After ingestion of H. perforatum
, hypercin gets absorbed from the gut and enters the bloodstream where it is distributed to all the tissues and skin. When the hypericin in the skin is exposed to sunlight, it interacts with the cells and causes tissue damage. Hypericin levels within the plant are highest in the spring, during rapid growth of the plant and remain high until the end of flowering.