Horsebrush is restricted to the drier sage brush range lands of the Rocky Mountains area. It usually grows in the same terrain as sage brush.
Poisoning in sheep occurs when horsebrush is gazed with black sage(Artemisia nova). The mechanism by which horsebrush and sage potentiate toxicity is not understood. T. glabrata is more toxic than T. canescens. About 0.5% of the animals body weight of green plant being the toxic dose. Tetradymol, a sesquiterpene, is the principle liver toxin. It is bioactivated by the liver to produce metabolites that disrupt oxidative phosphorylation in the liver cells.
Coarse, woody, branching shrubs up to 3 feet in height and 5 feet wide. Stems vary in color from grey to light green. Leaves and stems are covered with white hairs giving leaves a whitish cast. Leaves are linear,alternate and single. Flowers are produced in large numbers at the tips of branches. The yellow flowers are produced in composite heads, each flower consisting of 4 yellow florets. Straw colored bristles protrude between the florets, growing straight up from the fruits.
Unlike rabbit brush that blooms late in the summer, horsebrush blooms in early summer.
T. canescens - spineless horsebrush
T. glabrata - littleleaf horsebrush
There is no specific treatment as liver disease is frequently severe by the time the problem is recognized. Sheep should be removed from the horsebrush and given supportive therapy.
Photosensitization occurs secondarily to the liver disease. Redness and edematous swelling of the tissues around the head giving the name
Severe anorexia, depression, dyspnea, prostration and death result from acute liver failure.
Serum liver enzymes are elevated in acute horsebrush poisoning. Horsebrush causes liver disease characterized by central-lobular necrosis and fatty degeneration.
Sheep seldom eat horsebrush when other forages are available.