White snakeroot (Eupatorium rugosum
or Ageratina altissima
) is an erect, branched, perennial herb.
White snakeroot blooms during the late summer months, where numerous small heads of white flowers appear at the top of the stem and the ends of the branches. It's fruits are distinguishable as brown or black cigar-shaped capsules, single-seeded and tipped with a tuft of white hairs. It's leaves are 3 to 5 inches long and petioled, are sharply toothed on the margins, dull on top and shiny on the bottom.
White snakeroot contains tannins and the chemical compound tremetol, which is a poisonous alcohol. Poisoning in livestock is sporadic due to the considerable variation in the quantity of the toxin present in the plant---as it varies from location to location and also differs depending on current stage of growth. Tremetol causes hypoglycemia and ketoacidosis by impairing the tricarboxylic acid (TCA) cycle in horses. It causes degeneration of the skeletal and cardiac muscles and injures the liver and kidney. The major effects are related to congestive heart failure. Horses develop a condition known as "trembles" that may cause death. Lactating mares excrete the toxin in their milk, which they can then pass to their foals.
The onset of clinical signs in white snakeroot poisoned horses is within 2 days to 3 weeks after initial ingestion. The effect of consumption of white snakeroot is cumulative. Horses may die from eating a large amount of the plant at one time, or from eating small amounts over a long period of time.