White Snakeroot and Crofton, Jimmyweed, and Burrow Weed
White snakeroot or richweed (Eupatorium rugosum) has been known for many years to cause a disease called ''trembles'' in horses and other livestock, and milk sickness in people who drink the milk from cows that have eaten white snakeroot. Sporadic cases of white snakeroot poisoning occur in horses allowed access to areas where the plant grows or when it is present in their hay. ''Trembles'' is also encountered in horses and other livestock that eat Jimmyweed or rayless goldenrod (Haplopappus heterophyllus) and Burrow weed (H. tenuisectus), which grow in the southwestern United States and northern Mexico.
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White snakeroot is an erect perennial growing about 5 ft (1.5 m) tall and having opposite ovate leaves with markedly toothed edges. Flowers are white and produced terminally in clusters of 20 to 30 small five-petaled flowers 1ž 8 to 1ž 4 in (3 to 5 mm) across. The seeds contain white hairs at one end that aid in wind distribution. The plant prefers to grow in moist wooded areas from Minnesota south to eastern Texas, and all the eastern States.
The principle toxin in white snakeroot is tremetol, a fat-soluble higher alcohol. Jimmyweed, Burrow weed, and also Crofton weed (Eupatorium adenophorum) also contain tremetol and cause poisoning in horses. The toxin is present in greatest quantity in the mature plant, particularly the leaves, and persists in frozen and dried leaves. Tremetol is cumulative in animals; consequently, the amount consumed at any one time is not critical. Small doses over a period of time will produce poisoning. An amount of green plant equal to 1 to 10% of body weight is potentially lethal to horses and other animals. Tremetol is readily soluble in fat and is excreted in milk, which is the primary means by which nursing animals and infants are poisoned. Nonlactating animals tend to show more severe signs of poisoning since tremetol can't be excreted from the body except by lactation.
When sufficient tremetol accumulates in the body, muscle tremors occur, as the name for the condition, ''trembles,'' implies. In addition horses may present with difficulty swallowing, excessive salivation, and the appearance of being choked. Depression, patchy sweating, incoordination greenish discharge from the nostrils, and death within a few days of the onset of clinical signs may occur in horses poisoned with white snakeroot. Other diseases that can resemble white snakeroot poisoning include botulism, rabies, lead poisoning, and esophageal obstruction.
There is no specific antidote to tremetol. Affected horses should be removed immediately from pastures or hay that contain white snakeroot or other tremetol-containing plants and laxatives given to help prevent further absorption of the toxin. Mares should be milked by hand frequently to help remove the toxin from the body. Foals should be removed immediately from affected mares to prevent further intake of the toxic milk, and fed as described for orphan and early weaned foals in Chapter 15.
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