Larkspur or Poison Weed
Larkspur poisoning is of greatest concern for cattle, causing more cattle fatalities in the western United States than any other naturally occurring plant species. Horses are not as susceptible as cattle to the alkaloids in larkspur, and are infrequently poisoned by larkspur. However, consumption of larkspur will cause sudden death of horses, as well as of cattle and other animals.
There are at least 80 species of larkspur or poison weed (Delphinium spp.) in North America, most of which grow in the western United States. They are erect, perennial herbaceous plants with simple or branched hollow stems and alternate, palmate divided leaves. The flowers are perfect and irregular and are carried in terminal racemes. They vary in color from white to red to a dark blue purple. The flower has five sepals, the uppermost one having an obvious spur. The corolla is composed of two sets of two petals each, the two lower ones forming a claw and the upper two extending into the spur.
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Larkspur poisoning is related to the quantity of toxic alkaloids in the plant. This varies with the plant species, the stage of growth, the amount ingested, and the duration in which the plant is eaten. Young, rapidly growing plants are the most toxic, the highest concentration of toxic alkaloids being in the leaves and flowers. Delphinium barbeyi is the most toxic of the larkspurs. As little as 0.5% of body weight of green D. barbeyi is lethal to cattle. Sheep can tolerate 3 to 4 times as much larkspur as can cattle, and horses appear to be between cattle and sheep in susceptibility to larkspur poisoning. A horse would therefore be poisoned by eating an amount equal to 1 to 2% of its body weight of green larkspur, depending on the alkaloid content of the plant.
Larkspur alkaloids act primarily at the nerve-muscle junction, causing a curare-like blockade of nerve impulses with resulting muscle weakness and paralysis. Clinical signs of larkspur poisoning are best described for cattle but are similar in affected horses, sheep, and goats. Sudden death is often the first indication of larkspur poisoning. Poisoned cattle initially show uneasiness, increased excitability, and muscle weakness that causes stiffness, staggering, and a base-wide stance. The front legs may be most severely affected, causing the animal to kneel before finally going down. Frequent attempts to stand are uncoordinated and result in rapid exhaustion. Muscle twitching, colic, regurgitation, bloat, and constipation are common. Inhalation of regurgitated rumen contents commonly leads to severe pneumonia and death. Cattle frequently die within 3 to 4 hours of consuming a lethal dose of larkspur.
Early diagnosis of larkspur poisoning through recognition of the clinical signs and observation that horses have eaten larkspur is essential for successful treatment. Stress and excitement of the affected animal should be avoided as it will exacerbate respiratory distress and hasten death.
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