Oak
Horses are susceptible to oak poisoning caused by the tannic acid that accumulates in new leaves and acorns that horses will eat when normal forages are scarce. Although all 60 species of oak that grow in North America are potentially toxic, most livestock poisoning is attributed to Gambels oak (Quercus gambelii), Shinnery oak (Q. havardii), and Q. breviloba. Oaks range from shrubs to large trees. All have alternate, simple, toothed or lobed, dark green glossy leaves that become red in the fall. The plants are monoecious, with the staminate flowers occurring in long catkins and the pistillate flowers occurring singly or in small clusters. The fruit, an acorn, is a nut partially enveloped by an involucre of scales.
Tannins found in the leaves, bark, and acorns of most oak species are presumed to be the toxin causing poisoning in animals. Tannins are potent precipitators of cellular protein (astringents), which when ingested cause severe damage to the intestinal tract and kidney. Oaks at any stage of growth are poisonous, but are particularly toxic when the leaf and flower buds are just opening in the spring. As the leaves mature, they become less toxic. Ripe acorns are also less toxic than green acorns. Cattle, sheep, horses, and pigs are susceptible to oak poisoning. Ruminants frequently browse on oak without apparent problems, provided they have ample access to normal forages.
Clinical signs of oak poisoning vary according to the quantity of oak leaves, bark, or acorns consumed. Initially animals stop eating, become depressed, and develop colic. The feces are hard and dark, but a bloody diarrhea often occurs later in the course of poisoning. Some horses may appear to have choked with ingested food, and saliva passes out the nose. Mouth ulcers may also be present. Severe gastrointestinal, liver and kidney damage and a low blood calcium and phosphorus concentrations are usually present. Horses may die within a 24-hour period after eating large quantities of acorns, or may live for 5 to 7 days after the onset of clinical signs.
Affected animals should be removed from oak pasture and given supportive care in the form of fresh water and hay. Mineral oil, intravenous fluids and drugs to control pain are particularly indicated for colicky horses.
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