Onions
Poisoning from wild onions (Allium spp.) and domestic onions (A. cepa) will occur in horses that have eaten large quantities of the plants or the bulbs. Cattle are the most susceptible to onion poisoning, with horses and dogs being less so. Sheep and goats can tolerate eating considerably more onions than can other animals. Although species of wild onion are found in moist areas of most states, onion poisoning is most often caused by feeding culled domestic onions.
Onions contain an alkaloid which inhibits a red blood cell enzyme, causing hemoglobin denaturation and precipitation. This is recognized by the development of Heinz bodies in the red blood cells. The body removes these altered red blood cells from circulation, causing anemia and the clinical signs.
Depending on the quantity and rate at which onions are eaten by a horse, effects vary from mild anemia without clinical signs to severe anemia, icterus, increased heart and respiratory rates, weakness, a dark coffee-colored urine due to hemoglobin in it, and death. The odor of onions is often detectable on the breath. The presence of Heinz bodies in the red blood cells detectable microscopically is highly suggestive of onion poisoning. However, Heinz body anemia also occurs from red maple leaf ingestion and may occur in horses with lymphosarcoma-induced cancer. The hematocrit is often as low as 10 to 15%.
Horses with onion poisoning should not be stressed; they should be removed from the source of the onions and fed a good diet. Severely anemic animals may require blood transfusions.
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