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Dr. Karen Hayes, DVM, MS, discusses whether or not to feed your horse garlic as a feed supplement or fly repellent.
It's heartening to see how passionate readers are about the care of their horses--and their garlic!
The toxic element in allium (a family of plants including both garlic and onions) is well known to be a chemical called N-propyl disulfide. By altering an enzyme present within the red blood cell, it depletes the cell of a chemical known as phosphate dehydrogenase (PD), whose job is to protect the cell from natural oxidative damage.
When the PD level gets low enough, the hemoglobin in the cell oxidizes and forms a "bubble" called a Heinz body on the outside of the cell--it's quite distinctive and readily seen under the microscope. The spleen--which acts as a red-cell "bouncer" of sorts--quickly removes the deformed cell from the bloodstream. As more and more red cells are prematurely damaged and removed, as will happen from consistent poisoning with N-propyl disulfide, your horse gradually becomes anemic. This is called Heinz-body anemia.
The "toxic dose" of N-propyl disulfide, which is not well worked out in any species, is the amount thought to cause obvious poisoning, a sort of "9-1-1" situation. Cows are thought to be more sensitive to the toxin than are horses, but in one study published in 1972 in the "Journal of the American Veterinary Medical Association," the toxic dose in horses turned out to be considerably less than the 5 grams per kilogram of body weight reported in cows...
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