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.
Here's what happened. When horses in a pasture dotted with wild onions came down with anemia (low red-blood-cell count), jaundice (increased bile pigments in the blood, causing yellowish discoloration of the gums and whites of the eyes), and reddish-colored urine, investigators decided to find out for sure whether onions had caused these symptoms.
To do so, they fed 1 pound of the onion tops per day to a healthy horse for 3 days, along with his regular feed, then 4 pounds on Day 4. From Day 4 through Day 8, his packed cell volume (or PCV, meaning his red-blood-cell percentage) dropped about 23 percent, from 30 to 23.
The investigators continued to give the horse onion tops on Days 9 and 10; by Day 11, his PCV had dropped to a life-threatening 13--he'd lost almost 60 percent of his red blood cells in 11 days!
Most of us with a rudimentary interest in equine toxicology have no quarrel with this report, but what would've happened to pasture horses if they'd eaten smaller doses of the toxin? Good question—and here's where the controversy comes in.
Some vets say the toxic effects are more gradual and insidious—but still very real—when a lower dose is consumed on a regular basis, resulting in a mild anemia without obvious symptoms. This has been my experience in a practice. I see a handful of cases of Heinz-body anemia every year in horses that grazed on wild onions growing amidst the grass in their pastures, or helped themselves to discarded onions and leeks in the compost pile, or raided the garlic patch in the garden.
No well-designed, formal research has been conducted on the ill effects of lower doses of garlic on horses. But, to be fair, there also hasn't been any well-designed, formal research on the benefits of garlic in horses. For example, I've seen lots of horses reeking of garlic and crawling with flies, though garlic is reputed to be an effective fly repellent.
I've no doubt those of you who are feeding garlic to your horses are doing so because you want only the best for them—the best health and the highest degree of comfort. That's why I feel it's important for you to understand it isn't enough to say garlic is safe just because you haven't seen any ill effects in your garlic-supplemented horse. Depending on the dose, and the frequency and duration of dosing, there could be low-grade deleterious effects, due to red-blood-cell damage that's not enough to cause a 9-1-1 situation, but just enough to cause a mild anemia that might not be outwardly evident. It might affect your horse's stamina, energy level, or resistance to disease.
Until these suspicions are investigated and repudiated, how much risk are you willing to take? Until well-designed, formal research is done on garlic's risks and benefits, specifically in horses, it seems the only safe avenue is the avenue of caution. At the very least, I wanted each of you to make your decisions with benefit of all available information, including longstanding reports from researchers far greater than me, indicating that the popular garlic bulb has a dark side.
Pierce, K.R., et al., Acute hemolytic anemia caused by wild onion poisoning in horses, "Journal of the American Veterinary Medical Association," 1972; pages 160/323 to 327.
Lewis L.D. "Equine Clinical Nutrition," Williams and Wilkins, 1995, page 480.
Kobluk, Ames, and George, "The Horse; Diseases and Clinical Management," Saunders, 1995, page 1,083.
Knight A.P. and Walter R.G. "A Guide To Plant Poisoning Of Animals In North America," Teton NewMedia, 2001, page 186.
Murphy M., "A Field Guide To Common Animal Poisons"; Iowa State University Press, 1996, page 160.
Dr.Hayes is an equine practitioner based in Idaho. She's a frequent contributor to Horse & Rider magazine and EquiSearch.