Join Date: Oct 2009
Location: Just south of sanity
Feeding Garlic - The Great Garlic Debate
By Karen Hayes, DVM, MS
There's a toxic element in allium (a family of plants including both garlic and onions) of 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. 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.
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.
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 911 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.