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Q: I am always looking for natural ways to repel flies in my barn and I have heard that feeding garlic can stop flies from bothering horses. Does this work and is it safe to feed garlic to horses?
A: Garlic is touted for several beneficial functions, including respiratory health (it is supposed to alter the physical properties of mucus), antibacterial, and antimicrobial properties, and its supposed ability to repel insects. The thinking is that feeding garlic creates an odor in sweat that helps repel flies.
Garlic and other members of the onion family are rich sources of organic sulfur compounds (in organic chemistry, “organic” means a compound contains a significant amount of carbon). One of the many sulfur compounds present in garlic is a thiosulfate, which has been associated with antibacterial and antiparasitic properties found in allicin (a pungent, oily liquid in garlic).
Despite work showing the antibacterial and antiparasitic properties of allicin, there is limited data on the efficacy of garlic as a fly repellent either when applied topically or given orally, and none I’m aware of in horses. A study in people looking at garlic oil when applied topically at a 1% dilution did show a 97% repellent effectiveness.
Potential Dangers of Feeding Garlic
A handful of studies have looked at the potential side effects of feeding horses garlic. Garlic and other alliums (plants in the onion family) contain several potential toxins, the principal one being N-propyl disulfide. This compound alters the enzyme glucose-6-phosphate dehydrogenase in red blood cells, which interferes with the cell’s ability to prevent oxidative damage to hemoglobin (a protein responsible for transporting oxygen in the blood). The damaged hemoglobin precipitates on the surface of red blood cells resulting in what is known as Heinz bodies. The liver and spleen both act to remove damaged red blood cells from circulation, leading to anemia due to reduced numbers of circulating red blood cells. This form of anemia is known specifically as Heinz body anemia.
Feeding more than 0.4 grams per kilogram body weight of freeze dried garlic has led to Heinz body anemia in horses. This is the equivalent to feeding a 500-kilogram (1,100-pound) horse 200 or more grams per day, which is just under half a pound. While this amount could easily be consumed by horses eating wild garlic growing in pastures it is unlikely that this amount would be given on purpose.
However, in a second study researchers showed a tendency for decreased hemoglobin and red blood cell count when feeding 32 milligrams (mg) of garlic per kilogram of body weight over an 83-day period. This suggests that in the same 500-kilogram horse feeding as little as 16 grams (about half an ounce) could cause alterations in blood chemistry.
While many people feed garlic and report no ill effects, the clinical signs might not be obvious and depending on the dose, frequency, and duration of the dose given there could be low-grade deleterious effects.
The National Research Council states that more data is need to fully assess the potential risks and benefits of feeding garlic but that feeding 15 mg per kilogram of body weight per day (or 7.5 grams) of dried garlic powder on a long-term basis should not lead to any adverse event in horses under normal circumstances.
Take-Home Message
While feeding 7.5 grams of garlic per day might not lead to side effects, one does have to wonder whether that amount would confer any benefit as an insect repellent. Applying dilute garlic oil topically might be far more effective.
