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Another option is to mix small amounts of apple cider vinegar with the animal's drinking water; consuming this solution is known to change the blood pH of the horse, thus keeping horseflies away. Cider vinegar, coupled with essential oils, is also used in home sprays as the sharp odor may repeal horseflies.
Ovitrol Plus spray kills and repels fleas, flea larvae, ticks, lice, mosquitoes, flies, and gnats. It also prevents flea eggs from hatching for up to 2 months and prevents sprayed botfly eggs from hatching. The shampoo is for cats and dogs; the spray may be used on these animals as well as horses.