During the summer and early fall months, you may notice the appearancs of small yellow-ish dots on your horse's coat. These are the eggs of Gasterophilus, the botfly, which the botfly lays on the hair shafts on the chest and forelegs of the horse. Botflies are particularly annoying to horses, causing them to stamp and bite at themelves.
This biting action of the horse stimulates the larvae to hatch from the eggs and the horse then, in the course of his licking and biting, takes the larvae into his mouth. Once in the mouth, the larvae burrow into the cheeks and tongue of the horse, where they remain for about three weeks. While in the mouth they do not appear to cause any discomfort or worry the horse.
From the mouth, the small larvae migrate to the stomach, where they develop into larger larvae. They attach themselves to the stomach wall by their teeth. They stay in the stomach all winter, and can cause gastritis and even perforation of the stomach. In the spring, they release their hold and pass out in the manure to develop into adult flies and begin the life cycle again.
Horse owners can control the infestation of their horses by use of a bot knife to remove the bot eggs from the hair on a daily basis during the fly season. In addition, modern dewormers containing ivermectin are efficient at killing the larvae that reach the stomach. For this reason, many veterinarians advocate the rotation of dewormers to include products containing ivermectin, such as Eqvalan, Zimecterin or Equest (UK), in the fall and again in the spring for maximum efficiency.