Horse lovers should all learn to recognize bot eggs. These are the eggs laid by botflies along your horse’s legs or possibly on the mane or coat. The eggs look like tiny lice. Botflies themselves look like honeybees at first glance, but they don’t bite or sting. Instead, botflies “bug” horses by swooping around to lay their eggs. One female botfly may lay as many as 500 eggs and they prefer horses, donkeys and mules so other animals are generally left alone.
The most common botfly for horses is Gasterophilus intestinalis. Their yellowish eggs are laid along the legs and flanks of your horse. As your horse rubs or itches those areas, the eggs hatch, sending larvae into your horse’s mouth. Eggs are primed to hatch on exposure to warmth, moisture and carbon dioxide – all present in your horse’s breath. Larvae can live in your horse’s mouth for up to a couple of weeks, then move down to their stomach. Here the larvae attach to the stomach and develop over the winter. In the spring, they will pass through your horse’s digestive system in their manure, pupate and become adult botflies. Parasitologists argue as to how much damage the larvae can cause to your horse, but at the very least, they must be an irritant and a drain on nutrients.
To keep your horse safe from any ill effects of bots, you can start by diligently trying to comb off new eggs daily. Bot “knives” or combs are used to scrape the eggs off. Getting all of them is virtually impossible however. Washing the egg covered areas with warm water may stimulate early hatching and prevent some larvae from getting into your horse.
Using a fly repellent for horses during warm weather may keep adult botflies away. Practicing good manure management is also important. Clean stalls and paddocks daily. Drag larger areas to break up manure. Rotational grazing practices will help reduce all parasite problems.
Ivermectin for horses and Moxidectin are two pet medications that will kill bot larvae. Quest Gel and Zimecterin are two dewormers that will attack bots. You should discuss the timing of the deworming with your veterinarian. This will vary across the US based on when you have your first frost. Adult botflies die in the frost, so cold weather means no more bot eggs and larvae. Be sure to have the correct dose for each of your horses. Drawing up a yearly deworming schedule for your horses should be a part of your horse’s annual veterinary examination.