Botflies are a member of the fly family Oestridae, they are common virtually everywhere horses are kept, with two major species found in the United States. Bots like any other parasite need a host to carry out their life cycle. They are specialists, in that they only attack horses, mules and donkeys. They do not use cattle or other livestock as a host.
The botfly lays its’ eggs on the horse, glueing the egg to the shaft of a hair one egg at a time. A single female can lay up to 500 eggs. The botfly does not bite the horse but rather irritates the horse with a tickling sensation as it deposits the eggs. Horses will be seen repeatedly stomping, tossing the head, swishing the tail and even running trying to get away from the botfly. In some cases the horse is so distracted by the botflies that it becomes a hazard to its’ handler and does not perform well.
There are three species of bots that affect horses, mules and donkeys. They are the common bot (Gasterophilus intestinalis), the throat bot (Gasterophilus nasalis), and the nose bot (Gasterophilus hemorrhoidalis). In our area we have common bots and throat bots. While botflies are a type of fly they actually look more like small bees. The adult fly has a short life, it cannot feed, it cannot bite and has small non-functional mouthparts. It’s only job is to lay eggs. It must live on stored energy and when the supply is exhausted the fly will die. The adult female bot fly glues her eggs to the hair shafts of specific body parts of the horse. The eggs on the hair coat are stimulated to hatch by a combination of warmth, moisture, and carbon dioxide. All of these are supplied whenever a horse nuzzles its lower legs or grooms the coat of a herd mate.
In our area botflies are most active during the fall months. Eggs of the common bot are yellow and are glued to the hairs along the forelegs and flanks. Five days after the eggs are laid, they contain first stage larvae that are ready to hatch rapidly as the horse rubs its’ muzzle and tongue over the area. When the horse scratches himself with his muzzle and teeth the larvae enter the mouth and burrow into the skin into the tongue.
The throat bot eggs are whitish yellow and are laid under the jaw and throat area. Larvae hatch within three to five days and then crawl along the jaw and enter the mouth.
The larvae invade the tongue and oral tissues around the molars. While residing in the mouth they molt. After a molt to the second larval stage, the juveniles are found within pockets between the gums and cheek teeth. Ultimately, the second-stage larvae are swallowed and carried to the stomach and intestine. In the stomach they attach to the lining and remain there until spring or summer. Stomach lesions produced by second & third stage larvae include deep pits in the stomach wall at the point of attachment of each bot. Thus many pits results as the larvae accumulate into the typical clusters which may measure several inches in diameter. Stomach pits heal quickly once bots are removed. When warm weather returns in the spring, bot larvae detach from the gastrointestinal tract and are passed out in the manure where they will pupate in the soil or fecal pile. In about one month the adult flies emerge, mate and the life cycle starts over as the female begins laying eggs on a horse.
Prior to the development of Ivermectin horses had to be dewormed using carbon disulfide or trichlorofon. These medications were dangerous if not dosed correctly and required tube worming where a nasogastric tube is passed into the stomach. In the “old days carbon disulfide and trichlorofon had to be given 1 month after a killing frost because only certain stages of the parasite would be responsive to these drugs.
With the advent of Ivermctin removal of bots is simple. Treatment of horses for removal of bot larvae is recommended after a killing frost in the late fall or early winter. Ivermectin is effective against all stages of larvae and can be done anytime after a killing frost.
Grooming can help to reduce parasite exposure. You can use a bot knife, clippers, razor, bot-removal block or sandpaper to remove egg cases. All of these practices are labor- intensive, must be repeated frequently, and are not likely to be entirely successful. Preventive products that might discourage female flies from depositing eggs on the hair coat have not been developed.
Bot infection cannot be diagnosed by fecal examination unless larvae are present in the sample. Characteristic egg cases on the hairs of the forelimbs and other parts of the body are indicative of internal infections of bot larvae.
Friends We Have Lost
The Jeff Carter family bid a sad farewell to LHR Cherokee, their 17 year old Miniature horse mare after a severe bout of colic.
Rad & Judy Fountain fought a tough battle with laminitis that finally claimed the life of their beloved TWH gelding Roy. Roy had been a fixture on the family farm and his passing leaves big shoes to fill.
Colic claimed another life when it struck Dorinda & David Hennings' Quarter Horse gelding Bar-O-Bear. Cisco had been a favorite of the family and will be missed very much.
Sadly I report the death of Red Keno Cody, a black Quarter Horse who was one of my first patients when I entered veterinary practice nearly 25 years ago. The spunky gelding was a perfect match for his equally spunky young rider Amanda Mandel who competed with him in three day eventing. At the age of 31 he was struck with severe neurologic disease. We will miss seeing him and the Mandel family.
Wishing everyone a joyous holiday season,