AffectedAnimals:
Horses.

Overview:
During warm weather, bot flies swarm around horses. These parasites, of the species Gasterophilus, lay numerous tiny white eggs on the haircoats of horses. Bot eggs are not only aesthetically unpleasing, but once they have hatched into larvae, they also may cause the horse pain and irritation in the mouth. Adult flies that have developed from larvae may also cause various gastrointestinal problems.

Horses infected by bots often develop no clinical signs of disease. There must usually be a large number of bot eggs present before the animal becomes sick from the infection. Deworming treatments are very successful in treating severe cases of infection, and there are several effective tools for removing the eggs from the hair.

Clinical Signs:
Clinical signs include the presence of tiny, white-colored eggs measuring roughly one to two millimeters. Bot eggs can be found on any part of the body covered in hair, but are usually located along the forelegs, shoulders, and face. Horses with bots may suffer mild gastritis and stomatitis, resulting in pain or discomfort during eating. However, most horses with bots remain asymptomatic, aside from the visible eggs on their haircoats.

Symptoms:
See Clinical Signs.

Description:
In the warm summer months, flies of the species Gasterophilus lay eggs on the haircoats of horses. These eggs, and the flies they produce, are called bots. There are three primary species of Gasterophilus. Flies of the G. intestinalis species lay their eggs anywhere on the horse�s body, but most commonly on the shoulders and front legs. G. hemorrhoidalis usually glue their eggs to the mouths of horses. G. nasalis deposit eggs on the bottom part of the lower jaw and throat. G. intestinalis are the most common form of bot flies.

After the eggs have been laid, they will hatch within a few days. The larvae migrate into a horse�s mouth, and bury themselves between the molars and in the tongue. The bots will stay there for approximately one month, causing the horse pain and irritation. Eventually, the bots are swallowed and move to the stomach, where they continue to develop for nine to 12 months. From the stomach, the larvae move throughout the rest of the intestines and are passed into the feces. Over the course of the next month, the larvae mature into adult flies. The adult fly�s remaining life span is typically about two weeks.

Larvae affect a horse most dramatically when they are living in its stomach, where they hook onto its lining and cause the formation of ulcers. While many horses with bots have no clinical signs, some severely infected horses will manifest symptoms that include gastritis and stomatitis.

Diagnosis:
In order to diagnose an infection of Gasterophilus, immature worms or larvae must be located and identified in the feces of the horse. A tentative diagnosis may also be made when white eggs are seen attached to the haircoat of the horse.

Prognosis:
Excellent.

Transmission or Cause:
Transmission occurs primarily in the summer months, when the Gasterophilus flies lay their eggs on the haircoat. Within a few days, the eggs hatch and the larvae migrate into the horse�s mouth. When a horse licks or chews its haircoat, it stimulates some species of Gasterophilus to hatch. Other species of bots are able to hatch on their own, without stimulation from the horse. After hatching, the bot larvae move into the mouth and are swallowed. Eventually, the bots will travel through the intestinal system and be ejected in a horse�s feces.

Treatment:
Horses infested with bot eggs should be dewormed with an effective medication, such as ivermectin. Treatment can be performed during any season, but optimally should take place during autumn or early winter, after the first frosts have killed adult flies.

Prevention:
Prevention relies on keeping the horse�s haircoat free of bot eggs. Dull blades, fine-toothed combs, and special pumice stones are effective devices. These objects help to scrape and remove the eggs from the haircoat of the infected horse. The use of fly repellents may be helpful in preventing flies from landing on the horse and depositing eggs. Also an effective tool, a fly mask is made from a mesh-like material, and fits around a horse�s face and ears while leaving its mouth free.