**Equine Botflies** (genus *Gasterophilus*) are parasitic flies whose larvae (bots) infest the gastrointestinal tract of horses, donkeys, and mules. The flies are not native to North America, but several species have become established pests of horses globally. While the adult fly is non-feeding and harmless, the internal larval stage causes significant damage to the host, including irritation, stomach lesions, impaired digestion, and, in severe cases, rupture and colic.
Taxonomy and Classification
Equine Botflies belong to the family Oestridae (botflies) in the order Diptera (true flies). They undergo complete metamorphosis. There are several species, including the common horse bot fly (*Gasterophilus intestinalis*) and the throat bot fly (*G. nasalis*), each with specific egg-laying preferences. The larvae are obligate parasites, meaning they must live inside the host animal to complete their development.
Physical Description
Adult Botflies are stout, hairy, brownish flies, resembling honeybees, but they lack the typical fly mouthparts as they do not feed. They are rarely seen, as their flight period is spent primarily laying eggs.
The **larvae (bots)** are the damaging stage. They are fleshy, pale yellow to pinkish, cylindrical maggots, measuring $1/2$ to $3/4$ inch long, with two prominent, black oral hooks at one end and a spiny body surface. They attach themselves firmly to the lining of the horse’s stomach or small intestine.
The **eggs** are tiny, yellowish, and firmly glued to the hairs of the horse’s body, depending on the species: *G. intestinalis* eggs are usually laid on the forelegs, and *G. nasalis* eggs are laid under the chin.
Distribution and Habitat
Equine Botflies are distributed globally wherever horses are raised. Their external habitat includes pastures and stables. The larval habitat is the internal mucosa of the horse’s digestive tract. Adults are most active in late summer and fall, coinciding with the peak egg-laying period.
Behavior and Life Cycle
The botfly life cycle is synchronized with the seasons, lasting about one year. The female fly lays eggs on the host’s hair. The horse ingests the eggs through licking or scratching, or the eggs hatch spontaneously near the mouth. The larvae penetrate the oral tissues (gums, tongue) and migrate for several weeks before migrating to the stomach, where they attach to the lining.
The larvae remain in the stomach for 8 to 12 months, overwintering and feeding. In the spring, they detach, pass out with the feces, and pupate in the soil. The adults emerge a few weeks later to begin the cycle again. The presence of the flies when laying eggs can cause panic in the horse (**gadding**), leading to injuries.
Damage and Health Impact
Damage is caused by both the migratory and stomach-dwelling stages:
- **Oral Damage:** Larval migration in the mouth can cause irritation and inflammation, interfering with feeding.
- **Stomach Lesions:** The bots attach by their oral hooks, causing crater-like lesions, ulcers, and hemorrhaging in the stomach lining.
- **Impaired Digestion:** Heavy infestations reduce the functional area of the stomach, potentially blocking the pylorus and causing severe digestive upset, including **colic** and potential rupture.
- **Gadding Stress:** The adult fly’s presence causes stress and unnecessary running by the host.
Management and Prevention
Control requires breaking the life cycle with veterinary care and physical removal.
- **Deworming:** The primary control is administering a bot-effective dewormer (typically Ivermectin or Moxidectin) in the **late fall or early winter**, well after the first heavy frost has killed the adult flies, but while the larvae are still in the stomach.
- **Physical Removal:** Regularly scraping or washing the eggs from the horse’s hair throughout the summer and fall prevents larval ingestion.
- **Sanitation:** Proper manure disposal can help reduce the number of emerging adult flies.
Conservation and Research
Equine Botflies are critical veterinary pests. Research focuses on optimizing anthelmintic (deworming) timing and dosage to minimize drug resistance and developing better diagnostics to track infestation levels.