Botflies in Horses

Botflies are extremely common in all breeds of horse at all ages. In general they are considered harmless, although the adults are irritating to the horse because the female buzzes around their mouth and nose and other parts of the body while she prepares to lay her eggs. Veterinary attention is useful to determine an effective parasite prevention plan if signs of botflies are present.

During the larval stage in which botflies are buried in the horse’s mouth, the mucous membranes may become inflamed, and may develop pockets of pus. The inflammation resolves itself when the larvae molt into the next stage and move to the stomach. Despite the irritation, horses usually show no symptoms of oral pain.

Botfly adults emerge during the spring and summer directly from manure, or surfaces/substances that have been contaminated with manure.

Adult botflies are about the size of a honey bee but do not bite or sting. During egg laying, female botflies buzz irritatingly around the horse, darting in to lay her eggs in the hair on the horse’s head, legs or body.

There are three types of botfly that infect horses: the common botfly which lays eggs on the legs and flanks, the throat botfly which lays eggs under the jaw or on the throat, and the nose botfly which lays eggs on the lips. Regardless of where the eggs are laid, the eggs hatch into larvae during warm and moist weather, or when the horse licks the eggs off their coat or lips.

The larvae spend several weeks buried in the soft tissue of the tongue, lips, and gums before migrating to the stomach for further development.

Mature larvae exit the stomach with the fecal matter and begin their life cycle again.

Horses typically show no symptoms of botfly infestation. During grooming, owners may notice yellow eggs on the horse’s hair, particularly on the legs.

Horses with botfly infections with no symptoms do not usually require treatment unless otherwise indicated by a veterinarian.

Diagnosis is based on the presence of botfly eggs on the horse’s face or body, larvae in its mouth, stomach (often noticed during gastroscopy for unrelated issues), or feces.

Treatment is antiparasitic drugs, typically administered in the fall or winter after fly season has ended. The use of a botfly knife to comb the eggs out of the hairs on the horse’s face or body is also recommended to reduce the number of larvae entering the horse.

Botflies are extremely common in horses. Typically, they are harmless and go through their year-long lifecycle without doing damage to the horse or its systems. Prognosis is typically excellent.

Prevention of botflies is challenging because adult botflies can cover large distances in search of an appropriate host. Even in cases where excellent management is undertaken, infection is still possible.

Preventative strategies aim to disrupt the lifecycle of the botfly. Strategies include:

• Using antiparasitics based on the advice of a veterinarian
• Combing eggs from the horse’s hair daily during the egg laying season
• Managing pastures by frequent mowing and chain harrowing
• Avoiding overcrowded pastures or paddocks
• Removing manure promptly and disposing of it properly

Botflies are extremely common.

• No treatment
• Anthelmintics