Bot Flies in Horses: Identification and Eradication

Aug 15, 2011
Chris Thomas

Bot Flies can Irritate Young and Old Horses Alike - C. Thomas

There is one parasite that is treated in horses amongst worms that isn't actually a worm. This is the bot fly.

Although a fly that closely resembles a bee, the bot fly is very different. Bot flies lay eggs on the horse’s body that are then ingested by the horse. From here, larvae form and use the horse as a host, feeding on it. In this way, the bot larvae are similar to worms and consequently are often treated within this group of parasites.

Treating Varying Species of Bot Flies

There are three species of bot fly. When determining a wormer for your horse and looking to treat bots, this should be considered. Bots should be treated all year round, with late autumn and winter being prominent times to treat the horse for this parasite.

Bot fly species can be differentiated by their size, the colour of the larvae and what region of the horse on which the eggs are laid. Bot fly eggs are commonly found on the horse’s legs, belly and mane.

Identification of the Bot Fly

Bot flies are yellow and black, similar to a bee. They can often be found in paddocks, hovering about the horses’ legs. They can be very irritating to horses and will often result in the stamping of feet, tail swishing and even horses running madly about their field or paddock.

Bot eggs on the horses’ legs are a telltale sign of a bot fly problem. They are especially prominent on the dark legs of bay or brown horses. These eggs are bright yellow and easily attach to leg hairs. The horse then scratches its leg with its head and ingests the eggs.

From here the larvae are able to hatch and wreak havoc to the horse internally. Much like worms, the larvae of bot flies can cause great damage to the gastro-intestinal tract, resulting in commonly found issues in horses that are parasite infested:

The loss of condition
Anaemia and lethargy
Blockages in the gut due to a build up of larvae

Treating the Horse for Bots – Worming and Bot Egg Removal

There are two main methods that will assist the horse owner in eradicating this harmful parasites. The first is to treat the horse with a wormer that kills bots. Not all wormers will do this – it’s important to check the active ingredients of your wormer and make sure it states on the packaging whether it kills bots or not.

The second practice that can be carried out is that of the physical removal of any bot eggs. It is not enough to just remove the eggs with a bot fly knife and allow them to drop to the ground in the yard, stable or paddock where the horse is housed. If they are left where the horse could be grazing or otherwise eating food, they will still ingest the eggs and the life cycle will continue. The horse owner will need to be vigilant and collect any removed eggs and dispose of them away from where horses may be eating.

Although managing horses for bot flies can be time consuming, this practice is a better alternative to a horse that has had its health compromised due to internal parasites. Being observant of your horse’s condition as well as any signs of bot flies externally can help to keep on top of the bot fly population in your area.