What are the symptoms of bot fly in horses?

What is a Bot Fly?

A ‘Bot fly’ is a common name for a hairy fly that looks a bit like a bumble bee.  Its larvae live as parasites within the body of a horse.  They fly in on a pleasant, warm day and lay their eggs on the horse. They secrete an annoying substance that attaches the eggs to the horse’s body. 

What do they look like?

The eggs look like small, yellow specs on either the hairs or skin.  These are easier to spot on bay or chestnut horses but almost impossible to see on greys.  Female Bot Flies do not have a mouth, so they cannot feed. They live on stored reserves only long enough to lay eggs but they must reach a horse to procreate, so nothing short of death will stop them from getting there!  Between 150 and 1000 eggs can be laid on a horse's body.  This typically occurs during the early summer months. Have a look at the picture below to see what the eggs look like.

What types of Bot Flies are there?

There are three types of bot flies:

The Common horse bot (Astrophiles intestinalis). The eggs are laid on the horse’s legs, flanks, or shoulders and ingested while they’re grooming themselves.

The Throat bot (Astrophiles nasalis). The eggs are laid on the neck and beneath the jaw.  The larvae make their way into horse's mouth.

The Nose bot (Astrophiles haemorrhoidalis). This type is rare. The eggs are laid around lips.

What are the symptoms?

Once in the horse’s mouth, the larvae eventually move down the digestive tract, attaching themselves to the walls of the stomach, duodenum, or the rectum.   Once in the digestive system, they often cause malnutrition, ulceration of the digestive tract, and even complete blockage of the intestine.   Mature larvae then leave their host by passing out in their faeces. They pupate in the soil and emerge as adults several weeks later. See below for a picture of their life cycle.

How do I protect my horse from Bot Flies?

In order to break the life cycle outlined above and the number of larvae that are ingested, you must carefully remove the eggs when you groom your horse. This can be done with a grooming tool called a Bot Knife.  You can easily and safely scrape the horse’s coat or skin to remove the eggs without injuring the horse.   

Regular and liberal applications of fly spray is another way to control the bot fly life cycle.

Some over-the-counter wormers can be used to kill bot larvae providing they state on them that they can actually do so but if you’re in any doubt, consult your vet.

Remove manure from your field.  If you remove the manure before the pupae can mature, you can break the bot fly life cycle.

Anything else?

Yes - if you have a horsey friend, share this article with them so they are aware of the symptoms and can protect their horse. Use the sharing buttons below!