The Bot Fly 101

What is a Bot Fly?

The Bot Fly is an “ectoparasite”. Ectoparasites live on the skin surface of an animal, in this case, the horse. Examples of other primary ectoparasites of horses are houseflies, stable flies, mosquitoes, and, to a lesser extent, horse and deer flies. Ticks, lice and mites are also common ectoparasites in horses.

The horse Bot Fly in the UK is a bumble bee-sized insect that buzzes around the head and lower legs of horses as they glue their eggs, or nits as they are often known, to the horse’s coat hair.

Did you know that there are two different types of Bot Fly. The eggs that they lay differ in colour and placement of where they lay their eggs:

G. intestinalis lays up to 1,000 pale-yellow eggs on the horse's forelegs and shoulders. Moisture and friction from the horse's licking itself cause the eggs to hatch in about seven days. After hatching, G. intestinalis larvae are licked into the mouth.

G. nasalis lays about 500 yellow eggs around the chin and throat of the horse. These eggs are not dependent on the horse's licking them to hatch. They burrow under the skin to the mouth, wandering through it for about a month before migrating to the stomach for overwintering. Then the cycle begins again.

Life Cycle 

Much is understood about the life cycle of the Bot Fly in horses, the duration and impact of each phase.  

The first stage of the Bot Fly’s evil plan occurs in early summer when they deposit yellowish grey coloured eggs during their short 7 to 10-day life span. The eggs are easily recognisable as small yellow dots are glued individually to the hairs in the horse's coat. They are approximately 0.05 inches or 0.127 centimetres in length.

The second phase is particularly unpleasant. After the incubation period, the larvae migrate to the stomach. Here they grow and attach themselves to the stomach wall and intestinal tracts of the horse.

They dig into the stomach tissue and mostly attach themselves to the lining of the stomach in the non-glandular region, or the upper area of the stomach. At this stage they’re thick and tough skinned with a blunt posterior end and with a taper at the other end with two strong mouth hooks. There is a ring of prominent spines around each body segment giving the 1/2 to 2/3 inch-long whitish maggot a screw-like appearance.

Your horse is now officially “an alien host” for the Bot Fly for the next 8-10 months, ie from late Autumn to early Spring.

The third and final stage is when the larvae remain in the stomach until the spring. It is then they pass with the faeces, pupating on the ground to emerge as adult flies one or two months later.

After the fly emerges from the pupa, it quickly finds a mate. The mating activity typically occurs in the early afternoon during warm, sunny weather in relative proximity to horses. Within hours, the female begins to seek a host. Dispersal of eggs by the female is not restricted to one horse but can occur on many horses within an area. The female increases the chance of larval survival by not limiting her eggs to one horse. The adult female lifespan lasts seven to 10 days.

What are the symptoms of a Bot Fly infestation?

Symptoms vary and can include loss of condition, a dry coat, increased temperature, restlessness, kicking at the belly and lack of appetite. This may be accompanied with intermittent diarrhoea or constipation. The larvae can cause gastritis, stomach ulcers and in severe cases perforation of the stomach causing fatal peritonitis.

How do I treat Bot Fly?

This partly depends on the stage at which you notice it.

Stage 1 - A good Bot control program includes eliminating Bot eggs from your horse daily. The Bot Knife's special rounded blade is designed to reach into crevices where bot eggs are found and it’s serrated edge is able to remove the sticky eggs that are “glued” the strands of the horse’s coat.

Stage 2 - Although the Bot is not a "worm", an increasing number of wormers are effective against Bot larvae infestation.

What can I do to avoid Bot Flies?

Working on the basis that prevention is easier than cure:

* remove Bot eggs from the horse's coat daily with a Bot knife

* remove manure since this is the place where the final development occurs before the fly emerges

* worm horses regularly with a suitable wormer

* turning horses out after dark and bringing in early in the morning or providing a darkened shelter will help as the Bot Fly will not be about as it does not like these conditions