Bots
Bots- Gasterophilus species
Adult Bot flies are flying insects found very commonly around horses. The “bot” is actually the immature form (larva) of the adult fly.
If you have something resembling “maggots” in your horse’s poo and wondering if they need treating you might want to read on……
Ask us what treatment we recommend for your horse
The Parasite:
WIthin the UK there are three main species that affect horses and they rely exclusively on the horse to complete their life cycle. The prime purpose of the adult female fly is to lay their sticky eggs on the hairs around the host- legs, necks, and faces for example.
Note: The adult botfly does not bite the horse.
Although often considered a nuisance pest they can present as an irritant to the horse. In extreme cases, the bots can lead to disease or damage to the intestinal tract.
The bot fly will use horses, donkeys and other equid species as hosts.
Old-timers often call these gadfly/gadflies but this often, confusingly, also includes horse flies
Details and Treatment
What do they look like?
Adults– The adult bot fly is quite bee-like in appearance- brown thorax (body) with darker stripes, and around 1 -2cm long. Anatomically they appear hairy and have a single pair of wings.
Female bot flies have no mouth parts so they cannot feed. Instead, they live off stored reserves long enough to lay their eggs around the horse. The adult female lifespan lasts seven to 10 days only.
Bots (larva)-The bot larva is around 2 cm long (20mm), with a narrow, hooked end and a broad, rounded body. They are often reddish-orange in colour, although they can appear pink.
The lavrva are often described as “maggot-like” when seen in real life.
Eggs– The egg colour is dependent on the actual species of Gasterophilus present. With the most common bot, G.intestinalis, the eggs will look like small, yellow drops of paint deposited on the hair of the animal. The eggs are around 1.25mm, so visible to the naked eye.
The eggs of G.nasalis (throat bot) are similarly yellow in colour whilst those of the nose bot (G.haemorrhoidalis) are black. The latter of these two species is quite uncommon.
Egg laying begins in early summer, with each female depositing up to 1000 eggs, often moving horses to do so.
Detecting Bot Infection
Unfortunately, an infestation of bot fly larvae will not show up in a faecal worm egg count, so it is difficult to know whether or not your horse is affected. Remember the eggs are laid externally on the horse, not within it.
As the life cycle means the larval/pupal stages are passed out in the horses poo they are often seen in spring and early summer. If bot eggs are noted on a horse within a herd then it should be assumed that all horses are infected.
Often the most reliable way of detecting infection is part of a regular grooming and maintenance regime where the eggs may be seen by the naked eye.
Many horses show no outward signs of illness.
Life Cycle Stage
Treatment
Flies
POM-VPS insecticide
Over the counter fly repellants
Frost
Eggs
Physical removal- bot knife
Larvae
Chemical “wormer” based on ivermectin or moxidectin
POM-VPS
Pupae
Sensitive to the environment.
Frost and/or moisture
There are two effective ways of managing bot infestations.
Mechanical control– Regular removal of dung should be undertaken since this is the environment where the final development occurs before the adult fly emerges.
Bot fly eggs can be removed from the horse’s body by several methods. A bot knife which is a bladed tool with a sharp edge can be used to scrape away the bot eggs from the horse’s coat.
Both the above methods are essentially attempts to interrupt the life-cycle
Chemical control– An insecticide can be applied during the fly season to control adults. In the UK, for example Deosect® 5% Cutaneous Spray solution can be applied monthly for general fly control shortening to every two-weeks for severe infestation. Deosect® contains cypermethrin, a broad-spectrum insecticide, and is a POM-VPS medicine available from your vet or SQP
Once ingested, bot larvae are not treatable until they reach the horse’s stomach. Bot larvae appear sensitive to both ivermectin and moxidectin treatment although there have been reports of variable efficacy with the latter. The use of a “wormer” is recommended after the first hard frosts of the winter season have occurred as this weather will kill bot flies and ensure no re-infection. One advantage of using a moxidectin-containing wormer is that you can combine this into your program to treat encysted redworms.
Life Cycle
Horse bot flies undergo complete metamorphosis, that is:
- egg > larva > pupae > adult…………repeat
The current understanding is that only one generation is produced per year- so a 12 month lifecycle.
Due to the varied climates within seasons the life-cycle is not restricted, although certain stages are sensitive to temperature. Within the UK there is the potential for their occurrence to spread further northwards as the overall climate warms.
- Eggs- The female bot fly can lay up to 1000 eggs in her lifetime. These are small (<2mm) and stick to the hair shafts of the horse. Ovipositing (egg laying) typically begins in the early summer months. The eggs can survive for up to 14 days on the horse.
The eggs are commonly laid on the front legs and shoulders for G.intestinalis, the chin and throat area for G.nasalis and around the mouth/lips for G.haemorrhoidalis.
It’s at this point that the eggs can be physically removed using a bot knife or similar implement.
- Larvae- The eggs hatch into a maggot (larval stages) within 7 to 10 days of being laid. The larvae are stimulated to emerge by the horse licking or biting the attached, fully developed eggs. The larvae either crawl to the mouth or are ingested by the horse as it grooms itself.
The very small larvae subsequently bury themselves in the tongue, gums, or lining of the mouth where they remain for approximately 28 days. The latter two locations are the most common.
After 28 days in the oral mucosa the larvae enter a further motile larval stage and travel to the horse’s stomach. These 2nd and later 3rd stage larvae typically attach to or burrow into the lining of the stomach. Once attached the larvae remain immobile for the following nine to 12 months whilst feeding off the stomach contents.
Once matured the larvae detach from the stomach and are passed out in the horse faeces.
The larval stages cannot be routinely treated till they reach their final destination within the horse i.e. the stomach.
- Pupae – Once the larvae have been excreted they burrow into the soil/dried manure/pasture where they undergo pupation. This stage of the life cycle usually lasts for 3 – 5 weeks, and has been known to occur anywhere between late winter – early spring/early summer.
At this stage the larva/pupae are not mobile and the amount of pupae in muck piles can become rather significant and unsightly.
- Adult – The adult horse bot fly emerges from the pupal stage after 3 – 10 weeks during the mid to late summer in the UK. After the fly emerges from the pupa, it quickly finds a mate so as to ensure females are fertile. Mating is more likely around muck heaps where pupae numbers are large. Within hours of copulation, the female begins host-seeking for the laying of eggs to complete the life-cycle
The adult female lifespan is thought to last between 7 and 14 days only.
The University of Florida have a very good page on Bots which can be found here https://entnemdept.ufl.edu/creatures/livestock/horse_bot_fly.htm
Are bot flies the same as “Horse flies” ?
No, bot flies and “horse flies” have different taxonomic classifications and very different life cycles. Horse flies can lead to painful bites on both horses and humans. Apart from being a real nuisance they are known vector of some diseases and the the bites can become infected.
We’ve covered horse flies in a separate topic.
Geography
If you live in the north of the UK, and that doesn’t mean anywhere above the Watford Gap, then bots are less likely to be an issue.
Bots are much more active in the relatively warmer and dryer South/Midlands. Cold and wet conditions are not the favoured environment for bots.
Although often grouped together in “worming”, remember that although bots spend a certain aspect of their life cycle inside the horse they are not worms.