18/03/2025
🪰 Bot Flies 🪰
This season in Canterbury has seen a reoccurrence of a number of bot flies - it will all be down the weather ——> humidity + warmth = perfect bot fly environments!
It may also be due to owners treating known infected horses at the incorrect time.
Bot flies in Canterbury are the species Gasterophilus and spend part of the larval life cycle in the stomach of the horse.
They only produce one generation a year. The adults lifespan is short lived as the adult flies cannot feed as they have no mouthparts.
The adult flies lay eggs on the forelimbs, and less commonly, jaw, neck and mane of the horse.
The eggs cleverly hatch when they come into contact with the horses lips and tongue.
The first stage larvae are embedded into the folds of the tongue, then moult to a second stage and move to the gingival pockets around the molars and premolars.
This can cause periodontitis (inflammation of the periodontal tissue) and clinical signs can include salivation, head shaking, lingual irritation and chewing issues.
A number of equine dentists have spied these little guys hanging out around teeth this summer.
The second stage larvae emerge and are swallowed and passed to the stomach. They then attach to the mucosa of the non- glandular part of the stomach and develop into the third stage larvae.
Even though these larvae can cause slight inflammation where they attach, large numbers can be present (hundreds) with no appreciable effect on clinical health in the individual horse.
This third larval stage is the stage everyone is familiar with - cylindrical brownish larvae with spikes.
This larval stage overwinters in the host, to be expelled in spring and summer in faeces. They’re easy to see in poo piles!
The larvae then pupate in loose soil to emerge as flies 1-2 months later. The adult flies then mate and females then find a host to lay eggs upon.
Once the eggs are laid on the host by the adult flies, the best way to control the lifecycle is to remove the eggs.
Using a bot knife is the most common method, but eyebrow blades appear to be effective.
Ensure removal is not performed in the paddock or an area where the eggs still may be ingested.
There is some other egg destroying sprays that people have used (white vinegar et al) - I do not have any experience with these but using warm water to mimic the horses mouth to encourage hatching of the eggs to larvae, which are then removed seems logical.
Now to the main discussion point - when do we deworm horses to control the larvae actually in the horse?
Ivermectin based dewormers are labelled with activity against early larval stages seen in the mouth (tongue and periodontal regions).
However it makes more sense to deworm in late autumn and treat all larvae that could be present at once, even if it means you wait until the larvae mature to stage three in the stomach.
This is part of the reason why I keep harping on about not doing your autumn deworming too early, as you may be left with residual larvae if you treat too early.
As any eggs not removed at the time of the deworming treatment or laid after treatment are then not treated before winter and mature and then they exit the horse in spring time, continuing the lifecycle for next season.
With all deworming treatments it is about TIMING AND TYPE!
ALWAYS DEWORM WITH THE CORRECT DEWORMER ACCORDING TO INTERNAL PARASITE SPECIES PRESENT (or suspected).
So ivermectin, abamectin and moxidectin all have an effect against bot fly larvae.
Ivermectin is the only one that has a label claim for the oral stages (L1& L2) but it is incredibly likely that abamectin and moxidectin have the same effect but just haven’t been studied to make that claim.
Faecal egg counts cannot pick up bot larvae infections, so as I always say, the autumn deworming treatment is absolutely pivotal but in Canterbury, later autumn is always better, especially with known bot infections and high shedders.
