Bot Flies 

Now that we are finally getting some warm weather and the rugs come off our horses and us for that matter, we await the dreaded Bot Fly. There is nothing worse than watching your horse dance around their paddock trying to avoid that horrid bite or the annoyance of bringing your horse in, only to find hundreds of horrible eggs all over their legs. So, what are Bot Flies and what impact do they have on our horses. 

There are three different types of Bot Fly 

  1. Intestinalis – they lay eggs mostly on the legs and shoulders 
  2. Haemorrhoidalis – their eggs are black in colour and they laid on the hair on the horses lips 
  3. Nasalis – eggs are laid mainly in the hair on the jaw or throat 

Facts about Bot Flies 

  • Active in the warm summer months 
  • Can lay from 150-1000 eggs 
  • ‘Bee like’ in appearance 
  • Lay eggs mainly around nose, legs, mouth, throat and shoulders 
  • Horses bite or lick the eggs which hatch into maggots at approximately 5 days 
  • Maggot’s crawl or are ingested by the horse and they then imbed themselves around the tongue and gums for about 1 month to develop. They will then migrate to the stomach 
  • Larvae attach to the mucous lining of the stomach and the intestinal tract, they stay there during the winter then detach and are passed out through the manure 
  • They burrow into the ground to mature for about 3-5 weeks then emerge as adults to start the cycle all over again 


How does this affect my horse?  

Horses may show no outward signs of infestation, and even if that’s the case, they can still cause discomfort to your horse in several different ways 

  • Ulcers of the stomach lining and damage to stomach tissue 
  • Inflamed mouth 
  • Dental issues (pus pockets and ulcerations) 
  • Loss of appetite 
  • Sinus infections 
  • Compromised nutrition and weight loss 
  • Colic due to blockages in intestinal tract 


Treatment and prevention 

  • Treat horses at the end of autumn and beginning of spring to kill larvae and to interrupt the life cycle of the fly and larvae.  
  • Using wormers containing Ivermectin and Moxidectin which are designed to kill bots 
  • Remove visible eggs from your horse’s coat 
  • Fly repellents and sprays 

Prevention is always better than a cure so if you have any concerns about bots and how they will affect you horse, have a discussion with your vet about a worming program that suits you.  

Written by Selena Phillis


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