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  • Gasterophilus intestinalis

  • Bots are insects but we class them with worms as they develop in the horses stomach

  • Bots are more prevalent in certain areas of the UK


Parasite lifecycle


The bot fly pupates on the ground, the fly hatches and lays eggs on the horse’s legs and abdomen. The eggs are sticky and yellow. The horse then ingests the eggs by cleaning itself. The eggs hatch out in the mouth and the larvae burrow into the tongue and migrate to the stomach. The larvae attach to the upper, non glandular, region of the stomach by hooks. There they develop for approximately 10 months. After this time the larvae drop off and are excreted in the dung for the cycle to start again.

























What do they look like?


Bot larvae (left) are red and ridged with small hooks allowing them to attach to the non-glandular area of the stomach lining.


















Bot eggs (right) are yellow/white and around 2mm in size


What damage do they cause?


  • When bot eggs have hatched into larvae they can cause pain and irritation in the mouth

  • Bot larvae cause some mild ulceration where they hook onto the stomach lining

  • Can cause other gastrointestinal problems

  • There are no links between bot larvae and serious parasitic disease


Clinical Signs


Bot infestations in horses are generally asymptomatic and there has to be a very large number of bot eggs present to cause illness.


Symptoms include

  • Tears in the skin that can become infected

  • Ulceration of the mouth

  • Mild gastritis causing discomfort when eating

  • Colic


Diagnosis and Detection


  • The presence of botfly eggs on the legs and abdomen suggest there may be an infection

  • Bots cannot be detected in faecal egg counts

  • Diagnosis can only be reliably performed when botfly larvae are passed in the dung




  • Bots are extremely sensitive to wormers and the presence of bots usually means that it has been too long since a wormer has been administered.

  • Wormers effectively remove any bots present; particularly well treated by ivermectin and moxidectin based wormers.


Preventative Measures


  • Always use insect repellent during the summer months to prevent the botflies from laying their eggs on your horse

  • Good management practice of removing bots eggs as soon as they are seen over the summer months

  • Wormers treat any bots present, they do not provide further bot protection. An annual bot treatment is usually administered over the winter months as the frost will kill off any flies and the wormer will address any larvae present



Current Issues in 2015


This parasite is not considered a widespread threat in the UK












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