Gasterophilus spp.

Horse Bot Fly - Dennis Ray - Wikimedia Commons
  • Also known as the horse bot fly
  • Obligate parasites of equids
  • Spend most of life cycle in the equine stomach
    • Of little pathogenic significance
  • Three important species (in the UK)
    • G. intestinalis which is the most common
    • G. nasalis
    • G. haemorrhoidalis which is rare
  • Two other important veterinary species
    • G. nigricornis
    • G. inermis


Recognition

  • Medium to large flies at 10-20mm long
  • Look similar to drone bumble bees
    • Body covered with dense yellow hair
    • Dark coloured hairs produce a banding pattern
  • Clear wings with brown patches


Life cycle

  • Adults are most active in late summer
  • Eggs are laid on the body and hatch spontaneously or are stimulated to hatch through an increase in warmth and moisture from the animal self-grooming. They are laid in different areas according to species
    • G. intestinalis eggs are laid in the hair of the shoulders and fore legs
    • G. nasalis eggs are laid in the intermandibular area
    • G. haemorrhoidalis eggs are laid around the lips
  • Creamy-white eggs
  • 1-2mm in length
  • Larvae crawl into the mouth and penetrate the tissues of the tongue and buccal mucosa
  • Larvae then emerge after a couple of weeks and are swallowed
  • Larvae pass into the stomach and attach to the gastric mucosa
    • Larvae are now known as bots
  • Each species attaches to a specific part of the stomach
    • G. intestinalis attaches to the cardiac region
    • G. nasalis attaches to the pylorus
  • After 10-12 months in the stomach, the larvae detach and are passed out in the faeces
    • G. haemorrhoidalis attaches to the rectal mucosa before being passed out
  • Larvae pupate on the ground
  • Adults hatch after 1-2 months and survive from a few days to 2 weeks
    • Adults have non-functional mouthparts so cannot feed
  • There is only 1 generation per year in temperate regions of the world


Pathogenesis

  • Adults cause annoyance when egg laying
    • Disturbance and panic can ensue
  • Larvae cause a marked inflammatory reaction when attached to the gastric mucosa
    • Ring like thickening around the base of each attached larvae
  • Large numbers of larvae may interfere with the passage of food and action of gastric sphincters
  • G. haemorrhoidalis can cause mild irritation to the rectal wall
  • Host reaction to larvae in the mouth is minimal


Control

  • Treatment of horses with insecticides over winter
    • Breaks the life cycle as all the population are present as bots in the stomach
  • If eggs are present in late summer, the horse's coat can be sponged with an insecticide
    • Stimulates hatching
    • Kills larvae