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