Equis ISSN 2398-2977

Fly bite

Contributor(s): Dennis E Brooks, Sheelagh Lloyd, Graham Munroe, John Parker, David Senter


  • Cause: numerous species of biting and non-biting flies.
  • Signs: irritation and self-inflicted damage to eyelid   →   chronic blepharoconjunctivitis. Painful and pruritic papules and wheals on many areas of the body.
  • Treatment: topical antibiotic/corticosteroid preparations. Fly control is essential. Destruction of fly breeding places.
  • Prognosis: guarded due to difficulty of long-term management.
  • See also insect bite hypersensitivity   Insect hypersensitivity  .



  • Various fly species have been implicated.
  • Biting flies   →    pain and pruritus at the bite sites   Insect hypersensitivity      Biting and nuisance flies  :
    • Culicoidesspp   Culicoides spp   - prefer topline and ventrum. Vector forOnchocercaspp   Onchocerca spp  .
    • Simuliumspp (Black flies)   Simulium spp   - prefer thinly haired areas of the body, eg face, ears, neck, ventrum and legs.
    • Haematobia (Horn flies) - feed on the dorsum or ventrum. Cause of ventral midline dermatitis. 
    • Stomoxysspp   Stomoxys calcitrans   (Stable flies) - neck, back, chest, groin and legs.
    • Mosquitoes.
    • Tabanids(Horse flies, deer flies, breeze flies) - ventrum, legs, neck and withers.
    • Hippoboscaspp (Louse flies) - feed on perineal and inguinal regions.
  • Nuisance flies that feed on discharges:
    • Muscaspp   Musca domestica  .
    • Hydrotaeaspp.

Predisposing factors

  • Summer months.
  • Geographically warmer/wetter areas.
  • Environmental contamination - poor hygiene.
  • Horse is kept near water sources, eg ponds, streams, etc.
  • More common in growing foals/yearlings and broodmares due to management factors.


  • Peri-ocular:
    • Flies often feed on ocular secretions, especially at the medial canthus causing direct irritation and also serving as an intermediate host for habronemiasis   Habronemiasis   and thelagiasis.
    • The irritation    →   self-inflicted alopecia, excoriation and depigmentation of the canthi (medial).
    • In severe untreated cases the blepharitis   Eyelid: blepharitis - overview  may be accompanied by conjunctivitis   Conjunctivitis: parasitic  and dacryocystitis   Eye: dacryocystitis  , with complete eyelid closure reported as an infrequent complication.
  • Rest of body:
    • Different species of flies have favorite regions of the body to bite causing painful and pruritic papules and often wheals causing great annoyance to the horse.
    • Heavy exposure can   →    anemia and potentially shock and death.

Specific species

  • Antigens in saliva may induce hypersensitivity response. Inflammation, crusts (serous or bloody), nodular reaction.
  • House flies and face flies:
    • Nuisance feeding on lachrymal secretions and wounds.
    • Clustering around head.
    • Face flies persistent and fairly resident on horse.
    • Vectors ofHabronema(summer sores)   Habronema spp  andThelazia(eyeworms).
  • Stable flies:
    • Nuisance bite fairly painful.
    • Feed primarily legs, neck, shoulders and abdomen, also shoulders and neck.
    • Wheal, crust, papules and nodules may develop.
    • Can mechanically transmit various pathogens but not as important as horse flies in this.
    • Vector ofHabronema  Habronema spp  .
  • Horn flies:
    • Normally parasites of cattle and buffalo but will feed on horses nearby.
    • Spend most of adult life clustered on host feed particularly on the ventral abdomen of horses but also chest and shoulders.
    • May contribute to a seasonal, well demarcated ventral midline dermatitis in umbilical area and even cause ulcers where clustered on abdomen.
  • Black flies:
    • Inject toxin that causes local necrosis and hemorrhage and vesicle at bite (very, very large numbers can produce death from toxemia).
    • Feed inside ears, head, neck, chest, inside thighs and forelegs, abdomen.
    • Pin-point hemorrhages and crusts, most obvious in ears.
    • Flies, particularlyS. equinum, may be found clustered in ears.
  • Horse flies:
    • Very painful bite, often disrupted but return to feed immediately interrupted feeders so excellent mechanical transmitters of viruses and hemoprotozoa.
    • Lesion oozes blood (attracts house flies and myiasis flies); possibly nodules.


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Further Reading


Refereed papers