Hypoderma spp.

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  • Also known as warble flies
  • Important cattle parasites
    • Also parasitise horses, small ruminants and sometimes humans
  • The two main species in cattle are
    • H. bovis
    • H. lineatum
  • H. diana parasitises deer
  • Infestation is largely confined to the Northern Hemisphere


Recognition

  • Large 13-15mm long
  • Similar to bumble bees in appearance
    • Yellow abdomen
    • Band of black hairs
  • 1 pair of wings


Life cycle

  • Adult flies emerge on warm, sunny days between June and August
  • White eggs are laid around the belly and legs of the animal
    • Eggs are attached to hairs by cement and a small terminal clasp
    • H. lineatum lays a whole row of eggs as it approaches the animal stealthily
    • H. bovis only deposits a single egg before the animal runs away (gadding) as it makes a loud buzzing noise
  • The adult lives for 3 weeks
  • Females can lay several hundred eggs in their life
  • A few days after eggs are laid, larvae emerge and crawl down the hairs into the hair follicles
  • Larvae penetrate the skin through wounds made by biting flies
  • Larvae migrate through the subcutaneous tissue towards the diaphragm
    • Enzymatic secretions and mouth hooks aid larval migration
  • After 3 months larvae reach winter resting sites where they remain from November to February/March whilst moulting to the L2 stage. These sites are different between the 2 species
    • Epidural fat in the spinal canal for H. bovis
    • Wall of the oesophagus for H. lineatum
  • Larvae then migrate back to the subcutis along the midline of the back where they bore a breathing hole through the skin and moult to the L3 stage
    • Larvae are palpable as distinct swellings called warbles
  • L3 larvae emerge after 4-10 weeks where they fall to the ground and pupate under loose vegetation
  • Adult flies emerge from the pupa after 1 month where they copulate, lay eggs and die within 2 weeks
  • H. lineatum are on the wing 6-8 weeks before H. bovis
  • There is only 1 generation of warble flies per year


Pathogenesis

  • Production losses
    • Condemnation and down-grading of hides
    • Reduced milk yield and reduced weight gain
    • Injury from stock panic
    • Trimmed meat losses from H. lineatum
      • 'Butcher's Jelly' around warbles which is green due to mass eosinophil attraction
  • Paraplegia resulting from
    • Toxin release
    • Pressure on the spinal cord (H. bovis)
    • Bloat from pressure on the oesophageal wall (H. lineatum)


Control

  • Total eradication should be aimed for
  • Old methods include popping out warbles
    • But this could lead to anaphylactic shock
  • Timing is crucial for treatment
    • Larvae residing in winter resting sites, if killed, can lead to bloat and paraplegia
    • It is safe to treat in the autumn before larvae reach their winter resting sites and in the spring when the warbles have migrated to the midline of the back
    • Ivermectin can be given at any time without risking host infection as larval antigen is released much slower


Legislation in the UK

  • 'Warble Fly Order 1978' requires all clinically affected animals to be treated
  • Notifiable disease
  • 'Warble Fly Infected Area Order 1983'
  • For more information on the warble fly orders, see here