Hypoderma spp.

Warble Fly - Geo. H. Carpenter, Cambridge University Press 1913 - Wikimedia Commons

• 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

• Causes myositis

• 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

• Ectoparasiticides
  • Systemic organophosphorus insecticides in pour-on formula
  • Avermectins and milbemycins in pour-on and injectible formulations

• 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