Blowfly Strike

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Epidemiology
Blowflies are divided into categories depending on their ability to initiate strike

  • Primary flies are capable of initiating a strike on living sheep. Larvae can penetrate intact skin
    • Lucilia
    • Phormia
    • Calliphora spp.
  • Secondary flies cannot initiate a strike. Larvae attack an area already struck or damaged, extending it
    • Calliphora spp.
    • Chrysomya spp. (in warmer climates)
  • Tertiary flies attack lesions on carcasses which have become dry
    • Musca
    • Sarcophaga spp.


Predisposing factors to flystrike

  • Temperature
    • Temperature in the spring will determine when the overwintering larvae hatch
    • High temperature and humidity will create a microclimate in the fleece, attracting adult flies to lay eggs
  • Rainfall
    • Persistent rainfall will make the fleece microclimate attractive to adult flies. Females lay eggs after the rain ceases
    • Breeds with long, fine wool are the most susceptible
  • Host susceptibility
    • This is increased when putrefactive odours develop on the fleece due to bacterial decomposition of organic matter following soiling with urine or faeces
    • Merino sheep have a narrow breech area with excessive wrinkling making them more susceptible to soiling
    • A narrow opening of the penile sheath in rams and wethers may result in accumulation of urine and increase blowfly strike in this area
    • Cuts during shearing, fighting and barbed wire will also increase the incidence of blowfly strike


Blowfly season

  • Temperate regions in June to September
    • Mostly in unshorn sheep in June
    • Lambs from July to September
  • Warmer regions have a more prolonged season due to greater number of blowfly generations


Pathogenesis

  • Severe skin damage
    • Larvae lacerate skin with oral hooks and liquefy host tissue by secreting proteolytic enzymes
  • Skin lesions are extended and deepened
    • Secondary blowfly attack
    • Flies attracted to odour of decomposing tissue
  • Secondary bacterial infection
  • Production losses
    • Irritation and distress associated with skin lesions
    • Poor weight gain (often the first clinical sign)
  • Risk of strike is highest in warm, moist weather


Clinical signs

  • Anorexia
  • Listlessness
  • Animals standing apart from flock
  • Fleece may appear darker, be damp and have a foul smell


Control

  • Prophylactic insecticide treatment
    • Must kill larvae and remain in fleece to prevent flies from laying eggs
    • Applied by spraying, dipping, spray race or jetting
  • Insect growth regulators
    • Pour-on
    • 2-4 month protection depending upon the product used
  • Effective worm control
    • To minimise diarrhoea and therefore soiled fleece
  • Crutching to prevent soiling
  • 'Mule's operation'
    • Surgical removal of breech skin in Merino breeds
  • Tail docking of lambs
  • Proper carcass disposal
    • Eliminates fly breeding sites
  • Vaccination
    • Experimental use against Luculia cuprina in Australia