Blowfly Strike

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