Blowfly Strike

Introduction

Blowfly strike is a serious disease worldwide in sheep of all ages. It is caused by 'blowflies' laying eggs in damaged areas of skin. Larvae then hatch and eat the skin causing severe and debiliatating disease. The disease can also occur in rabbits and is incredibly distressing to rabbit owners.

Blowflies are divided into categories depending on their ability to initiate strike. Primary flies are capable of initiating a strike on living sheep and larvae can penetrate intact ski. These are Lucilia, 'Phormia and 'Calliphora spp. Secondary flies cannot initiate a strike. Larvae attack an area already struck or damaged, extending it. Ca'lli'phora spp. and 'Chrysomya spp. (in warmer climates)are examples of these. Tertiary flies attack lesions on carcasses which have become dry; Musca and 'Sarcophaga spp. cause this.

Predisposing factors to flystrike include:

• Temperature -Temperature in the spring will determine when the overwintering larvae hatch and 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 cease. 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, such as in cases of diarrhoea, caused by worms or other alimentary tract diseases. 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. Also, cuts during shearing, fighting and barbed wire will also increase the incidence of blowfly strike.

Blowfly seasonin temperate regions is June to September. It will be seen mostly in unshorn sheep in June and in 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