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 debilitating 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. Calliphoria 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 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, 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 increase the incidence of blowfly strike.

Blowfly season in 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 will predispose flies to lay eggs. Larvae lacerate skin with oral hooks and liquefy host tissue by secreting proteolytic enzymes, which extend and deepen skin lesions. Secondary blowfly attack occurs as flies are attracted to odour of decomposing tissue. This may then lead to secondary bacterial infection. Production losses, irritation and distress associated with skin lesions and poor weight gain (often the first clinical sign). The risk of strike is highest in warm, moist weather.

Clinical Signs

Animals are usually first seen standing away from the flock. Anorexia, depression and listlessness will then be noted. The fleece may appear darker, be damp and have a foul smell. At closer inspection the skin will appear sore and maggots may be present.

Treatment and Control

Humane slaughter may often be the only treatment in severe cases. Scraping the affected tissue and removing the maggots has also been performed. The sheep will need to be isolated and kept inside in a dry area so as to prevent more infection.

Control measures are so much more effective than treatment. These include:

Prophylactic insecticide treatment. This must kill larvae and remain in fleece to prevent flies from laying eggs. These can be applied by spraying, dipping, spray race or jetting. Insect growth regulators in pour on form, such as cyramazine will provide 2-4 month protection depending upon the product used.

Effective worm control is vital to minimise diarrhoea and therefore soiled fleeces. Crutching (removing wool from the perineal are) can also be performed especially in ewes to prevent soiling.

'Mule's operation', which is the surgical removal of breech skin in Merino breeds will help prevent the disease in this breed.Tail docking of lambs is readily practiced in many countries and this also decreases the incidence of soiling of the fleece.

Proper carcass disposal eliminates fly breeding sites on farms.

Vaccination has been used against the gut lining of blowflies in Australia, but this is only in the research stages.