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What is Flystrike?
Blowfly strike (cutaneous myiasis) in sheep is defined as the invasion and feeding on living tissues of humans or animals by dipterous (fly) larvae. It causes misery and death in sheep if not protected.
The Journal of Australian Veterinary Association stated that flystrike is, “a major animal health and welfare problem for sheep production in Australia, estimated to cost A$280m annually”.
Australian sheep blowfly present in 96% of all strikes
Australian sheep blowfly (Lucilia cuprina) is the primary sheep blowfly responsible for initiating most blowfly strikes. Hairy maggot fly (Chrysomya rufifacies) is a secondary blowfly that will only strike sheep after the Australian sheep blowfly has initiated a strike. Blowflies thrive in warm, humid conditions.
A large scale survey in the late 1970s, in which flies were bred from over 1400 strikes around Victoria, found that the green blowfly or Australian sheep blowfly (Lucilia cuprina) was present in 96% of all strikes and was the only species present in more than 50% of these. Not only does it initiate most strikes, it is the main species involved in their continuation.
Fystrike occurs where the fleece has become soiled or infected
The wool of a sheep with active fleece rot or a soiled breech harbours seething masses of bacteria. This provides an environment somewhat similar to that found in a fairly fresh carcass. Some of the more opportunistic flies, Lucilia cuprina in particular, have been able to make the transition from breeding on carcasses to developing on suitable live sheep.
The ammonia in urine is particularly attractive to the flies, and the young larvae can become established where the skin of the sheep has become irritated by the urine. Fly eggs, once laid, hatch within a few hours or days. The emerging larvae or maggots are very active and secrete enzymes that liquefy the skin and flesh of the sheep upon which they are feeding. Larvae feed for a variable time period depending on the availability of food. The mature maggots crawl off the fleece and pupate in the soil. In suitable weather conditions in the summer it takes less than a week for the fly to emerge, but often it is two or three weeks. With flies laying thousands of eggs, it is easy to see why the population of blowflies increases so rapidly with the right weather conditions.
Sheep die from septicaemia and toxaemia
Sheep suffering from flystrike show obvious signs of distress. They spend less time grazing and more time tail wagging and rubbing the affected area and biting the struck areas of the fleece they can reach. If these signs go unrecognised and secondary strike occurs, the wounds can become very extensive and bacterial infection may lead to serious complications such as death from septicaemia and toxaemia. When sheep are examined there is often a foul smell from the wound and visible signs of maggots.
These flies cost the Australian sheep industry well over $100 million per year in deaths, lost production and treatment costs. But more importantly they cause pain, misery and often death.
The future of fly strike in Australia?
The Journal of Australian Veterinary Association has stated:
“In 2004, an undertaking was made by several sheep industry agencies to phase out surgical mulesing by December 2010. The organisation responsible for coordinating research in the wool industry, Australian Wool Innovation (AWI), initiated an intensive program of investigating alternatives to assist woolgrowers manage the risk of breech-strike without mulesing. In July 2009, AWI announced that, although there had been significant progress, cessation of mulesing by the end of 2010 was unlikely and the assessment of alternatives was continuing. In the longer term, genetic selection of Merino sheep with less breech wrinkle, and decreased scouring are strategies that will reduce the risk of breech-strike. However, it will take time for this to be implemented, first in ram-breeding flocks, then in wool-growing flocks using rams purchased from ram breeders. Consequently, in the short- to medium-term there is a need to assess other management strategies that effectively control breech-strike on unmulesed Merino sheep.”