Flystrike is a major welfare concern and an important cause of ill thrift in individual affected sheep.
Blowfly strike is very common in the UK; it affects more than 75 percent of farms, where an average of 1.5 per cent of ewes and three per cent of lambs may be struck in an average year, despite the use of prophylaxis by most farmers.
The economic cost of fly strike to the UK sheep industry is considerable as it includes mortality, production losses, hide and wool damage, treatment and control costs, as well as time and labour involved in the frequent inspection of flocks.
For a farm with 250 ewes, lambing in March, with an output of 1.5 live lambs per ewe, the untreated flock might expect 19 ewe strikes with one death and 23 lamb strikes with two deaths over the entire season in a moderate risk area in the UK, according to published data.
Assuming the total cost of a breeding replacement ewe is £200 (including, vaccinations, transport, etc.) and the average loss of a lamb that dies from strike is £80, the total cost of not treating a flock this size would be £1,027.
This figure includes the cost of labour, reactive treatment, deaths and lamb production losses.
The main species of blowfly that infests sheep in most of the UK is the common greenbottle, Lucilia sericata.
Primary blowflies are attracted to protein food sources for egg laying.
These include carcasses, blood from wounds and wool soiled with urine, faeces, or inflammatory exudates. Maggots hatching at such sites can invade an existing wound or create a new superficial one by lacerations with their mouth parts.
These lesions can then be struck by more primary blowflies, or by secondary blowflies. Severe cases can end in death.
Most of the blowfly life cycle occurs off the host, and adult flies can travel large distances.
Adult females live for about 30 days during which they can lay up to 3,000 eggs.
These are deposited on dead animals or soiled fleeces, and hatch into first stage larvae within about 12 hours, moulting twice and developing into mature maggots in three to ten days.
Third stage maggots then drop to the ground and pupate, mature flies emerging between May and September after three to seven days.
Flies over-winter in the soil as pupae, and emerge as soil temperatures rise during the spring.
Effective fly control and prevention of fly strike relies on good management practices in addition to the use of preventative products.
Shearing was shown to be associated with a 95 per cent reduction in the risk of ewe strike by the common greenbottle fly and the presence and length of a tail have also been recognised as important risk factors for strike.
Strike incidence was shown to be over five times higher in lambs where the tail had not been removed, compared to docked lambs. Faecal soiling has been recognised as a primary risk factor for breech strike.
Lambs with watery faeces have been found 8.5 times more likely to be struck.
Strike risk can therefore be reduced by good parasitic worm control.
Other control measures include foot rot control, dagging and crutching as well as appropriate insecticide use.
Sheep should be inspected frequently, especially at high risk times, and timing of chemical application is crucial.
Baited fly traps can be used to monitor the activity of blowflies, so that chemicals can be applied before problems arise.
Dysect® Sheep Pour-On Solution is licensed for the prevention of strike up to eight to ten weeks and can be applied directly to the site in struck animals.
Fly-struck sheep need to be treated immediately. Struck areas are sensitive to sunburn, so should not be clipped other than to gain access to the wound.
The use of antimicrobials may be indicated for secondary bacterial infection.
Flies are attracted to a warm, humid environment, so grazing on particularly damp, well-sheltered pasture during the risk period should be avoided if possible.
As larvae may also feed on decaying organic matter and carcasses, measures should be taken for prompt removal and disposal of dead stock and good management strategies for dealing with faecal material and rotting vegetation such as winter bedding.