The risk period for blowfly strike has begun and will continue through to October, and beyond, where conditions allow for the problem to occur. Unchecked blowfly in sheep can cause welfare issues, production losses and sadly mortality.
Blowflies are one of the most widespread external parasites affecting sheep in the UK, with surveys from SCOPS suggesting 80% of flocks will have one or more cases of blowfly strike every year on an average farm.
Estimates from SCOPS have shown that the number of sheep with this problem range from 500,000 to 700,00 with mortality rates of 2-3% (10,000-20,000 sheep per annum). The problem of blowfly strikes can vary hugely from year to year, this is due to weather conditions.
Unfortunately, failure to treat a strike promptly will lead to the sheep having reduced performance, secondary infections and, in severe cases, sepsis and cardiovascular collapse. This can mean at the least a loss for farmers, and at worst, death of the affected animals.
What is blowfly strike?
Blowfly strike is caused by the larvae of Lucilia sericata (greenbottle flies), Phormia terraenovae (black bottle flies) and Alliphora erythrocephala (bluebottle flies). The flies such as greenbottles like to strike on living sheep with soiled fleece or wounds, while secondary flies such as bluebottles and black bottles only attack areas which are already struck or damaged.
Female flies are attracted by the smell of wounds, and they enjoy areas such as soiled fleece or dead animals, and often lay batches of up to 250 eggs at a time. These eggs start to hatch after about 12 hours and it then takes around three days for the larvae to mature and drop off the sheep, where they will pupate in the soil. The lifecycle can take about four to six weeks depending on the conditions.
When can blowfly strike occur?
Blowfly strike was often expected between May and September but changing weather patterns mean lowland flocks are often at risk from March through to December.
What is the lifecycle?
Adult female flies deposit eggs on dead sheep or soiled fleeces. These eggs hatch into first stage larvae within about 12 hours. These larvae feed on skin and faecal material, becoming mature third-stage maggots in as little as 3 days if temperature and humidity are at optimum levels. Third-stage maggots then drop to the ground and pupate; mature flies emerge after three to seven days.
What clinical signs should I look for?
Adult flies are attracted to areas of soiled fleece surrounding the tail, wounds, foot rot lesions, lumpy wool lesions on the skin, and urine scalding around the prepuce. The main clinical signs to watch out for include:
- Isolation from the flock
- Discoloured wool, often with “oozy” skin underneath
- Agitation and kicking or nibbling at the affected area
- Disturbed grazing
- Tissue decay
- Toxaemia, collapse
- Death
Unlike sheep scab and lice, most of the blowfly lifecycle occurs off the sheep and adult flies can travel large distances between farms locally.
When looking for blowfly strike lesions, they can range from small areas of skin irritation with just a few maggots to hundreds of maggots that can cause major problems to the skin. Most commonly the back end of the sheep will be affected, but lesions may also be seen over the withers, back, shoulders and head. It is important for smallholders as much as farmers to keep an eye out on all areas of the sheep.
Can I prevent blowfly strike?
There is ways to help prevent blowfly strike, having a close inspection daily is a really effective method, plus this also means that you would be able to spot any other potential problems.
Application of a preventative treatment at the start of the main blowfly season.
The product you choose will depends# on the class of sheep, age, withdrawal periods, shearing and lambing dates. It is important that you speak to your SQP/Vet for options which would suit you. It is important to remember that more than one application may be required.
Minimise soiling of backends
If you notice any fleece dragging the ground then dagging/shearing together with an effective worm control strategy, are important ways risk can be reduced. Tail docking is also used to reduce the incidence of strike in lowland flocks.
Treat lame sheep promptly
It’s often forgotten that flies are attracted to wounds caused by footrot and to the feet themselves; foot strikes can be especially damaging so prompt attention is required.
Reduce the fly population
If the fly population is controlled early in the year, there is a reduced challenge as the grazing season carries on, this can include fly traps, prompt removal of any deadstock and prompt treating of any sheep showing signs of flystrike.
How can it be diagnosed?
Diagnosis is based on visual inspection of the sheep: large numbers of adult flies are seen on the discoloured fleece with maggots on the blackened skin once the surrounding fleece has been lifted clear. There is also a pungent smell coming from the area.
If I notice blowfly strike is their treatment?
Treatment of affected sheep involves physical removal of maggots, cleaning and disinfection of wounds and supportive treatment such as antibiotics, fluids and non-steroidal anti-inflammatories (NSAIDs) under direction from your veterinary surgeon.
Blowfly products
There are essentially three types of topical product recommended for use for blowfly control.
Insect growth regulators (IGRs)
These products allow the female blowfly to lay her eggs but then prevent the larvae developing, stopping the lifecycle. They prevent strike for eight to 16 weeks depending on product choice. They do not treat an existing strike.
Cypermethrins
These products will treat an existing strike and give protection from further strikes for six to 10 weeks, depending on the product.
Deltamethrin
These products only treat an existing strike.
Plunge dipping in an organophosphate (OP) will also treat and prevent blowfly strike. However, SCOPS have recommends using one of the topical products where possible to target blowfly, rather than using the broad activity of the OP unless it is necessary. If an OP is used, it must be as a plunge dip; the use of any other method for application is not legal.
If left untreated, what economic and welfare implications could blowfly strike have?
Blowfly strike is a horrible condition and has a huge effect on animal health and welfare, as well as emotional and financial implications for you as the farmer or smallholder. If left untreated, it will cause death.
Any sheep that do survive can take a long time to recover, which in turn has a long-term effect on growth rate and productivity. If you have any concerns at all about blowfly prevention, treatment or any other health concerns then it is important to contact your veterinary surgeon or SQP for further information or treatment protocol.
Remember
- When using any products for blowfly strike, always read the manufacturers’ instructions on dose rate, storage, package disposal and withdrawal periods. Always check the data sheet. If you are unsure then contact your SQP/Vet.
- Check you have the correct application equipment for the product and that it is working properly and is calibrated.
- Make sure you know how to apply the product and have the appropriate PPE. If in doubt, ask your vet or SQP for help.
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