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Blowfly Prevention in Sheep - Strike First

In recent years the UK has experienced increasingly unpredictable weather conditions making the timing of preventative blowfly strike treatments more challenging. Up to 80% of farms are affected and up to half a million sheep are still struck annually.

High risk fly strike season was historically reasonably predictable - beginning in May and ending in September. Lately we have seen the season for blowfly strike beginning as early as March and ending as late as December when winters have been mild.

This variation in risk period means farmers must be more observant and more targeted in the timing of preventative treatments. It is no longer possible to predict treatment timings on management tasks, such as post shearing, but must instead be strategically employed when environmental conditions determine that the risk is high.

The life cycle of the female green bottle fly (Lucilia sericata) begins when they hatch from their pupae in the soil - this is governed by environmental factors and generally falls around mid-April but can be earlier or later.

One adult female Greenbottle fly (Lucilia sericata) will deposit batches of approximately 200 eggs in the fleece. She will seek out damp, humid places to lay eggs. Fleeces which are contaminated with faeces and urine or even which are damp and humid make the perfect conditions for egg laying. One female fly can lay up to 3000 eggs in her 28 day life!

BlowflyLifeCycleThese eggs quickly hatch into larvae (or maggots) which produce enzymes. These enzymes digest the host tissue leading to skin damage and cause the painful wounds associated with blowfly strike. The developing maggots feed on the dead and dying tissue and produce powerful odours. These odours attract other egg-laying females and this quickly increases the numbers of eggs/larvae present.

If left untreated, the wounds will increase in size, become infected and ooze. This will cause the sheep to enter a state of shock and perish. This further complicates the situation as an undetected carcass would be an excellent host for more larvae to develop and exponentially increases the number of flies in the area.

If you delay preventative treatment for the flock until animals have become struck you will suffer losses (whether from death or reduced productivity). The most significant impact is on the welfare of the animals affected. By the time fly strike is observed in a small number of animals, many more may already have high numbers of eggs deposited in the fleece and may still go on to develop strike in spite of treatment.

Risk Factors for Fly Strike:

Presence of organic matter in the fleece
Fleeces soiled with faeces or urine provides the perfect environment for fly strike and the presence of organic matter can reduce the efficacy of preventative products leaving the animals at high risk.

Open wounds
This could be from footrot, a dagging injury or potentially even a fly strike wound. Even where preventative products have been applied, animals with open wounds are still susceptible to strike and need to be monitored more closely until the wound has healed completely.

Thick fleeces
Humidity is a key risk factor for strike and thick fleeces can create the perfect, humid micro-climate for maggot development.

Environmental conditions
Prolonged periods of hot weather can lead to an explosion in the fly population resulting in a very high challenge to livestock.


  • Annual shearing and regular dagging of soiled fleeces. Shearing results in a 95% reduction in the incidence of strike.
  • Docking- where appropriate, and when done in conjunction with the law. Docked lambs are 5 times less likely to suffer from fly strike.
  • Check animals frequently.
  • Use preventative products to reduce the risk.
  • Remove any carcasses promptly.
  • Reduce scouring (gastrointestinal parasite control, good nutrition).
  • Any wounds should be monitored closely until resolved.
  • Reduce the incidence of footrot.

Which Preventative Product is right for my flock?

There are a number of factors to consider when choosing a fly prevention product for your flock.

The withdrawal period and duration of action is very important as lambs may be going for sale in a matter of weeks and this will be a key factor in deciding what treatment is appropriate for the lamb crop. Consider the duration of protection you need and the cost associated with that treatment. It is also worth considering whether you require more than blowfly control? Would it be beneficial to also treat for other ectoparasites such as ticks, lice and scab?

For adult sheep ask your vet/SQP for a breakdown of the cost per treatment and which other parasites are covered to help decide which product is most appropriate for you.

Table1. Active ingredients licenced for the treatment and/or prevention of blowfly strike
ActiveParasites controlled
Blowfly PreventionBlowfly TreatmentTicksLiceKedsSheep ScabHeadflies
Diazinon (dip)

Data sheets for all products are available on the VMD website. Please consult them for the duration of action and additional information on the parasites controlled.

About the Author

Rachel Mallet is a Veterinary Surgeon, who now works as a Professional Services Vet providing technical support to vets, SQPs and farmers in the UK. Rachel is passionate about animal health and about promoting best practice and preventative medicine amongst farmers.

Date editorial prepared: February 2018
Use medicines responsibly. Noah.co.uk/responsible.
*Ectofly contains 12.5mg/ml cypermethrin Cis 80: Trans 20 and is a POM-VPS medicine.
Please consult your SQP or vet to determine which is the most appropriate preventative treatment for your flock and consult the SPC data sheets on the VMD website for further information.

Sustainable Control of Parasites in Sheep