Fly Control – Don’t Get Struck!

With Blowflies being one of the most common ectoparasites to affect sheep in the UK, flystrike is likely to affect 80% of UK sheep flocks every single year. Even with preventative measures in place across many farms, an average of 1.5% of ewes and 3% lambs may be affected each year in the UK. With mild weather this year, the season is likely to be longer and fly numbers are likely to increase. This makes being on top of your fly control even more important if you want to prevent production losses caused by blowfly strike.

Blowfly image courtesy of Dr Philip Scott and NADIS

In the UK, fly strike is caused by the maggots of greenbottle, blackbottle and bluebottle flies. They are attracted to long, damp, soiled fleece, wounds or animal carcases. Female flies can each lay around 250 eggs on the surface of the skin, and within 12 hours these have hatched to begin feeding on the animal’s flesh. These larvae attach with hook like mouthparts and begin secreting enzymes to liquify and digest the tissue. These struck areas often attract more flies and leads to further waves of infestation. This fast-moving progression from eggs to maggots means regular monitoring of the flock is essential to catch any cases early, before serious damage or death occurs.

What to look for:

  • Signs of irritation, scratching, teeth grinding, leading to inappetence, dullness and depression. Affected sheep may separate themselves from the rest of the flock as well.
  • Darker areas of fleece – these darker areas of foul-smelling moist wool are commonly seen around the backend, chest and feet but can occur anywhere on the body. When the fleece is parted these will reveal the presence of maggots feeding on the flesh below.
  • Kicking of hind limbs at body and tail shaking
  • Skin lesions – underneath the fleece these lesions can be anything form inflamed reddened skin, to deep penetrating wounds that enter body cavities. The severity of these is entirely dependant on how long the maggots have been left untreated. The longer the infestation is left to establish itself on the sheep, the poorer the prognosis becomes for survival.
  • Tissue decay, toxaemia and death – left untreated the affected regions of tissue die and start to decay leading to blood toxaemia. At this point death is highly probable without swift intervention, and treatment is often not successful at this stage.
Darker areas of foul-smelling moist wool are commonly seen around the backend, chest and feet

How to treat it:

  • Remove dirty contaminated fleece from around the whole area of the strike – in bad cases this may involve shearing the entire sheep. Make sure to follow any dark stained patches of fleece as these may lead to more pockets of maggots.
  • Wash the skin clean of as many maggots as possible, and clean with hibiscrub or other skin disinfectant.
  • For areas of broken skin, use a topical antibiotic spray.
  • Use a fly strike treatment product around the areas affected. These should have deltamethrin or cypermethrin as an active ingredient e.g. Crovect, Spotinor, Ectofly or Deltanil. It is important to note that CLiK does not treat established strike.
  • If there is broken skin and open wounds, and the animal is uncomfortable you should consider the use of systemic antibiotics and anti-inflammatories, if in any doubt over treatment plans contact your vet who will be able to advise you.
  • The animal should be closely monitored for the next few days and the rest of the flock checked thoroughly for signs.

How to prevent it:

Prevention of strike should be an integral part of your flock health plan and there are various strategies that can be employed to reduce the risk to your flock:

  • Examine animals regularly during the risk period – twice a day if possible.
  • Shear and dag ewes prior to the high-risk period to reduce faecal soiling of the fleece.
  • Control internal parasites and minimise changes in diet that may lead to digestive upsets and cause diarrhoea which in turn will increase soiling of fleeces.
  • Preventative treatments such as dipping or use of pour on to prevent strike or inhibit the growth of the fly larvae before the anticipated challenge from flies:
    • Cypermethrin pour ons e.g. Crovect etc – protection lasts 6-8 weeks
    • Plunge dipping – can provide protection for 3-8 weeks but requires precision and good technique to ensure efficacy.
    • Insect Growth Regulators – e.g. CLiK give 16 weeks protection by preventing larvae from hatching.
    • It is important to note withdrawal periods for each of these products and that none of these have been licensed for use in animals producing milk for human consumption.
  • Dispose of carcases promptly to minimise attraction to flies.
  • Ensure wounds and footrot lesions are treated promptly.

Future risk?

Sheep susceptibility and fly abundance are both heavily influenced by prevailing weather patterns. As such, global warming may well lead to a prolonged risk period for flocks, with the disease being seen earlier in the year. As ever, remaining vigilant to the risk it poses to your flock will help to reduce its incidence and impact.

If you are concerned that flies may be posing a significant threat to animal welfare and economic losses on your farm, please speak to your vet. 

Written by Alice Goble BVM&S MRCVS