Blowflies are one of the most common and widespread ectoparasite which affect sheep in England, with an estimated that 80% of flocks having at least one or more cases of Blowfly Strike every year. 

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With the warmer weather now here, it is the perfect condition for flystrike to occur, meaning prevention is now needed to help avoid this increasingly fatal disease. Blowfly strike is a big economic concern for farmers with considerable prevention costs involved for all at-risk sheep.  Sheep affected with blowfly strike have disrupted grazing patterns and rapidly lose weight, especially if untreated for several days. It’s also a very painful condition and can be fatal, meaning it’s just as important to hobby farmers and smallholders, as no-one wants to see a sheep in distress.

What is blowfly strike in sheep?

In England, blowfly strike is mainly caused by the maggots of the green bottle fly (Lucilia sericata), black bottle fly (Phormia terraenovae) and blue bottle fly (Calliphora erythrocephala); the flies are attracted by the sheep’s fleece, wounds, footrot, soiled fleeces and even dead sheep.

Flies such as the green bottles start the strike on living sheep with soiled fleece or wounds, whilst flies such as bluebottles and black bottles only attack areas which are already struck or damaged.

In the right conditions, it is possible for the female to lay up to 250 eggs on the surface of the sheep’s skin and in the fleece. They then hatch after about 12 hours. After this time there are many maggots present on the sheep which start to attack the flesh.

Understandably, this can be profoundly serious, so serious it can kill, resulting in the affected animals are effectively being “eaten alive”. Sheep are the main animals on the farm that can suffer from strike. The flies’ eggs need specific optimum conditions (such as temperature and humidity) to hatch. These optimum conditions are found in the wool, just away from the skin, on the soiled back ends of sheep.

When does strike occur?

Blowfly populations are greatest during the summer months, although due to changes in climate, the risk period can be from March to December in some lowland areas.

What is the lifecycle of the fly to cause strike?

The adult female flies deposit eggs on dead sheep or soiled fleeces. Eggs hatch into first stage larvae within about 12 hours. These fly larvae will then feed on skin and faecal material. They can become third stage larvae in as little as three days if there is the correct temperature. 

These third-stage maggots then drop to the ground and pupate with mature flies, emerging after 3 to 7 days between May and September. 

Flies are hardy parasites as they can” hibernate” in the soil as pupae and then emerge (from this state) when the soil temperatures rise, usually springtime. The entire life cycle from egg to adult can occur in less than 10 days in optimal conditions.

What do the maggots do?

Maggots are active eaters, so they can quickly cause skin and muscle damage. Bluebottles and black bottles are then attracted to the smell of the decomposing tissue. Chemicals such as ammonia are secreted by the maggots, and then absorbed through the skin lesions and into the sheep’s bloodstream, causing illness and even death. Secondary bacterial infections are common and may also cause death if untreated.

Does blowfly strike a concern?

Blowfly strike is a welfare concern for sheep: an average of 1.5% of ewes and 3% of lambs in England may be affected each year, despite preventative measures undertaken by most farmers. Unlike sheep scab and lice, most of the flies’ lifecycle occurs off the sheep, with adult flies travelling large distances between farms.

What are the signs of blowfly strike?

Adult flies are commonly attracted to areas of soiled fleece surrounding the tail and most commonly to wounds, foot rot lesions, lumpy wool lesions on the skin, and urine scalding around the prepuce. It is essential during the flystrike season that the flock is checked daily. 

The main clinical signs include:

  • Signs of irritation leading to inappetance, dullness and depression.
  • Foul smelling areas of moist, stained wool – often dark green in colour. This is most common around the back end, chest and feet.
  • Sheep isolated from the flock and looking unwell.
  • Kicking of the hind limbs and tail shaking.
  • Maggots present at skin level upon parting the fleece.
  • Skin lesions (from reddened areas to “tears” in the skin to deeper wounds with muscle and even bone exposed).

How is strike diagnosed?

Confirming a case of flystrike is based on visual inspection: 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. You may also notice a decaying smell.

How is flystrike treated?

Treatment should be done immediately once found on the sheep. It 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 vet. Always keep a close eye on flystrike cases as they can soon spread on the sheep and cause further problems.

Prevention and control

Prevention of blowfly strike is an important part of any health plan that you have for your flock, however large or small. 

The main costs of blowfly strike are associated with prevention of disease; frequent inspection of sheep and labour-intensive preventative treatments usually cost more than treating a few affected animals, however failure to protect the flock is a major welfare risk.

There are numerous strategies that can be used to reduce the risk of blowfly strike in the flock:

  • Examine flock regularly during at risk periods – ideally twice a day.
  • Shearing from early April, especially in warmer areas or lowland flocks.
  • Tail docking of lambs.
  • Controlling intestinal parasites and trying not to change the diet as this could cause diarrhoea which flies would be attracted to.
  • Correct disposal of carcass disposal.
  • Ensure all wounds and footrot lesions are treated promptly.
  • Use of the NADIS blowfly alert to identify the periods of highest risk and take preventative action.

Medical prevention

There are several chemical products which can be used to help with fly prevention, these include:

  • Cypermethrin pour on product – e.g. Crovect, which lasts between 6-8 weeks.
  • Plunge with a Diazinon dip gives 6 weeks protection – excluding young lambs.
  • An insect control product e.g. CLiK that give 16 weeks protection by preventing larva from hatching

Remember the withdrawal period 

This should always be noted and adhered to, as should the application and disposal of the product.  It is possible to use aids to trap the flies; this can potentially help to reduce the overall population.

Safe use:

Before using any products, please check all application equipment is working properly and is calibrated. Replenish dips according to instructions and follow safety guidelines.

Flies can be a real problem for sheep so being ready is key. Being prepared for them can help prevent a fatal disease from happening. If you need any advice or guidance, always speak to your veterinary surgeon or your qualified Registered Animal Medicine Advisor (RAMA).

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