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Blowfly Strike Alert : High Risk

Blowfly Strike Alert : High Risk
By Wynnstay Animal Health Team 4 months ago

The NADIS blowfly forecast suggests that weather conditions in your area mean that local flocks are at MEDIUM-risk of blowfly strike caused by female flies being active and laying eggs. We are therefore sending this as a reminder for you and your clients for early prevention and treatment of strike. Carcasses, dirty backends, foot rot lesions and open wounds are all good candidates for egg laying sites. Strike can develop very quickly, with the first maggots appearing within 12 hours of eggs being laid.


Stock should be checked daily for any signs of strike

  • If a preventative treatment has not yet been applied, encourage farmers to do so ASAP
  • By using preventative treatment early on in the blowfly season, the risk of strike will be dramatically reduced.
  • Advise the use of the most appropriate product, based on labour resources, age of the lambs during the risk period, withdrawal periods and anticipated slaughter dates.
  • Remind clients that application area and method will vary between products so to check the instructions before use.
  • Reduce dirty backends - Dagging, crutching and timely shearing are all important.
  • Tail docking lambs is a debated but accepted procedure to reduce strike in lowland flocks.
  • Control worm burdens. Discuss with your client an appropriate faecal egg counting and parasite control plan. Encourage them to consult their vet if appropriate.
  • Manage the fly population: Reducing the fly population early in the year has the greatest impact on the challenge to your clients’ flocks for the grazing season. Flies can lay up to 3000 eggs in a 3 month period so populations can increase quickly
          1. Inexpensive fly traps have been shown to reduce strike incidence by 80% in a season.
          2. Consider how muck heaps are sited and managed.
          3. Encourage prompt disposal of deadstock.
  • In high risk periods, advise grazing more exposed pastures which are less favourable to the flies.


Stock should be checked daily for any signs of strike

  • Discomfort
  • Irritation/Restlessness
  • Separation from flock
  • Damp/Discoloured fleece
  • Lameness and wounds
  • Dull and sick animals
  • Found dead animals

The earlier strike is recognised; the more effective treatment will be. Often the struck area is not obvious until the animal is handled and the fleece parted.


  • Prompt treatment is a necessity if it is to be successful.
  • Veterinary advice should be sought for severely affected or sick sheep.
  • Clip entire affected area.
  • Apply effective product to kill the maggots.
  • Remind clients that Insect Growth Regulators (IGRs) do NOT kill maggots.
  • Ensure you advise clients on the most effective products to use.
  • Please encourage farmers to discuss any treatments with their vet.

Be part of the solution

Help NADIS in conjunction with Elanco nationally assess and improve the surveillance of Blowfly Strike. If you hear about a case, please ask the farmer to log it at: alerts.nadis.org.uk All data is confidential and not shared with vets or any other bodies. The information will be used to build an active national map of when strike is occurring.

The NADIS Parasite Forecast is based on detailed monthly Met Office data for each of the 40sq km areas across the UK. Weather conditions directly affect the likely levels of parasite activity. Disease incidence will also depend on farm management, grazing and treatment history. Indi-vidual farm and field conditions may vary, so consult your vet as part of a veterinary parasite control plan.