Animal Health Skills

Blowfly Alert

Issued: 18/06/2018 14:16:45

The Blowfly Alert in conjunction with Elanco predicts the emergence of blowflies based on Met Office Data. This can help to predict risk of clinical cases and help treatment timings.

Elanco

Strike risk is currently starting to increase more rapidly in all areas. While ewes are likely to have been sheared in many regions, reducing their strike risk, lambs are now at growing risk and increases in worm burdens leading to scouring will increase this further. If not already employed, strike prevention measures for lambs should now be considered urgently.


Map Key

Low Risk
Medium Risk
High Risk
Severe Risk

Low risk means there is no significant risk of strike. Medium risk represents a risk that approximately 1 in 2500 animals might be struck over the period of the forecast. High risk represents a risk that 1 in 500 animals will experience strike and severe risk represents a risk that 1 in 100 animals will experience strike over this period.

Blowfly strike causes serious discomfort to sheep. Flies are attracted to the faecal stained fleece; resulting in frequent tail swishing, nibbling at the back end and disrupted grazing.

Blowfly Strike is a serious disease thought to affect over 80% of farms in the UK.

‘Blow’ refers to the laying of eggs by flies and ‘Strike’ is the damaged caused by the larvae (maggots).

The Cause

In the UK, strike is caused primarily by the green bottle fly, Lucilia, which seeks decomposing matter to lay her eggs. Carcasses, dirty backends, foot rot lesions and open wounds are all good candidates for egg laying sites. 

Female Green Bottle Fly

Female flies lay batches of 200 eggs at each oviposition, so fly populations will quickly increase over a short period of time in the summer months.

Blowfly Life Cycle

Welfare/Economics

Blowfly strike has a serious impact on the welfare of sheep within the UK, as well as having a major impact on productivity. Figures from 2015 suggests blowfly strike costs the sheep industry £2.2 million per year.

Losses are incurred from:

  • Welfare
  • Loss in productivity (weight loss and decreased milk yield)
  • Fleece damage
  • Deaths
  • Treatment costs; including product, labour and time

Disrupted grazing occurs when sheep are affected by blowfly strike, this in turn leads to weight loss/decreased weight gain

Disrupted grazing occurs when sheep are affected by blowfly strike, this in turn leads to weight loss/decreased weight gain 

Flocks should be carefully checked at least once a day throughout the blowfly season to look for any signs of blowfly strike. It is often necessary to handle animals and part the fleece to fully appreciate the extent of disease.

Early Signs of Strike

  • Irritation
  • Nibbling at tail head
  • Increased swishing of tails
  • Rubbing
  • Further signs of discomfort in lame animals

Nibbling of tail head in this lamb affected by strike is affecting its grazing, which in turn will lead to decreased daily live weight gains.

Nibbling of tail head in this lamb affected by strike is affecting its grazing, which in turn will lead to decreased daily live weight gains.

Signs of Severe Strike

  • Discoloured/damp fleece
  • Fleece loss
  • Separation from flock
  • Sick animals
  • Death (due septicaemia from secondary bacterial infection and release of toxins)

Untreated strike has resulted in the death of this animal.  The feeding maggots have damaged extensive areas of skin, this will attract more flies

Untreated strike has resulted in the death of this animal. The feeding maggots have damaged extensive areas of skin, this will attract more flies

Sponsor Content

Blowfly Strike: The Facts

Blowfly strike is more likely to occur in warm, humid weather (higher than 9°C). Changing climate patterns in the UK and Ireland have meant the blowfly season is starting earlier, lasting longer, and becoming increasingly difficult to predict. Agitation, dejection, odour and shedding of wool are all potential symptoms of a struck animal.

Read More

In order to prevent Blowfly strike, the following steps are recommended:

  • Application of a preventative product in advance of the main risk period for flies
    • Discuss with your vet or SQP the most appropriate product, based on labour resources, age of your lambs during the risk period, withdrawal periods and anticipated slaughter dates
  • 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 vet an appropriate faecal egg counting and parasite control plan.
  • Treat lame sheep promptly. Flies are attracted to wounds caused by footrot
  • Manage the fly population: Reducing the fly population early in the year has the greatest impact on the fly challenge during the grazing season
    • Inexpensive fly traps have been shown to reduce strike incidence by 80% in a season
    • Prompt disposal of deadstock
    • In high risk periods, consider grazing more exposed pastures which are less favourable to the flies.

Long fleeces and tails, as well as dirty back-ends (probably due to worms) all increase the risk of blowfly strike

Long fleeces and tails, as well as dirty back-ends (probably due to worms) all increase the risk of blowfly strike

All cases of blowfly strike should be treated promptly if the animal is to survive and recover quickly. Euthanasia should be seriously considered in severely affected or sick sheep for welfare reasons.

  • Clip entire affected area
  • Apply effective product to kill the maggots.
    • Be aware that Insect Growth Regulators (IGRs) do NOT kill maggots. Seek advice from your vet/SQP as to effective products to use.
  • Consider injecting non-steroidal pain relief and spray or inject antibiotic to cover secondary bacterial infection. Please discuss any treatments with your vet.

The damage caused by blowfly strike can be extensive. Even with effective treatment, it takes time for wounds to heal, fleece to grow back and condition to be restored.

The damage caused by blowfly strike can be extensive. Even with effective treatment, it takes time for wounds to heal, fleece to grow back and condition to be restored.

Strike Watch!

Help monitor blowfly activity in the UK. If you have a case of blowfly strike in your flock, record it here.

Report A Case

Sources

The model used to generate these predictions uses Met Office recordings of daily temperatures and rainfall, along with a detailed understanding of fly biology and sheep susceptibility to strike, to forecast the patterns of strike incidence. The model is highly accurate and tests have shown that it can effectively explain patterns of strike in sheep flocks.

Wall et al., 2000, Sheep blowfly strike: a model approach. Res Vet Sci, 69, 1-9.

Wall et al., 2002 Development and validation of a simulation model for sheep blowfly strike. Med Vet Entomol, 16, 335-346.

Acknowledgements

Risk data provided by Professor Richard Wall BSc, MBA, PhD from University of Bristol.

Images and videos provided by Dr Philip Scott, DiplECBHM, DiplECSRHM, FHEA, MPhil, DVM&S, FRCVS.

Map Key

Low Risk
Medium Risk
High Risk
Severe Risk

Low risk means there is no significant risk of strike. Medium risk represents a risk that approximately 1 in 2500 animals might be struck over the period of the forecast. High risk represents a risk that 1 in 500 animals will experience strike and severe risk represents a risk that 1 in 100 animals will experience strike over this period.

Strike Watch!

Help monitor blowfly activity in the UK. If you have a case of blowfly strike in your flock, record it here.

Report A Case

Blowfly Strike: The Facts

Blowfly strike is more likely to occur in warm, humid weather (higher than 9°C). Changing climate patterns in the UK and Ireland have meant the blowfly season is starting earlier, lasting longer, and becoming increasingly difficult to predict. Agitation, dejection, odour and shedding of wool are all potential symptoms of a struck animal.

Read More