Silent Killers:  Blowfly strike

Blowfly strike presents a substantial economic concern for farmers and is a major animal welfare concern.

Blowfly strike presents an economic concern not only for the impact it can have on the health of a flock, affecting profitability,  but also because of the considerable costs associated with prevention.  Traditionally, this has been an issue in summer months, when blowfly populations are at their height, but with changes in climate, the risk period can extend from March to December in some lowland areas.

Of equal significance is that blowfly strike is a major animal welfare concern: an average of 1.5% of ewes and 3% of lambs in the UK may be affected each year, despite preventative measures undertaken by most farmers. This number will be much higher if no control measures are adopted. At least 75% of sheep farms report cases of blowfly strike each year. Flystrike of foot lesions causes severe non-weight bearing lameness, compounding the welfare implications of lameness alone. Death can result in neglected cases, with mortality associated with fly strike estimated at 5% of affected animals.

Blowfly strike results from the opportunistic invasion of living tissue by the larvae of Lucilia sericata (greenbottle flies), Phormia terraenovae (blackbottle flies) and Calliphora erythrocephala (bluebottle flies). Primary flies such as greenbottles initiate strike on living sheep with soiled fleece or wounds, while secondary flies such as bluebottles and blackbottles only attack areas which are already struck or damaged.  Sheep affected with blowfly strike have disrupted grazing patterns and rapidly lose weight, especially if untreated for several days.

The entire blowfly life cycle from egg to adult can occur in less than ten days in optimal conditions.  Adult female flies deposit eggs on dead animals or soiled fleeces and eggs hatch into first stage larvae within about twelve hours. These larvae feed on skin and faecal material, becoming mature third-stage maggots in as little as three days if temperature and humidity are at optimum levels.  Third-stage maggots then drop to the ground and pupate; mature flies emerge after three to seven days between May and September. Flies can over-winter in the soil as pupae and emerge as soil temperatures rise during the spring.

Maggots are active and feed voraciously, causing skin and muscle damage by secreting enzymes. Secondary blowflies are attracted by the smell of decomposing tissue. Toxins released by damaged tissues and ammonia secreted by the maggots are absorbed through the lesions into the sheep’s bloodstream, causing illness and death in severe cases. Secondary bacterial infections are common and may also cause death if untreated.

Unlike the situation for sheep scab and lice, most of the blowfly lifecycle occurs off the sheep and adult flies can travel large distances between farms.

Clinical signs 

Adults flies are attracted to areas of soiled fleece surrounding the tail or breech, and less commonly to wounds, footrot lesions, lumpy wool lesions on the skin, and urine scalding around the prepuce. The main clinical signs include:

  • Isolation from the flock
  • Discoloured wool
  • Agitation and kicking or nibbling at the affected area
  • Disturbed grazing
  • Tissue decay
  • Toxaemia
  • Death

Blowfly strike lesions may range from small areas of skin irritation with a few maggots to extensive areas of traumatised and devitalised skin resulting in death of the sheep. Most commonly the back end of the sheep will be affected, but lesions may also be seen over the withers, back, shoulders and head.

Legal requirement

It is a legal requirement to inspect all sheep daily during the highest risk periods for signs of blowfly strike; disease is easily detected by observing sheep whilst grazing. Gathering sheep into a group with dogs will obscure this abnormal behaviour, meaning that early signs of disease may be missed.

Treatment

Treatment of individual affected sheep involves physical removal of maggots, cleaning and disinfection of wounds and supportive treatment such as antibiotics, fluids and non-steroidal anti-inflammatories (NSAIDs) under veterinary direction.

Treatment by plunge dipping using an organophosphate preparation may be undertaken but it is more usual to treat individual animals with dip wash applied directly to the struck area after first clipping away overlying wool.

Prevention and control

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

  • Use of the NADIS blowfly alert to identify the periods of highest risk and take preventative action
  • Shearing ewes prior to the onset of the high-risk period
  • Control of parasitic gastroenteritis caused by roundworms in lambs to reduce diarrhoea and therefore faecal contamination of the fleece
  • Dagging or crutching of fleece around the tail area to reduce fleece soiling
  • Dipping or use of pour-on chemical formulations to prevent strike or inhibit larval growth
  • Correct disposal of carcases in order to minimise suitable areas for flies to lay eggs
  • Ensure all wounds and footrot lesions are treated promptly
  • Trapping flies to help reduce overall fly populations - this must be used in conjunction with other control methods

It should be noted that the use of dipping to prevent blowfly strike may also help to treat or prevent infestation with sheep scab and lice, reducing the need to use injectable products against these infections.

Nadis Blowfly Alert

The Nadis 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:  see httpps://alerts.nadis.org.uk.