Blowfly strike (cutaneous myiasis) is one of the most serious welfare emergencies in sheep, causing severe pain, tissue destruction, and rapid death if untreated. It is also largely preventable with appropriate management.
The green bottle fly (Lucilia sericata) is the primary species responsible for sheep fly strike in the UK. Adult flies are attracted by moisture, wounds, and the ammonia from fleece contamination with urine and faeces. Eggs are laid in groups of hundreds; they hatch in 12–24 hours in warm conditions. First-stage larvae (maggots) begin feeding on the skin surface, producing enzymes that liquefy superficial tissue and creating the warm, moist environment that attracts further oviposition. Within 24–48 hours, larvae penetrate deeper tissues, causing rapidly expanding subcutaneous wounds.
The breech (perineal area) is the most common site, particularly in sheep with dagginess (faecal soiling of the fleece). Strikes can also occur around wounds, foot rot lesions, sores, and occasionally on the back in wet, unshorn conditions. Risk factors include: dirty fleece especially around the breech, worm burdens causing diarrhoea, skin wounds, weather (warm and humid conditions favour fly activity), and delayed shearing creating dense, potentially wet fleece conditions.
Fly strike causes extreme suffering. Affected sheep show intense restlessness, stamp their feet, bite at or rub affected areas, and adopt abnormal postures. The pain and systemic effect of toxin absorption cause progressive depression, anorexia, and deterioration. Untreated sheep die within three to five days from toxaemia, secondary sepsis, and shock. Even sheep that survive may have permanently damaged tissue and require extended recovery. The welfare impact per affected animal is among the most severe of any routine farming health problem.
Early detection requires twice-daily inspection of all sheep during the fly season (April–October). Early signs include: restlessness, rubbing or biting at the affected area, and visible discolouration or wetness of the fleece. Smell — the characteristic unpleasant sweet-rotten odour — may be detectable before lesions are obvious. Any sheep showing these signs should be isolated and examined immediately. The 'wet, smelly, restless' trio is a reliable early warning combination.
Insect growth regulators (IGRs): Cyromazine (Vetrazin) and dicyclanil products provide 8–16 weeks protection by preventing larval development on the sheep. Applied by back-line or dip, they represent the most reliable individual animal protection available and are recommended for all sheep at high risk during peak fly season.
Breech clipping: Removing fleece from the perineal area reduces the dirty, moist conditions that attract egg-laying. This is particularly important in breeds with wool around the breech.
Dagginess control: Managing internal parasites to reduce diarrhoea and faecal soiling reduces strike risk. Regular faecal egg counts, targeted selective treatment, and pasture management reduce worm burdens.
Breed selection: Breeds with less wool around the breech (bare breech) have lower strike risk. Selective breeding for reduced breech wrinkle in Merino sheep has been adopted in Australia as a welfare improvement strategy, reducing strike risk without requiring mulesing.
Mulesing: Surgical removal of wrinkled skin from the breech, used primarily in Merino sheep in Australia, prevents perineal strike but is itself a welfare compromise due to pain. RSPCA Australia recommends pain relief at time of procedure; industry is moving toward alternatives including plain-breech breeding.
Affected sheep must be treated immediately. Clip all fleece from around the strike area (far beyond visible damage). Remove all maggots manually if possible. Apply licensed treatment (organophosphate or IGR dressings as appropriate) to the wound. Provide pain relief (meloxicam) and antibiotic cover for severe strikes. The depth and extent of tissue damage determines prognosis; deeply penetrating wounds with extensive tissue destruction may require euthanasia on welfare grounds.
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