Blowfly Strike in Sheep: Signs, Treatment & Prevention

Blowfly strike, also called cutaneous myiasis, happens when blowflies lay eggs on a sheep's fleece or damaged skin and the hatching larvae feed on the skin surface and underlying tissue. In sheep, strike most often starts around the breech where wool is soiled with feces or urine, but it can also develop in wounds, around the prepuce, near the horns, or where foot problems contaminate nearby fleece.

This is a true veterinary emergency. Once larvae are established, tissue damage can expand quickly, especially in warm, humid weather. Affected sheep may become intensely painful, weak, dehydrated, and systemically ill. Advanced cases can contain very large numbers of maggots and have a strong, characteristic odor.

In North America, several blowfly species can be involved. Merck Veterinary Manual notes that black blowflies and the green bottle fly are important primary flies in the US and Canada. Moisture, odor, fleece contamination, and skin disease all make sheep more attractive to egg-laying flies.

For pet parents and flock managers, the key point is speed. Early recognition and prompt veterinary care can be lifesaving, while delays can turn a localized skin problem into a severe welfare crisis.

Blowfly strike develops when adult flies are attracted to moisture, odor, and skin debris in the fleece and then lay eggs. The highest-risk situations are warm, humid weather and any condition that leaves wool wet, dirty, or inflamed. Merck Veterinary Manual describes fecal and urine contamination of the breech as a classic trigger, while fleece rot after soaking rain can set up body strike.

Common predisposing problems include diarrhea from gastrointestinal parasites, urine staining, long or dense fleece, skin folds, wounds, footrot or other causes of lameness, and skin infections such as dermatophilosis. Rams may also be struck around the horns or prepuce. Any area where damaged skin meets damp wool can become a target.

Management factors matter too. Delayed shearing, inadequate crutching or dag removal, poor carcass disposal, and infrequent flock checks during fly season all increase risk. NADIS also highlights prompt treatment of lame sheep and control of worm burdens because dirty back ends and wound contamination strongly attract flies.

Not every sheep in a flock has the same risk. Animals with chronic diarrhea, poor body condition, heavy wool, or repeated breech soiling often need closer monitoring and a more proactive prevention plan with your vet.

Diagnosis is usually based on history, season, physical exam, and direct inspection of the fleece and skin. Your vet will part or clip the wool to define the full extent of the lesion, because the visible opening is often much smaller than the area affected underneath. The presence of maggots, moist inflamed skin, foul odor, and wool discoloration is often enough to confirm the problem.

Your vet may also look for the underlying reason the strike started. That can include checking for diarrhea, parasite burden, urine scalding, footrot, wounds, fleece rot, or skin disease. Finding and addressing the trigger matters, because treatment is less successful if the original cause is left in place.

In more serious cases, your vet may assess hydration, body temperature, pain level, and signs of toxemia or shock. Sheep that are depressed, off feed, weak, or recumbent may need more than local wound care. Some cases also need repeat examinations over the next few days to make sure hidden larvae are gone and the skin is healing.

Larval identification can be done when needed, but in day-to-day practice the immediate priority is stabilizing the sheep, removing larvae, and stopping further tissue damage.

Prevention works best when it combines flock management, close observation, and seasonal planning. During fly season, inspect sheep at least once daily if weather is warm and humid or if your flock has active risk factors. Pay special attention to the breech, tail area, feet, wounds, and any sheep that is lame, thin, or has diarrhea. Early detection is one of the most effective ways to limit suffering.

Keep fleece as clean and dry as possible. Timely shearing, crutching, and dag removal reduce soiling and improve airflow through the wool. Merck Veterinary Manual notes that crutching or tagging can help control breech strike for several weeks, and complete shearing helps when body strike risk is high. Prompt treatment of diarrhea, footrot, urine scalding, and skin disease is also essential because flies are strongly attracted to odor and moisture.

Preventive products can be useful, but they need to be chosen carefully for the flock, season, and production goals. MSD Veterinary Manual describes insect growth regulators such as diflubenzuron, flufenoxuron, and cyromazine as preventive tools rather than rapid treatment for established strike. Your vet can help you choose products that fit your region, handling schedule, and food-animal withdrawal requirements.

Good farm hygiene matters too. Remove carcasses promptly, manage high-risk pastures when fly pressure is heavy, and consider fly traps as part of an integrated plan. NADIS reports that inexpensive blowfly traps can reduce strike incidence in some settings. The best prevention plan is the one your flock can follow consistently through the whole risk season.