How to Prevent Fly Strike in Sheep: Summer Treatment Guide

Why Fly Strike Is a Summer Emergency

Blowfly strike (myiasis) is one of the most serious welfare emergencies in sheep farming. In warm, humid conditions, the green bottle fly (Lucilia sericata) can lay eggs on a sheep in minutes. Maggots hatch within hours and begin feeding on living tissue — a severely struck sheep can die within 3–4 days if untreated. In the UK, peak risk runs from May through September, with July and August typically the worst months.

The good news: fly strike is largely preventable with the right products and flock management.

Which Sheep Are Most at Risk?

Any sheep can be struck, but risk is highest in:

• Sheep with dag (faecal soiling around the breech) — the number one risk factor
• Sheep with foot rot or wounds — maggots are attracted to any moist, warm area
• Unshorn or heavily fleeced sheep in hot weather
• Sheep with fleece rot or dermatitis
• Lambs with dirty tails or navels
• Any sheep that is lame, cast, or unable to move freely

Prevention: Treatment Options

There are three main categories of preventative treatment in the UK:

1. Pour-On Products

Applied along the back and breech, pour-ons are the most widely used preventative. Key products include:

• Crovect / Ectofly (cypermethrin): Provides 6–8 weeks protection. Suitable for use on shorn and unshorn sheep. Withdrawal period applies for meat.
• Dysect (alpha-cypermethrin): Similar spectrum, 8 weeks protection on shorn sheep.
• Zermex / Extinosad (spinosad): Newer mode of action, useful where pyrethroid resistance is suspected. Up to 13 weeks protection.

2. Plunge Dipping

Whole-body dipping with an organophosphate or synthetic pyrethroid dip gives the most comprehensive coverage, including the head and legs. It is labour-intensive but highly effective for high-risk flocks. Requires a registered dipping facility and correct disposal of spent dip.

3. Spot Treatments

Products such as Clik Extra (dicyclanil) are applied to the breech area and provide up to 16 weeks protection — the longest available. Particularly useful for ewes and lambs going out to high-risk pasture for the summer.

Timing Your Treatment

• Apply preventatives before fly activity peaks — don't wait for the first struck sheep
• In the UK, treat from late April/May in southern regions; May/June in northern and upland areas
• Re-treat according to the product's stated duration — mark your calendar
• Shearing before treatment improves product efficacy and reduces risk significantly
• Dagging (crutching) sheep with dirty breeches before treatment is essential — don't apply product over soiled fleece

What to Do If You Find a Struck Sheep

Act immediately — every hour counts.

1. Catch and restrain the sheep — struck sheep are often dull and easy to catch, which is itself a warning sign
2. Clip away the affected fleece — expose the full extent of the strike. It is often larger than it appears on the surface.
3. Remove all maggots — use a blunt instrument or gloved hands. Be thorough.
4. Apply a wound treatment — use an approved fly repellent/wound spray to the affected area
5. Treat with a systemic product if severe — speak to your vet about injectable treatments for badly affected sheep
6. Provide supportive care: fresh water, shade, and monitor closely for the next 48–72 hours
7. Record the case in your flock health records

Sheep with deep tissue damage or signs of toxaemia (rapid breathing, high temperature, collapse) need urgent veterinary attention.

Flock Management to Reduce Risk

• Dag sheep regularly throughout summer — especially after any scouring
• Control intestinal worms to reduce scouring and dag build-up
• Treat foot rot promptly — lame sheep are high-risk
• Check the whole flock at least twice daily during peak fly season — morning and evening
• Pay particular attention to sheep that are lagging behind or standing apart from the flock
• Consider fly traps around the farm to monitor and reduce local fly populations

Resistance Awareness

Pyrethroid resistance in Lucilia sericata has been confirmed in parts of the UK. If you are seeing treatment failures with cypermethrin-based products, discuss alternative actives (spinosad, dicyclanil) with your vet or animal health advisor.

Browse our Animal Health Products & Wormers range for fly strike preventatives, wound sprays, and flock health supplies.

Animal Health Information: This guide provides general information only and is not a substitute for veterinary or SQP advice. Always follow product labels and seek professional advice when selecting or administering animal medicines.