Flystrike is a condition where blowfly larvae (maggots) feed on the living flesh of a sheep. It begins when certain species of blowfly lay eggs in a sheep’s fleece, typically in warm, moist areas soiled by urine or diarrhea. The eggs hatch within 12 to 24 hours, and the emerging larvae burrow into the skin, releasing enzymes that break down tissue. Left untreated, flystrike can kill a sheep within days.
How Flystrike Starts
Not all blowflies can initiate a strike. “Primary” flies, most notably the green blowfly (Lucilia cuprina in Australia and Lucilia sericata in the UK), are the species that lay eggs on living sheep. They’re drawn to the smell of damp, soiled wool, particularly around the rear end where feces and urine collect. A single female can deposit 100 to 200 eggs in a cluster, and in warm weather those eggs hatch in as little as 12 hours.
Once the first larvae begin feeding, the wound they create attracts “secondary” and “tertiary” fly species like bluebottles and blackbottles. These flies can’t start a strike on their own, but they pile onto an existing one and dramatically accelerate the damage. This layering effect is a major reason why a small initial strike can become fatal so quickly.
Where on the Body It Happens
The most common site is the breech, the area around the tail, rear end, and between the back legs. Diarrhea and urine staining keep the wool in this zone wet and odorous, which is exactly what blowflies seek out. Wrinkled skin folds trap even more moisture, compounding the problem. Breech strike accounts for the majority of cases in most flocks.
Flystrike can also occur on the body (“body strike”), typically after rain saturates the fleece, or along the back where fleece rot develops. Wounds from fighting, shearing cuts, or foot injuries are other common targets. Any break in the skin or prolonged dampness in the wool creates an opportunity.
Signs to Watch For
Early flystrike is easy to miss. Affected sheep often separate from the flock, stop grazing, and repeatedly stamp their feet or bite at their hindquarters. You might notice a dark, moist patch in the fleece, sometimes with a faint sweet or rotten smell. As the strike progresses, the wool becomes discolored and lifts away from the skin. Underneath, the larvae create visible trails of damaged, raw tissue.
In advanced cases, sheep become lethargic, stop eating entirely, and may lie down and refuse to move. The skin turns black or green, and large numbers of maggots are visible when the wool is parted. Toxins released by the larvae enter the bloodstream, causing shock and organ failure. At this stage, even treatment may not save the animal.
Weather Conditions That Raise the Risk
Flystrike is strongly seasonal. Blowflies need warmth and humidity to be active, and research in southwest England found that egg-laying essentially stops when minimum temperatures drop below about 9°C (48°F). The highest-risk periods are warm, humid stretches in spring and summer, especially after rain. A few consecutive days of temperatures above 15°C with high humidity can trigger a wave of strikes across a flock.
Regions with mild, wet summers face the longest strike seasons. In parts of Australia, the risk window can stretch from September through April. In the UK, peak risk typically runs from May to September but has expanded in recent years with warmer autumns.
What Makes Some Sheep More Vulnerable
Diarrhea and the resulting buildup of soiled wool (known as “dags”) around the breech are the single biggest risk factors. Blowflies are attracted to the smell of prolonged wetting from feces and urine, and once they land, the warm, moist environment is ideal for egg survival. Sheep with wrinkly skin around the breech trap more moisture and are struck at higher rates. Fleece density and coverage around the crutch area also matter: more wool in the breech zone means more places for moisture and odor to build up.
Both wrinkle score and dag score are heritable traits, meaning some bloodlines are inherently more susceptible. Ewes are more vulnerable than wethers because urine can stain the wool below the vulva. Lambs with docked tails that are too short lose the ability to lift the stump away from the body when defecating, which increases soiling. The recommended docking length is at the caudal fold, roughly where the tail tip would cover the vulva in a ewe. Docking shorter than this also raises the risk of rectal prolapse by about 8 percent.
Prevention Strategies
Crutching and Shearing
Crutching means removing wool from around the breech, tail, and back legs to keep the area clean and dry. It’s one of the most effective non-chemical ways to reduce strike risk because it eliminates the damp, soiled wool that attracts blowflies in the first place. Timing matters: crutching should ideally be done before the start of the blowfly season, and again before any expected period of wet weather or when diarrhea is a problem in the flock. Full shearing also provides protection by removing the entire fleece, but the effect is temporary as wool regrows.
Chemical Protection
Preventive chemicals applied to the fleece can provide weeks of protection. Insect growth regulators are the most widely used. One active ingredient, cyromazine, prevents larvae from developing normally and protects sheep for roughly 12 to 13 weeks under field conditions. A newer compound, dicyclanil, extends that protection to at least 20 weeks. These products are typically applied as a spray-on along the backline or over the breech area, either right after shearing or onto longer wool before peak fly season.
Genetic Selection
Breeding for flystrike resistance has become an increasingly practical long-term strategy, particularly in Merino flocks. Traits like low wrinkle score, reduced breech wool coverage, and resistance to dags are all heritable and genetically linked to lower strike rates. Australia’s national research body, CSIRO, developed best-practice guidelines for incorporating these traits into breeding programs about a decade ago. Since then, more breeders have been selecting rams with both flystrike resistance and wool productivity traits, and a growing number of properties have been able to stop mulesing (the surgical removal of skin folds around the breech) as the national flock shifts toward less susceptible animals.
Treating an Active Strike
Speed is everything. The longer a strike goes untreated, the worse the tissue damage and the higher the chance of death. The basic steps are straightforward but need to be done thoroughly.
- Clip the affected area: Remove wool well beyond the visible edges of the strike, leaving at least a 5 cm margin of clean wool around the wound. This dries the skin and exposes the full extent of the maggot trails.
- Remove all maggots: Every larva needs to come off the animal. Bag the infested wool in black plastic and leave it in the sun to kill any remaining maggots and prevent them from maturing into adult flies.
- Apply a registered flystrike dressing: Use a low-pressure applicator to treat the wound thoroughly. Choose a product that lasts long enough to protect the area through the entire healing process.
- Isolate the sheep: Remove struck animals from the mob so you can monitor their recovery and prevent the strike smell from attracting more flies to other sheep nearby.
After treatment, keep a close eye on the rest of the flock. Where one sheep is struck, others are likely at risk from the same conditions. Animals that are struck repeatedly despite good management are worth culling from the breeding program, since their susceptibility is likely genetic and will be passed to their offspring.