Strike is a very serious condition - it causes great pain and suffering, representing poor animal welfare, and will lead to death of the host if left untreated. It is the hatching of larvae and maggots from fly eggs laid on the skin.
Each pregnant female fly (of the species greenbottle - Lucilia sericata, bluebottle - Calliphora erythrocephala or black blowfly - Phormia terra-novae) lays roughly 10 batches of approximately 300 eggs in 3-4 weeks. The flies are attracted to soiled fleece or damaged skin and this is where they will lay their eggs. The eggs hatch within hours and the larvae begin to penetrate the flesh, using enzymes to digest a path ahead of them. More flies are attracted to lay eggs on this damaged flesh. The larvae complete a pupal stage in the soil, which is as short as 3 days in the summer, before emerging as adults. In the winter, the pupae enter an inactive state, remaining as pupae until the soil temperature reaches 7°C.
The fly season in the UK runs from April to about November and this is the period of increased risk for strike, due to the increased humidity within the fleece. Strike is most common around times of high humidity, such as warm weather after heavy rain. Strike around the breech can occur at anytime, since soiling of the fleece produces the perfect conditions for larval hatching.
The first sign of fly strike is often depression, combined with agitation. A depressed animal will stand away from the group, move less than its companions, and may hang back during feeding. Animals with breech strike often stamp their hindlimbs and shake their tails. The primary lesion appears as a foul and discoloured area of wool, often with early stage maggots present.
- Flocks should be examined twice daily during the high risk period.
- Shearing reduces the high humidity environment near the skin. Crutching or dagging from the start of the fly risk period reduces the incidence of fly strike.
- Docking tails of lambs reduces soiling of tails and hence fly strike.
- Reduction of diarrhoea is very important to reduce faecal coverage - make sure you know the intestinal parasite status of your flock, and treat if necessary.
- Footrot is another attractor of flies to otherwise healthy animals. Make sure you treat footrot in a timely fashion.
- Dispose of carcases quickly - these will attract tissue decomposing flies and it is the same species which cause strike.
Strike can be treated with a suitable plunge dip, spot-on or pour-on treatment.