Flystrike can be a major Summer problem in sheep, especially lambs, particularly following a period of warm wet weather. Four species of flies are involved in NZ as causes of this disease. They lay eggs or live larvae (maggots) in the wool close to the skin mainly along the back, rump, breech and tail of the sheep. The eggs hatch to maggots 12 – 24 hours later and the maggots eat through the skin and feed on the underlying tissues for 4 to 6 days. This causes irritation to the sheep, they loose condition and may die due to fluid loss or infection via the wound. The flies are particularly attracted to damp wool, especially if it has been soiled with blood, urine or faeces; or by secretions from foot or pizzle rot. Initial signs are irritation and restlessness, followed by the wool in the struck areas becoming discoloured. This wool falls off and exposes raw tissue underneath; at this stage the sheep is obviously off colour. Without treatment, the sheep will die.
Preventing flystrike in sheep involves several strategies.
- Prevent dags.
- Crutch or shear sheep in summer.
- Have a good worm control strategy.
- Remove potential fly breeding grounds.
- Burn or bury deeply any dead animals (including sheep, rabbits, opossums) on the farm; cover offal pits; avoid creating dung heaps or composting vegetation.
- Keep sheep away from high risk areas. Blowflies love sheltered areas such as scrubby gullies. They dislike windy exposed areas.
Chemical prevention – dipping. There are several different chemical families used to prevent fly strike; and they are applied in different ways. Application can require saturation on the sheep fleece to skin level (plunge or shower dips); pour-on or spray-on application to the back and breech of the sheep; or hand jetting (the most effective and economical). The chemical used will depend on the method of application, the length of time of protection required, and whether lice treatment is required as well as fly protection. It is very important to read the label in full and follow the manufacturers directions for use, and to observe meat and wool withholding periods.
Treatment of flystrike requires clipping the wool surrounding the struck area from the outside in (to prevent maggots escaping), and treating the affected area with a suitable product (an organophosphate chemical). Note many prevention products are not effective at treating flystrike. Some sheep may require antibiotic treatment as well. When clipping the affected area, avoid removing all the wool – if an area of skin is exposed, it will need sunburn protection as well.