Vet's View with Ravensdown Vet Gavin Goble BVSc MRCVS

Flystrike is the most important ectoparasitic disease of sheep in New Zealand. It is caused by blowflies laying their eggs on sheep, which then hatch to maggots and invade the animal. It's a very serious disease affecting millions, and killing hundreds of thousands of sheep annually.

An animal may take six weeks to regain lost weight after treatment for flystrike, and up to eight months for its fleece to recover.

Flies must be attracted to lay their eggs on sheep, so such factors like faecal/urine stains, fleece rot, footrot and wounds predispose to flystrike. Sheep that have already been ‘struck', ie have live maggots, should be treated as soon as possible with an insecticide product (as well as removing wool around the area) to reduce pain and suffering and minimise production losses. Ideally, maximal effort should be directed towards preventative measures.

Flystrike has historically been prevented by ‘dipping', which saturates the entire fleece or body regions most prone to flystrike, in a product (usually IGR) that persists for long enough to cover the fly period. However, several other aspects of control are used.

Well-fed sheep, with an effective worm control programme – less dags – rotationally grazed on pasture previously grazed by cattle or deer are less susceptible to flystrike. Shearing is effective at preventing flystrike for a few weeks because the eggs and maggots dry-out with exposure to UV light.
Treat shearing cuts to prevent fly attraction and keep diseases such as footrot and fleece rot under control. Grazing management is also important; keep sheep away from areas that have a particularly high fly challenge, including areas that are warm and sheltered, such as scrub-filled gullies, bush margins and in the lee of shelterbelts – windy exposed paddocks are less favourable to flies. Remove potential fly breeding grounds; burn, or bury deeply, dead animals, cover offal pits and avoid creating dung heaps or composting vegetation – for example rotting hay.

Finally, get expert advice on the appropriate dip chemicals to use, along with the correct application method and timing.