Screw-worm fly

Screw-worm fly larvae (maggots) burrow into living flesh of the host and feed until maturing and dropping off the host to pupate on the ground.

Screw-worm fly (SWF) larvae are pale, off-white maggots found deep in the wounds of mammals (including people) and birds. Likely sites for wounds infested with SWF on animals include:

• husbandry wounds – from dehorning, castration, branding, tail docking and ear tagging
• newborns – navel
• just after lambing/calving – the vulva or perineum of the mother
• skin puncture – from vaccinations, ticks or buffalo flies
• trauma – from barbed wired, horns and other penetrating objects
• sheep – the corners of the eyes and breech region without obvious trauma (not typical breech fly-strike)
• dogs – weeping skin sores, fight wounds, anal gland abscesses, eye and ear discharges and lip fold eczema.

Confirmation of the species of maggot requires specialist expertise.

Adult screw-worm flies are blue-green blow flies that look very similar to common Australian blow flies.

Old world screw-worm (Chrysomyia bezziana)

• Commonly found in countries north of Australia.
• Found in the coastal swamps of Papua New Guinea adjacent to the Torres Strait and throughout much of Indonesia, Malaysia and the Philippines.

New world screw-worm fly (Cochliomyia hominivorax)

• Most often found in the tropical areas of America.
• Eradicated from the southern states of the United States.

• all domestic and wild mammals – including people
• birds.

The screw-worm fly life cycle includes the eggs, larvae, pupae and adults

The life cycle can be completed in less than 20 days under ideal environmental conditions

Eggs

• 100–200 are laid in rafts beside wounds.

Larvae (maggots)

• Hatch about 12 hours after laid.
• Enter living tissue around wounds.
• Create cavernous, foul smelling lesions up to 10–12cm in diameter.
• Feed on host tissue until mature—about 5–7 days.
• When mature they leave the wound and fall to the ground.

Pupate

• Pupate for 2–60 days before the flies emerge.
• Do not survive temperatures below freezing.

Adult

• Free flying.
• Prefer warm humid environments.
• Optimum temperature range is 20–30ºC.

• Maggot infested wound.
• Cavernous, foul smelling lesion up to 10–12cm in diameter.
• Progressive liquefaction, necrosis and haemorrhage.

Secondary bacterial infection, oxaemia and fluid loss may lead to death of the animal.

If screw-worm fly were to reach Australia, it would have a devastating effect on wild and domestic animals in northern areas. Up to $100 million would be lost annually through livestock production losses and control costs. The pain and distress of wounds would also have severe impacts on pets, wildlife and people.

Because screw-worm flies are established in neighbouring countries they could be carried to Australia by:

• storm fronts from countries to the north.
• boats or planes.
• maggots within a wound.

The average distance that screw-worm flies can travel is 11km and the furthest distance is 100km with assistance from the wind. Survival of flies at the destination will depend on the presence of animals that they can infest.

All year round.

Early detection will enable us to minimise the costs of an incursion and eradicate SWF from Australia more quickly.

Queensland livestock owners and private veterinarians can assist by reporting and submitting any suspicious maggots. Sampling kits are available from your local Biosecurity Inspector.

Maggots collected in Queensland can be posted to the Biosecurity Sciences Laboratory. Entomologists specifically trained in SWF identification will examine maggots and will inform you of the results.

How to submit samples to the Biosecurity Sciences Laboratory:

1. Confine the animal for sampling and photograph the wound.
2. Gently flush the wound with clean running water to wash off any secondary-strike maggots.
3. Using tweezers collect up to 10 maggots from the wound and place in a small sturdy container.
4. Pour hot water (just off the boil) over the maggots and wait for two minutes for it to kill the maggots.
5. Using tweezers, transfer the maggots to a small container with a screw-top lid and cover the maggots (1cm) with either 70% ethanol or another alcohol based product, for example hand gel/sanitiser.
6. Screw the lid onto the container firmly to close it and place the container into a zip-lock bag with some absorbent paper.

Ports and northern Australia are under surveillance for adult screw-worm fly.

Australia's screw-worm fly surveillance border program is delivered by the federal government. The program is designed to minimise the risk of undetected entry of fly struck animals or adult flies from countries to the north of the Torres Strait.

Screw-worm fly is difficult to control, as adults are free flying and can travel long distances. Spread of the fly could be rapid if it is not detected early.

Control measures may include:

• quarantine and movement controls to prevent movement of infested animals
• decontamination and disinfection of contaminated areas
• frequent mustering to treat affected animals with chemical insecticides or pesticides
• tracing and surveillance to determine the extent and distribution of the fly
• zoning to define infested and disease-free areas.
• releasing sterile male flies in infected areas

Eradication programs have been successful in other countries using the sterile insect technique.