Screw-worm Fly

What is it?

Screw-worm fly is an insect parasite of warm-blooded animals, including humans. Related to the blowflies that cause fly-strike in Australian sheep, it prefers hot, humid climates and cannot survive in frost-prone areas. 

There are two species of screw-worm fly: ‘Old World’ (Chrysomya bezziana) and ‘New World’ (Cochliomyia hominivorax).

Where is it found?

Old World screw-worm fly occurs in almost all tropical countries except Central and South America and Australia, including much of Africa, India, Southeast Asia and New Guinea. Screw-worm flies are also present in the coastal swamplands of Papua New Guinea adjacent to the Torres Strait. Because of their proximity to Australia, these pose the greatest risk of entry through either sea trade or a fly strike wound on animals or people. 

The New World species is found in the western hemisphere, including Central and South America. Long-distance spread of the insect generally depends on the movement of infected animals.

What are its effects?

Screw-worm flies breed in wounds on mammals, including humans. The female screw-worm fly lays up to 250 eggs on the edge of an injury, scratch, branding mark or castration wound.

Larvae (maggots) hatch within 24 hours, enter the wound and chew their way into healthy underlying flesh to feed. The wound becomes a mass of maggots, causing extensive tissue damage and leaving the flesh susceptible to a secondary fly strike. After about a week the larvae drop from the wound to pupate in the soil. Adults emerge in another seven days to mate and repeat the cycle.

If left untreated, infested animals can die from infection and loss of tissue fluid.

Screw-worm flies look like Australian blowflies, with a shiny, blue-green body and red eyes, and can only be distinguished from native flies through microscopic examination or DNA analysis.

What’s the risk to Australia?

Screw-worm flies could cost close to $500 million a year in lost production and control measures if they entered Australia. They would have a devastating effect on northern livestock production, particularly cattle and sheep industries. In some herds, 10 to 15 per cent of cattle could be struck at any time; the greatest loss would be through the deaths of newborn calves as a result of navel strike.

The potential spread of this pest from Papua New Guinea is one of the major quarantine threats to northern Australia. Eradication would depend on the release of millions of sterile male screw-worm flies to reduce breeding numbers. This could take up to five years, with health implications for remote northern communities.

AQIS has a monitoring program in the Torres Strait, on Cape York Peninsula and in other strategic locations to ensure early detection of a screw-worm fly incursion.

The program includes lure traps for adult screw-worm flies around airports, seaports and other high-risk areas, and inspection of livestock for fly strike. Quarantine officers also inspect returning livestock vessels, and sentinel cattle herds are used to monitor for maggots.

Northern Australia Quarantine Strategy (NAQS) fact sheet on Screw-worm fly