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Screw Worm Fly Surveillance & Preparedness Program

Since 2002, Animal Health Australia (AHA) has managed the work program to coordinate Australia’s preparedness for the early detection of an incursion from exotic Old World screw-worm flies.

Types of screw-worm flies

Screw-worm fly is an insect parasite of warm-blooded animals, including people and birds. Its eggs hatch to become flesh-eating maggots or larvae that invade all types of wounds or moist openings on animals and people. In severe cases, it can even cause death.

There are two commonly known screw-worm flies:

  • Old World (OWS) screw worm fly (Chrysomya bezziana)
  • New World (NWS) screw worm fly (Cochliomyia hominivorax)

Suspicion of screw-worm fly infestation in animals is notifiable under state and territory animal health legislation (see the Australian Government’s Department of Agriculture list of nationally notifiable animal diseases).

The OWS and NWS species have similar biology and fill similar ecological niches in Africa and Asia, and the Americas respectively.

Old World screw-worm fly

The OWS is considered to be a very serious exotic pest threatening the economic viability of Australia’s northern livestock industries. It is endemic in a number of Australia’s closest northern neighbours. It is found in the coastal swamps of Papua New Guinea adjacent to the Torres Strait and throughout much of Indonesia, Malaysia and the Philippines.

The likelihood of an OWS incursion into Australia is low but the potential consequence of establishment of OWS in Australia is estimated to cost the livestock sector hundreds of millions of dollars.

Figure 1: Global distribution of screw-worm flies

screw-worm flies

OWS is found from Ethopia to northern South Africa, the Middle East Gulf region, the Indian subcontinent, much of south-east Asia, the Malay Peninsula, the islands of the Philippines and Indonesia, and Papua New Guinea. NWS was eradicated from the United States of America, Mexico, Curacao, Puerto Rico, the Virgin Islands and Central America. NWS can be found in the parts of the Caribbean islands, northern countries of South America to Uruguay, northern Chile and northern Argentina.

Screw-worm Fly Surveillance & Preparedness Program

In consultation with livestock industry and government stakeholders, AHA’s program includes:

  • trapping adult flies at sea ports and in the Torres Strait
  • promoting awareness of the OWS risk to landholders and the public
  • sampling of myiasis wounds of animals (livestock and wildlife).

A national advisory committee of the program also monitors Australia’s entomology requirements and risk-based surveillance requirements appropriate to Australia’s veterinary emergency plan (AUSVETPLAN; Australia’s response plan for the detection of an incursion of screw-worm fly).

Access the Emergency Animal Disease Bulletin and Exotic Animal Disease Alerts on the Australian Government Department of Agriculture website for more information about screw-worm fly.

Screw-worm fly toolkit

AHA has developed a number of tools to assist with the identification, diagnosis, reporting, and control of screw-worm fly.

What does screw worm fly do?

The screw worm fly lays its eggs on the dry edge of all types of wounds including tick bites, scratches, healing umbilical cords on new born animals, and wounds caused by dehorning or castration. The damage is done by the eggs hatching, the maggots burrowing deep into the wound and feeding on host tissues.

Screw worm fly can only survive in the healthy tissue of warm-blooded animals.  When the maggots mature, they exit the wound and drop to the ground where they burrow into the soil. Their skin hardens and forms a protective layer for the maggot while it develops and emerges as an adult fly. This process can be completed within 20 days.

Screw worm fly would be difficult to control, especially as adults are free flying and can disperse over large distances. So spread of screw worm fly is rapid if it is not detected early.

Figure 2: Lifecycle of a screw-worm fly (click on figure to view a larger image in your web browser)


Figure 3: A screw-worm fly laying its eggs in an animal’s wound


Photo: Philip Spradbery

Figure 4: A screw-worm fly egg masses (white) deposited in an animal’s wound


Photo: Philip Spradbery

Figure 5: Hatching of screw-worm fly maggots or larvae


Photo: Philip Spradbery

Recognising screw worm fly

Screw-worm fly looks like an average sized blue blowfly and is found in almost all tropical countries except Australia. It is related to the blowfly that causes fly-strike in Australian sheep.

Screw-worm fly maggots appear whitish to cream in colour. They have bands of dark spines growing on each body segment giving them the appearance of a screw. They grow to approximately 15 mm in length and 3 mm in diameter.

In the wound, screw-worm fly maggots stay bunched together while feeding, with their heads are down and tails up, showing two dark spots on the end of each tail. This bunching behaviour causes the wound to become larger as the maggots grow. The wound usually emits a strong pungent, sickly smell.

Figure 6: Shoulder wound on a dog infested by screw worm fly

Photo: Dr Don Sands

How does screw-worm fly spread?

Although the pest has not yet reached Australia, the adult screw-worm fly could enter Australia in a storm front, arrive on a boat or as maggots in an existing wound on animals or people arriving from countries to our north.

The risk of a screw-worm fly incursion is low, but the risk of screw-worm fly occurring in northern Australia is heightened by:

  • increased movement of people and animals (particularly through illegal entry)
  • increased live animal exports to the South East Asian and Middle East regions and subsequent return of transport vessels to northern Australian ports.

What do I do if I see screw-worm fly?

If you spot an animal that you suspect could have screw-worm fly:

  • isolate the animal immediately
  • call your local vet or a government animal health officer
  • check the herd or flock or other animals that may have come in contact with the suspect animal
  • check working dogs, domestic pets or co-workers.

Spread of screw worm fly is rapid if it is not detected early.

Who do I report it to?

If you spot anything unusual, report it immediately.

Contact your local vet, a government animal health officer or stock inspector, or phone the Emergency Animal Disease Watch Hotline 1800 675 888.

Early detection is important. Containment and eradication programs can be started before screw-worm fly can spread into other areas.

How do we treat screw-worm fly?

Screw-worm fly eradication, while theoretically possible, can be achieved by using the sterile insect technique (SIT), but this is technically difficult and very expensive.

The use of chemical pesticides or insecticides to treat screw-worm fly infestations would be a key component of any response plan to address a screw-worm fly incursion.

Insecticides that could be used for the treatment of animals with screw-worm fly infestations include the organophosphates – diazinon, chlorfenvinphos and fenthion, macrocyclic lactones, applied topically or systemically, spinosad and possibly some synthetic pyrethroids.

Ivermectin 2 Chemicals for Old World screw-worm administered systemically are effective against early OWS larvae but may not reliably kill older larvae. Topical application of ivermectin may be more effective against late stage larvae.

Spinosad is a relatively new registered compound that has the attraction of a nil withholding period and which is approved for use on organic properties by a number of certifying bodies. It could also be used to provide short-term protection for animals during transport to market.

See  Chemicals for the control of the Old World screw worm fly in Australia (pdf - 354 KB) .


Page reviewed: January 9, 2017