About Screw Worm Fly

Screw worm fly is an insect parasite of warm-blooded animals, including people and birds. It’s 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 three types of screw worm fly – Old World Chrysomya bezziana (OWS); New World (Cochliomyia hominivorax) (NWS) and Palaearctic screw-worm (Wohlfahrtia magnifica).

The Old World screw-worm, Chrysomya bezziana (OWS) or screw worm fly (SWF) 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 our closest northern neighbours and is found in the coastal swamps of Papua New Guinea adjacent to the Torres Strait and throughout much of Indonesia, Malaysia and the Philippines.

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 and the maggots burrowing deep into the wound.

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.

 

 

 

 

(please click for larger image)

Screw worm fly laying its eggs in a wound

Screw worm fly egg masses (white) deposited in a wound

Hatching of screw worm fly into maggots
(Photos courtesy of 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 occurs in almost all tropical countries except Australia.

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

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

Shoulder wound on a dog infested by screw worm fly
(Photo courtesy of 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 with the increased movement of people and animals, particularly through illegal entry and with increasing live animal exports to the SE Asian region and subsequent return of transport vessels to northern Australian ports, heightens the risk of screw worm fly occurring in northern Australia.

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. Don’t forget to check working dogs, domestic pets or even 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 call 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 is 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 newly 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.

Chemicals for the control of the Old World screw worm fly in Australia

Page Updated: 20 August 2009