Screwworm fly

Old World and New World screwworm flies are a type of blowfly whose larvae (maggots) burrow into wounds and moist tissues of animals and humans. The maggots feed on the surrounding living flesh, causing the wound to grow and become foul-smelling and infected. Sometimes this leads to the death of the host.

Screwworm fly larvae are pale off-white maggots with bands of dark backward-facing spines on each body segment that give them a 'screw-like' appearance.

Adult screwworm flies are blue-green blowflies that look very similar to common Australian blowflies.

Identification of the species of adult fly or maggot requires specialist laboratory expertise.

Old world screwworm fly:

• is commonly found in countries neighbouring Australia to the north (including Papua New Guinea and Timor-Leste) and throughout much of Indonesia, Malaysia and the Philippines
• is prevalent across tropical and subtropical zones of sub-Saharan Africa, the Middle East and South Asia.

New world screwworm fly:

• is most often found in the tropical areas of the Americas (including South America, parts of southern Central America and some Caribbean islands)
• was eradicated from the United States, but since 2023 has been re-emerging in Central America and Mexico.

Screwworm fly's life cycle includes eggs, larvae, pupae and adults. The 2 species have similar life cycles, which vary in length from 18 days to around 2 months, depending on environmental conditions.

• Adult flies are attracted to wounds and moist tissues of warm-blooded animals.
• A female fly deposits a cluster of 100–300 eggs beside a wound or inside the nose, mouth, ears, eyes or naval of a newborn.
• The eggs hatch after 12–20 hours and the larvae burrow into and feed on the living tissue.
• After 4–8 days, the larvae are mature and drop to the ground to pupate.
• Adult flies emerges from the pupae after 7–60 days.

Infestation of, and damage to, tissues by screwworm fly larvae is called myiasis. Signs include:

• maggot-infested wounds
• cavernous, foul-smelling lesions up to 12cm in diameter
• wounds that fail to heal normally and emit a strong, pungent, sickly smell.

Affected animals may die due to secondary bacterial infection, toxaemia and fluid loss.

Likely sites for wounds on infested animals

• All animals:
  • Husbandry wounds – from dehorning, castration, branding, tail docking or ear tagging
  • Skin punctures – from vaccinations, ticks or buffalo flies
  • Trauma – from barbed wired, horns or other penetrating objects
• Newborns – the navel
• Ewes and cows – the vulva or perineum, just after lambing or calving
• Sheep – the corners of the eyes and the breech region without obvious trauma (not typical breech flystrike)
• Dogs – weeping skin sores, fight wounds, anal gland abscesses, eye and ear discharges and lip fold eczema

If screwworm fly were to reach Australia, it would have a devastating effect on wild and domestic animals. Most of northern Australia and the eastern seaboard offer a suitable climate for screwworm fly survival, and there is a genuine risk of the pest becoming established if it manages to gain entry in Australia.

The potential economic impact on the livestock industry includes production losses, animal deaths and control costs. The estimated direct annual cost to the livestock industry if screwworm fly were to become established in Australia is $500 million. The estimated total economic impact, including indirect and public health costs, is $900 million. Also, the pain and distress of wounds would have severe impacts on the health and welfare of pets, wildlife and people.

Australia has strong biosecurity measures in place to minimise the chance of screwworm fly entering Australia. However, it could enter through the arrival of either:

• infested live animals (including humans)
  or
• live adult flies or pupae (off-host stages).

Infested hosts

Both humans and animals with infested wounds could enter Australia via unregulated or illegal pathways. These include:

• movements across the Torres Strait
• foreign fishing vessels
• private yachts
• wildlife smuggling
• natural migration of birds or bats.

Live flies or pupae

Live adult flies or pupae might be able to enter Australia via:

• natural dispersal, flying across the Torres Strait (potentially assisted by prevailing winds)
• vessels or aircraft entering Australia.

All year round.

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

Livestock owners and private veterinarians can help by reporting any suspicious maggots to:

• Biosecurity Queensland on 13 25 23
  or
• the Emergency Animal Disease Hotline on 1800 675 888.

Also, take a photograph of the wound and maggots then, if possible, collect and send some maggots for laboratory identification.

To obtain a sampling kit from Biosecurity Queensland, contact us online, by phone or in person.

What to look for

• Cavernous, foul-smelling pocket-like wounds
• Maggots buried deep into the flesh of living animals
• Maggots with a 'screw-like' appearance that often feed in tightly packed clusters

How to collect maggots for identification

1. Confine the animal for sampling and photograph the wound.
2. Gently flush the wound with cool, clean running water to wash off any secondary-strike maggots.
3. Using tweezers, collect up to 10 maggots from the wound and place them in a container.
4. Pour hot water (just off the boil) over the maggots in the container and wait for 2 minutes for the maggots to die.
5. Using tweezers, transfer the maggots to a small watertight container and immerse the maggots in alcohol-based hand sanitiser or 80% ethanol.
6. Seal the container and place it in a ziplock bag with some absorbent paper.

Where to send the sample

Send maggots collected in Queensland to the Biosecurity Sciences Laboratory.

Entomologists specifically trained in identification of screwworm fly will examine the maggots and will inform you of the results.

National surveillance and preparedness

The screwworm fly surveillance and preparedness program, managed by Animal Health Australia, coordinates surveillance for the early detection of any screwworm fly incursion. This program includes:

• regular trapping for adult screwworm fly in the Torres Strait and at several major ports around Australia (including Townsville)
• targeted inspection of livestock wounds, conducted during animal health surveillance activities in northern Australia
• entomology training and development of reference materials
• awareness promotion to increase general surveillance for screwworm fly.

An AUSVETPLAN response strategy outlines the nationally agreed approach for responding to an incident or suspected incident of screwworm fly in Australia.

Given the significant economic and ecological impact an incursion of screwworm fly could have on the Australian livestock sector and wider community, the current policy is to eradicate the pest as soon as possible.

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.