Screw worm fly

Screw worm fly is a notifiable disease

Under Queensland legislation, if you suspect that domestic animals may be infested with screw worm fly, you must report it to Biosecurity Queensland.

Call us 13 25 23 or
Emergency Disease Watch Hotline 1800 675 888

Overview

Cause

Chrysomyia bezziana, the old world screw worm fly

Description

Screw worm fly maggots are parasites of all domestic and wild animals, birds and people.

The fly is not present in Australia. However it is in Papua New Guinea and Indonesia and is considered the most serious exotic pest threatening Australia´s livestock industries. This status is due to the difficulty in eradication requiring extensive use of sterile male release, the wide host range and the damage to the food production of northern Australia that is possible from this pest.

Where the disease occurs

There are two types of screw worm fly. The old world screw worm (Chrysomyia bezziana), is endemic in countries north of Australia. 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 new world screw worm fly (Cochliomyia hominivorax) is predominately found in the tropical areas of the American continent. It has been eradicated from the southern states of the United States.

The disease in animals

The disease is caused when fly larvae (maggots) burrow into living flesh of the host and feed until maturing and dropping off the host to pupate on the ground. Adult flies are free flying, preferring warm humid environments.

The female screw worm fly is attracted to any moist discharge on warm blooded animals such as wounds or natural body orifices. The female fly lay their eggs (usually between 100 - 200) on the edge of all types of wounds such as the navel of new-born animals, castration or dehorning wounds, tick bites, running eyes or peeling brands. Eggs are laid in rafts beside the wound to allow easy access for the maggots when hatched.

The larvae hatch in about 12 hours and penetrate the living tissues surrounding the wound. Egg laying by other screw worm flies is encouraged by the presence of larvae already in the wound. The larvae create a cavernous foul smelling lesion up to 10-12 cm in diameter, characterised by progressive liquefaction, necrosis and haemorrhage. Secondary bacterial infection, toxaemia and fluid loss contribute to the death of the animal.

Larval development is complete in 5-7 days after which they leave the wound and fall to the ground. The larvae pupate for 2-60 days depending on temperature before the flies emerge.

The optimum temperature range for fly activity is 20-30ºC. They do not survive temperatures below freezing. In optimal temperatures and humidity the cycle can be completed in less than 20 days.

How the disease spreads

Screw worm fly could arrive in Australia as an adult fly via a storm front from countries to the north or on a boat or plane. It is also possible for maggots to enter Australia in an existing wound on an animal. Constant surveillance is essential.

The mean distance that 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.

Prevention of the disease in Australia

Australia is free of the disease and the main control activity is constant surveillance for adult screw worm fly at ports and in the north of Australia. Queensland livestock owners and private veterinarians can assist by reporting and submitting any suspicious flies or larvae. Sampling kits are available from your local Biosecurity Inspector. The larvae should be dropped into water that has just boiled and left for 1-2 minutes to kill them and preserve their shape and colour. They should then be transferred to a container of 70% alcohol, such as methylated spirits. Samples should be sent to a Biosecurity Queensland laboratory for examination and identification. A plastic vial is best for encasing samples and should be packed in a sealed plastic bag to avoid leakage. Pack the inside of the plastic bag with absorbent material such as cotton wool and place the entire parcel in crush-resistant packaging for mailing.

Australia's screw worm fly surveillance border program is delivered by the federal government through the North Australian Quarantine Strategy. 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.

Fly traps are positioned at the sea ports to trap adult flies from overseas vessels. In 1988, several adult Old World SWF were trapped in an empty livestock vessel in Darwin harbour. The vessel had just returned from delivering cattle to Brunei. In 1992, New World SWF larvae were identified in a lesion on the back of the head of a person who had just returned to Australia from a visit to Brazil and Argentina.

Control of the disease in animals

If screw worm fly were to reach Australia, it could have a devastating effect on wild and domestic animals in the northern areas. The fly would be difficult to control, especially as adults are free flying and can disperse over large distances. Spread of the fly could be rapid if it is not detected early.

Control measures will include:

  • quarantine and movement controls to prevent movement of infested animals
  • decontamination and disinfection of larval-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 infected and disease-free areas.

Eradication programs have been successfully carried out in other countries using the sterile insect technique.

Can people get the disease?

People who travel overseas can be infested with screwworm fly larvae. Any maggot infested wounds, especially in someone who has just returned from abroad, should be reported to a doctor or a health unit. Infested wounds should be cleaned and treated immediately to prevent spread of the pest.

Last updated 06 February 2013