Blowflies

(Calliphoridae)

  • Blowflies vary in size, but their bodies have a metallic colour.
  • Blowflies lay their eggs in decomposing meat or animal flesh.
  • Flystrike occurs when larvae are found in wounds, it is uncommon in cattle in Australia.
  • Flystrike should be investigated to exclude exotic screw-worm fly by sending maggots to your local Departmental Veterinary Officer or private veterinarian

Screw-worm infestation is a notifiable disease. Any flystrike of cattle in Australia, and particularly in northern Australia should be investigated to rule out screw-worm fly. Any animals with maggots in wounds are to be reported to the Emergency Animal Disease Watch Hotline on 1800 675 888, and samples of maggots submitted to the local Departmental Veterinary Officer or private veterinarian.

Blowflies come in a wide range of sizes and colours (Figure 1), but one thing they all have in common is that they are attracted to decomposing meat or animal flesh where they lay their eggs. These flies can be found throughout Australia and can be attracted to wounds in cattle where they feed on the blood and associated fluids, and occasionally lay eggs.

Blowflies are more common during the warmer months when temperatures are above 17˚C.

Figure 1. Blowfly adults; left a sheep blowfly with inset larva and right an oriental latrine fly. Image courtesy of Mukund Madhav

Location of flystrike

The incidence of flystrike in cattle in Australia is low but it does occur occasionally in wounds such as from dehorning, castration and occasionally dog bites. The navels of newborn calves and any large wound can also be a target for blowflies. Similar to sheep flystrike, long-haired cattle breeds such as Scottish highland cattle may be susceptible to flystrike if the hair is heavily contaminated with faeces and urine.

Signs of flystrike

Signs of cattle with a fly-struck wound include:

  • weeping wound with a pungent, rotten smell
  • constant licking of the wound
  • restlessness
  • fever
  • lethargy

Although flystrike caused by blowflies is a common problem in sheep in Australia, it is relatively rare in cattle. However, screw-worms are commonly seen to strike cattle in neighbouring countries north of Australia as well as in the Middle East and South America. Screw-worms are of considerable biosecurity concern to Australia and maggots found in wounds in cattle should be carefully checked to ensure that they are not screw-worm maggots (see the sections on screw-worm flies and manage flystrike below).

Treatment of flystrike

There are no products currently registered for use on cattle for treating flystrike. Talk to your veterinarian as some diazinon-based compounds that are registered for treatment of sheep flystrike are registered for use on cattle for other purposes.

Screw-worm flies

Old World screw-worm (Chrysomya bezziana) and New World screw-worm (Cochliomya hominivorax)

Old World screw-worm flies are not present in Australia, but they are found in nearby Papua New Guinea and Indonesia. New World Screw-worms are found in a number of northern hemisphere countries and in South America, and have entered Australia in human infestations on at least 2 occasions. These flies are a serious exotic pest that can strike most warm-blooded animals, they are a serious threat to Australia´s livestock industries.

Screw-worm infestation is a notifiable disease. Any flystrike of cattle in Australia, and particularly in northern Australia should be investigated to rule out screw-worm fly. Any animals with maggots in wounds are to be reported to the Emergency Animal Disease Watch Hotline on 1800 675 888, and samples of maggots submitted to the local Departmental Veterinary Officer or private veterinarian.

Adult screw-worm flies (Figure 2) look very similar to other shiny blue-green blowflies commonly found in Australia (Figure 1). The difference between screw-worm flies and other blowflies is screw-worm fly maggots only grow in living flesh and flies are much more likely to be attracted to wounds.

Female flies lay their eggs near open wounds or moist orifices. The eggs hatch and the maggots then enter the wound where they feed for 6-7 days before exiting and dropping to the ground where they pupate and emerge as a new fly a week later.

Figure 2. Old World screw-worm fly, Chrysomya bezziana, adult and larva. Image courtesy of Mukund Madhav

Location of screw-worm strikes

Any wound including those from normal management operations such as castration, dehorning, and ear tagging, as well as from dog bites, cuts, scratches and lesions caused by ticks and buffalo flies (Figure 3). The navels of newborn calves and the vulval area of cows after birthing are very susceptible to screw-worm attack.

Figure 3. Flystrike on the ear caused by Old World screw-worm fly. Image courtesy of Peter Green, Queensland Department of Agriculture and Fisheries

Signs of screw-worm strike

Screw-worm maggots are hard to see as they often burrow deep into wounds.

Signs of a screw-worm fly-struck wound include

  • weeping wound with a pungent, rotten smell
  • constant licking of the wound
  • restlessness
  • fever
  • lethargy

Diagnosis of screw-worm strike

Larval stages present in wounds are the most likely way screw-worm flies will be encountered. The way to determine if the larvae are screw-worm fly is to remove larvae from the wound and preserve them by placing them in near boiling water for 30 seconds, then removing and placing in methylated spirits in a sealed bottle. These samples should then be submitted to the local Departmental Veterinary Officer or private veterinarian for identification.

Treatment of flystrike

There is currently only one chemical, ivermectin administered by subcutaneous injection, registered for use against Old World screw-worm fly.

You can review the products pages on this site to find out specific information about treatments, including their length of protection, dose rate, withholding period, export slaughter interval and manufacturer.

Management of flystrike

Screw-worm flies are not currently present in Australia. The best management to prevent screw-worm flies from becoming established here is through increased observation of cattle wounds and manage flystrike strategies.

More information

More information on screw-worm fly can be found in the Old World Screw-Worm Fly: A Diagnostic Manual produced by Animal Health Australia (AHA).