Old World Screw-worm Fly

Chrysomya bezziana belongs to the fly family Calliphoridae. This family comprises blow flies, carrion flies, and cluster flies. Other parasitic screwworms are found in this family, such as Cochliomyia hominivorax and Cochliomyia macellaria.

Adult. The body of the C. bezziana adult is typically metallic green or blue. The face of this species is typically yellow with soft, fine, yellow hairs. The length of an adult fly ranges between 8 and 12 mm. The abdominal tergites (the segments of the dorsal portion of the fly) have narrow darker bands and the legs are black or dark brown. Only the bases of the wings are infuscated; the wings have a glassy appearance. The anterior spiracles (small openings on the surface for the respiratory system) range from dark brown to orange. Another close relative, Chrysomya megacephala, occurs in many of the same regions as C. bezziana, so being able to distinguish between the flies is important. The male fly can be distinguished from C. megacephala by its eye facets. C. bezziana does not have a boundary dividing the upper and lower eye facets, while C. megacephala does have a boundary distinguishing them. The female does not have as distinct differences and cannot be as easily distinguished from C. megacephala, but can be identified by analyzing the frons (the uppermost part of the head of an insect). The frons of C. …

Chrysomya bezziana is widely distributed throughout tropical areas in the Old World. It is most prevalent in Southeast Asia, tropical and subtropical Africa, some countries in the Middle East, India, the Malay Peninsula, the Indonesian and Philippine Islands, and Papua New Guinea. C. bezziana is not found at altitudes higher than 2500 meters above sea level. In countries where C. bezziana does not exist, scientists and those in agriculture are worried that commercial flights, boats, or vehicles could distribute the fly. The climates in which C. bezziana thrives are present in Australia and the Americas; the fly requires climates similar to its New World relative, Cochliomyia hominivorax. Spread of the pest from Papua New Guinea to Australia has become a concern for Australia, as the presence of C. bezziana could cost livestock industries up to $500 million a year.

The female lays around 150–200 eggs at a time, in wounds and mucous membranes of a live mammal, and will hatch after 24 hours. The larvae feed on living tissue and are especially attracted to blood. The larvae feed on the host tissue for 5–7 days while they complete their development, then they fall to the ground to pupate. The pupal stage is temperature-dependent with warm weather favoring growth. Depending on the temperature, the pupal stage can last from 1 week to 2 months. The males become sexually mature after 24 hours of leaving their puparium (the hardened shell in which the pupae mature), while females take about 6–7 days to become fully sexually mature. If the weather is tropical (29 °C or 84.2 °F), the entire lifecycle will last about 24 days, but at cooler temperatures (below 22 °C or 71.6 °F), the life ycle can take 2–3 months to complete.

Chrysomya bezziana usually infects livestock, causing myiasis, which is the infestation of tissue (living or dead) on a living mammal by fly larvae. Mammals such as sheep, dogs, cattle, pigs, and even humans can become infested. The adult female lays her eggs on superficial wounds in live animals, preferring wounds that are several days old. Eggs of C. bezziana are commonly laid in the navel of newborn livestock species or on castration wounds in cattle. Eggs are also laid on open sores, ulcers, scratches externally, or mucous membranes throughout the body. Wounds as small as a tick bite are large enough for a female to lay her eggs. When the eggs hatch, the larvae burrow into the animal's living tissue and feed on it. Their common name, "screwworms", is derived from the maggots that embed themselves into the flesh of their host in a screw-like fashion. The larvae can burrow as deep as 15 cm (5.9 in) into the host's living tissue. As the maggots feed and cause tissue damage, the wound produces a characteristic odor, which can go unnoticed by humans. However, this odor entices more female flies to the wound and encourages them to lay their eggs there, as well, causing further infestation. C. bezziana is different from other fly species because tissue infestation can occur in the absence of necrotic tissue. The C. bezziana maggots may cause serious and permanent tissue damage. …