Chrysomya bezziana
(Old World screw-worm)

Preferred Common Name

• Old World screw-worm

International Common Names

• English: Old World screwworm; Old World Screw-worm fly; screw worm

English acronym

• OWS
• OWSWF

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Arthropoda
•             Subphylum: Uniramia
•                 Class: Insecta
•                     Order: Diptera
•                         Family: Calliphoridae
•                             Genus: Chrysomya
•                                 Species: Chrysomya bezziana

Globally, there are two Screw-worm fly species, the Old World SWF (OWS) (Chrysomya bezziana) and the New World SWF (NWS) (Cochliomyia hominivorax) (Spradbery, 1994). The genus Chrysomya contains 11 described species, many of which are important in forensic and medical criminal taxonomy (James, 1947). C. bezziana is the only species in the genus of primary medical importance and is distributed in sub-Saharan Africa, Middle East, Indian subcontinent, SE Asia and New Guinea, where the larvae parasitize warm-blooded animals including livestock and humans. Chrysomya bezziana was first described by Professor Mario Bezzi of Turin, based on material collected from cattle in the Belgian Congo (now CDR) who identified them as Chrysomya megacephala, but later they were correctly identified as a new species by Joseph Villeneuve de Janti, and named Chrysomya bezziana, in honour of Professor Bezzi (Villeneuve, 1914).

Chrysomya bezziana was first described from adults reared from larvae collected in cattle myiases in Kitoboli and Dolo, Belgian Congo (now CDR) by Joseph Rovère, a doctor and veterinarian (Villeneuve, 1914). The endemic distribution of C. bezziana includes much of tropical Africa, Arabia and the Middle East, the Indian subcontinent and SE Asia as far east as New Guinea. Introductions are made possible by the movement of host animals. For example, live sheep exports from Australia to Middle East ports resulted in sheep becoming infested at the port of Muscat in the Sultanate of Oman when unloading animals, followed by their subsequent offloading in Bahrain and Kuwait (Kloftet al., 1981). C. bezziana has been recorded in Australia on one occasion when a returning livestock vessel docked at Darwin after delivering cattle to Brunei. An electrocutor trap was set up on board the vessel after it had cleared customs and quarantine and 7 adult C. bezziana were subsequently captured (Rajapaksa and Spradbery, 1989). A passenger aboard a commercial flight from Sri Lanka to Kuala Lumpur in Malaysia was found to have a C. bezziana infestation (Pillai and Ramalingan, 1984). The accidental introduction of C. bezziana to Iraq in 1996 (Abdul Rassoul et al., 1996; Al-Izzi et al., 1999; Hall et al., 2001; Siddig et al., 2005) resulted in its rapid dispersal throughout the Tigris and Euphrates river systems with approximately 60,000 cases reported in livestock and humans within 2 years, with a mortality rate of 1 per cent in livestock (Spradbery and El-Dessouky, 1998).

Detailed descriptions of life stages and their morphology in C. bezziana and associated species are given in Spradbery (2002a) and geographical races of C. bezziana are described in Wardhana et al. (2012a).

Eggs: The egg is white, 1.25 mm long and 0.26 mm in diameter with a cylindrical shape, rounded at both ends with the anterior end more tapered. The eggs are laid in a mass of three or more layers thick, glued together in parallel, giving the appearance of a shingled roof.

Larvae: 1^st, 2^nd and 3^rd instar larvae are white to cream coloured 1.6 mm, 3.5-5.5 mm and 6.1-15.7 mm long, respectively, with characteristic bands of spines on most segments. The thorn-like spines are black with a single point. Prominent mouth hooks and heavily sclerotized peritreme surrounding the posterior spiracles with three slit-like openings.

Pupae: Colour changes from deep pink to dark brown as the larval skin changes into the puparium shell through sclerotization. Puparia are characterized by their barrel-shape and retaining many of the external morphological characters of the final stage larva such as spines and spiracles.

Adult fly: The adult fly is 8-12 mm in length and green to blue in colour.

In Zimbabwe (Rhodesia) Chrysomya bezziana is, with the exception of the tsetse fly, Glossina morsitans, the most important insect pest of cattle, horses, dogs and other domestic animals (Cuthbertson, 1933). When the tick control programme broke down during the 1973-1978 guerilla war in Rhodesia, more than 300,000 livestock were lost, the majority due to C. bezziana infestations (Norval, 1978). C. bezziana is considered a major obstacle to large-scale beef production in Malaysia (Basset and Kadir 1982). The economic impact of the 1996 Iraq incursion included an estimated FAO/AOAD budget at the time of US$8,555,000 to counter the invasion (Spradbery and El-Dessouky, 1998). The cost of a C. bezziana incursion resulting in the establishment of the pest in Australia was estimated as high as $AUD900 million per annum a decade ago (Kwabena Anaman in Spradbery, 2002b). The social impact of livestock production losses is matched by the misery of human infestations caused by C. bezziana, especially among children and the aged and infirm. The adverse impacts on wildlife would also be considerable.

Impact on Habitats

Generally low, if any, measureable adverse impacts on habitat.

Impact on Biodiversity

Because of its wide host range, Chrysomya bezziana is unlikely to have a major impact on biodiversity although, in the United States, the New World Screw-worm fly species killed 80 per cent of fawns of one species of deer, the white-tailed deer (Odocoileus virginianus texanus) in some years (Fuller, 1962). The introduction of livestock breeds naïve for screw-worm fly into endemic areas such as Antipodean sheep to India and Papua New Guinea can be catastrophic for the new arrivals (Basset and Kadir, 1982; Spradbery, unpublished observations).

• Invasive in its native range
• Proved invasive outside its native range
• Has a broad native range
• Is a habitat generalist
• Tolerant of shade
• Capable of securing and ingesting a wide range of food
• Highly mobile locally
• Benefits from human association (i.e. it is a human commensal)
• Fast growing
• Has high reproductive potential

• Host damage
• Negatively impacts agriculture
• Negatively impacts human health
• Negatively impacts animal health
• Negatively impacts livelihoods
• Transportation disruption

• Parasitism (incl. parasitoid)
• Pathogenic

• Highly likely to be transported internationally accidentally
• Difficult/costly to control