Classification: Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology, 2001, Springer-Verlag)

Metazoa (Animalia) (multicellular eukaryotes, animals)
Arthropoda (arthropods, segmented body, exoskeleton, jointed appendages)
Uniramia (with antennae, first mouthparts mandibles)
Insecta (insects, 3 body parts, 6 legs, many with wings)
Diptera (flies, mosquitoes, winged, complete metamorphosis)
Brachycera (short antennae, reduced wing venation, incomplete larval head capsule)

Family: Calliphagidae (Calliphoridae)
Diptera are the true flies, a large insect order with over 120,000 described species. They have three body parts (head, thorax and abdomen), six legs but only one pair of wings (the hind pair having been reduced to halteres). All species undergo complete metamorphosis whereby larval stages pupate to transform into adults. Dipterous flies can be parasites as larvae or adults, rarely both. Blow flies are important parasites of domestic animals as their larvae (maggots) cause fly strike (myiasis) in sheep (breech and body strike). Blow flies are categorized as causing primary, secondary or tertiary strike according to whether they initiate strike or occur later. Most primary strike flies belong to the subfamily Calliphorinae, including Lucilia cuprina (green bottle fly), Lucilia sericata (European green bottle fly), Calliphora augur (lesser brown blowfly) and Calliphora stygia (large brown blowfly), while many secondary strike flies belong to the subfamily Chrysomyinae, including Chrysomya rufifacies (secondary green blowfly or hairy maggot blowfly) and Chrysomya micropogon (mallochi) (steel blue blowfly).

Lucilia cuprina [this species causes myiasis (flystrike) in sheep]

Parasite morphology: Blow flies form four developmental stages: eggs, larvae, pupae and adults. Eggs appear as creamy-yellow ovoid bodies (~ 1mm long) deposited in clusters. Larvae (maggots, grubs or gents) have elongate opaque segmented bodies (8-14mm long) with a pair of oral hooks at the anterior end, and peritremes with spiracles at the posterior end. Pupae appear as ellipsoidal creamy-brown cocoons (10-14mm long) with barely visible transverse striations. Adult flies measure up to 10mm in length and they have a characteristic metallic bronze to greenish sheen. They have one pair of wings with a bare stem vein and have three pairs of dorsocentral thoracic bristles. Male and female flies are similar in appearance except the eyes of males are almost touching while those of females are clearly separated.

Host range: Lucilia spp. are facultative ectoparasites mainly on sheep, but other domestic and wild animals may be affected. They are considered to have a cosmopolitan distribution due to stock translocations and other anthropogenic activities, but L. cuprina if found more frequently in warm-temperate and subtropical areas while L. sericata is more common in cool-temperate regions.

Site of infection: Eggs are laid in batches of 50-250 on carrion or on moist wounds or the soiled wet fleece of live animals, causing body, breech or tail strike depending on where emergent larvae invade cutaneous tissues. Body strike is found commonly around the shoulders and along the back, and has been associated with bacterial dermatophilosis and pseudomonal fleece rot. In Merino sheep, breech and tail strike commonly occur due to the excessive wrinkled skin of the hindquarters that becomes fouled with faeces and urine.

Pathogenesis: Some dipterous flies are obligate ectoparasites and can only complete their development using a living host. The majority, however, are facultative ectoparasites and can develop in both living and dead organic material. They comprise primary facultative species, which can initiate myiasis but also live as saprophages in decaying organic matter or carcases, and secondary facultative species, which live normally as saprophages and cannot initiate myiasis but can secondarily invade pre-existing infestations. Larvae invade necrotic or living tissue causing traumatic cutaneous myiasis. Three larval instars develop over 4-6 days and they cause great suffering by feeding directly on host tissues. Maggots secrete proteolytic enzymes and tear at the tissues with strong oral hooks resulting in extensive invasive lesions. Light infestations may be well tolerated by sheep whereas heavily affected sheep become anorexic, dull and depressed. Fleece in the affected area becomes damper, darker and develops a foul odour, which appears to act as a chemo-attractant for subsequent infestations.

Mode of transmission: Adult flies are free-flying near animals and females require protein meal for eggs to mature. Females may lay 2-3 batches of eggs during their 2-3 week life. Eggs hatch on sheep in 8 hours to 3 days depending on temperature and humidity (desiccation is the main cause of egg mortality). Three larval instars develop in the tissues and the third larval instars drop from the host, burrow into soil and form pupae. Pupation may be completed within a week or delayed depending on environmental conditions. Overwintering may lead to synchronized emergence and fly swarms in spring.

Differential diagnosis: Infestations are diagnosed on the basis of clinical signs and the visual detection of larvae in lesions. The larvae of many fly species can be differentiated on the basis of their size, shape and appearance as well as the arrangement of their posterior spiracles. The peritreme (outer perimeter) of the spiracles is entire in primary strike flies and broken in secondary strike flies. It is thin in Lucilia spp. and thick in Calliphora spp. The spiracles of blowfly larvae have straight slits compared to sinuous slits in house fly larvae. First stage larvae have one slit, second stage larvae have 2 slits and third stage larvae have 3 slits.

Treatment and control: Sheep should be regularly inspected during fly season, and infested livestock identified and isolated. The skin surrounding the lesion should be clipped, the wound cleaned (taking care to kill emergent larvae as they may pupate producing another generation of flies) and then dressed by topical application of dilute insecticides, such as diazinon, cypermethrin or deltamethrin. Sheep can also be treated prophylactically to prevent infestations using organophosphorous and pyrethroid insecticides which persist in the fleece for up to 10 weeks. These formulations may be applied by whole body plunge dipping, jetting or shower sprays. Some insect growth regulators, such as cyromazine and dicyclanil, have been shown to give good protection as single pour-on applications but only when used in timely fashion before anticipated challenge. Certain breeds of sheep are more susceptible to strike because the wool and skin are too easily wetted with urine or remain wet after rain. Various management procedures have been developed to prevent strike, such as crutching (shearing hindquarters), tail docking and mulesing (removing hindquarter skin folds). Infections by gastrointestinal helminths should also be controlled to prevent diarrhoeal wetting of perineal wool. Any carcases in the field should be removed and destroyed to remove alternative breeding places for blow flies.