CHRYSOMYA BEZZIANA (OLD WORLD SCREWWORM FLY)

Morphology

Chrysomya bezziana is classified within the family Calliphoridae. The adults have dark metallic green bodies with abdominal segments with narrow bands along the posterior margins (Figure 1.1 A). The legs are black or partially brown and the face is orange-yellow. This morphology is quite similar to that of the secondary myiasis species, Chrysomya megacephala, but the body size of C. bezziana is relatively smaller than that of C. megacephala (DuPonte and Larish, 2003). In general, C. bezziana adults are 10 mm long with a 4.1 mm head width. There is no macroscopic characteristic to easily distinguish the fly by eye, so a microscope is needed to for identification (Sigit, 1978; Spradbery, 1991). The eyes of males are much closer together than those of females (Figure 1.1 B1; B2) (Spradbery, 1991). A microscope is needed for identification.

There are three larval stages, first (L1); second (L2) and third (L3) larval instar. Larvae have 12 segments: 1 head segment, 3 thoracic segments and 8 abdominal segments covered by broad encircling bands of spinules. In general, the body of the larva is of a typical muscoid shape that gradually tapers anteriorly. L1 often moves unnoticed because of its tiny size, around 1.65-3 mm in length. The L2 is quite similar to L3 but are smaller and shorter: 3.5-5.5 mm vs. up to 18 mm in length, and 0.5-0.75 mm vs. 1.1-3.6 mm in diameter, respectively. The anterior spiracle of L3 larvae is clear, and palmate in shape due to 4-5 papillae arranged in a single row. They are located at the dorso-posterior margin on each side of the prothorax. The L3 is pale pink in colour. The colour and the length become darker and shorter as the larvae pupate; the puparium (pupal case) surface is the hardened and darkened cuticle of the L3 larva (Spradbery, 1991; Sukontason et al., 2006).

Sukontason et al. (2006) showed that C. bezziana have some features that can be used to morphologically distinguish them from other blow fly species (Chrysomya megacephala, Chrysomya rufifacies, Chrysomya nigripes and Lucilia cuprina). Larvae of C. bezziana have a unique broad base and sharply pointed recurved spines on intersegmental spines bands between the pro- and mesothorax, an incomplete posterior spiracular peritreme, four to six papillae on the anterior spiracles, and strong and robust mouth hooks, which could explain their ability to penetrate deeply into live tissues. Those characters are diagnostic features of C. bezziana.

Figure 1.1 A. Morphology of adult C. bezziana; B. The head of C. bezziana; (B1) male, note that eyes are close together, (B2) female, note clear separation (pictures were produced using Leisca mycrosystems suite V3)

Unlike the larvae, pupae do not depend on the host for nutrition but are dependent on environmental factors for their development, such as humidity, temperature, and climate. Pupae are static (Spradbery et al., 1991). Based on scanning electron microscopy images, the caudal end of the posterior spiracular discs has three characteristic spiracular slits with peripheral posterior spiracular hairs and basal compressed buttons (Sukontason et al., 2006).  It was also revealed that the opening of each slit measured ca. 2 mm in width and some were closed with a membrane-like structure underneath. Sukarsih et al. (2000a) and Wardhana et al. (2004b) reported that normal pupal weight, when larvae were reared in laboratory conditions, was 39.7 mg on average, and the normal rate of emergence adults from one colony (27.5 mg) was 91.5-96.0 % (around 625 - 660 emerging flies).

Life Cylce

The life cycle of C. bezziana comprises 4 stages: egg, larva, pupae and adult. The entire larval stage from L1 to L3 at 36^oC needs 5-6 days, the pupal stage lasts 7-8 days, emerging adults produce eggs when aged 6-7 day old, and eggs hatch in less than one day (18-24 hours at 25^oC and 10 hours at 37^oC)  (Spradbery, 1991; Spradbery, 2002). At ambient temperature, the lifetime of C. bezziana can reach 24 days (Wardhana et al., 2003b) and some authors reported that C. bezziana is able to survive for 40 days at 28^oC (Spradbery, 2002), or even up to 51 days (Nielsen, 2003) (Figure 1.2).

Females become sexually mature and receptive to mating within the first few days after hatching. Females normally mate only once, but a male can inseminate several females during its lifetime. Females are able to lay several egg masses during their lifetime, although it is rare to find females in the field laying more than two masses (Spradbery, 1991).

Figure 1.2  Life cycle of C. bezziana

Gravid females are attracted to open wounds of domestic and wild animals and also humans. They lay their eggs in masses of up to 250 (on average 180 eggs) exclusively on the dry edges on the upper side of wounds; a more secure substrate than the moist lower edges. According to Spradbery (1979), oviposition by C. bezziana in the field occurs mainly in the late afternoon and continues until dusk and the eggs are cemented tightly together like a shingled roof. The egg mass is characteristically white and compact compared to secondary Chrysomya species (C. megacephala, C. rufifacies, C. albiceps and C. varipes), which lay yellowish eggs in loose masses which can be readily brushed off the host’s body (Spradbery, 1991).

The eggs will hatch in 12-24 hours at ambient temperature after deposition, or in 10 hours at 37^oC. The hatched L1 move forward to penetrate into the wound. They begin feeding superficially on the wound liquid, but fully penetrate the wound and develop into L2 within 24-48 hours. Larval development may take 5 – 8 days, but the majority of larvae evacuate the wound after 7 days of feeding. Wound departure occurs mainly during darkness with a peak between midnight and dawn, to protect them from ultraviolet light (Spradbery, 1991). Under laboratory conditions, it was demonstrated that the majority of larvae leaving the artificial media after 6 days of feeding are female whereas there is higher proportion of males on day 8 (Wardhana et al., 2003b). The phenomenon of females of C. bezziana emerging faster than males has been observed in other laboratory studies (Mahon and Leopold, 2002; Wardhana et al., 2003b).

After the larvae have vacated the wound, and fallen to the ground, they burrow 2-3 cm into soil and pupariate within 24 hours. Pupariation is the formation of a hard sclerotised pupal case from the larval cuticle by the tanning processes (Spradbery, 1991). Within the pupal period, the insect changes into an adult fly, which then emerges after 7 days at 28^oC but this may be considerably prolonged if the weather is cool (Spradbery, 1991; Siddig et al., 2005).

The majority of flies emerge just before dawn with little or no emergence during daylight hours when sunlight and diurnal predators could cause serious losses. Up to now, knowledge of the behaviour of the male screwworm fly is very lacking, probably because they are very reclusive and occur at a low population density compared to non-parasitic blow flies. Based on a study in America, researchers suggest that males form mating aggregations on bushes and trees and fly towards any small passing object. If it should be a female screwworm fly, the male catches the female and they fly to nearby vegetation where mating occurs (Spradbery, 1991).

Host

Chrysomya bezziana attacks healthy tissues of all warm-blooded animals and humans. Unlike other Chrysomya spp., that live in carrion or as scavengers, C. bezziana larvae can only survive and develop in live animal or human tissues (Spradbery, 2002; Neilsen, 2003).

 

Geographical distribution

The ability of C. bezziana adults to fly long distances (100 km) enables the fly to spread over wide areas in the absence of any barriers (Spradbery, 1994). Sutherst et al. (1989) and Spradbery (1991) reported that C. bezziana is found throughout much of tropical and subtropical Africa, the Indian subcontinent and South-East Asia from southern China in the north to New Guinea in the south (Figure 1.3). Traumatic myiasis caused by C. bezziana has been reported in the Middle East region such as Bahrain, Qatar, Baghdad, Saudi Arabia, Fujairah and Muscat, including Yemen where myiasis has been reported in an outbreak situation since late 2007 with populations peaking in 2008 (Rajapaksa and Spradbery, 1989; Sutherst et al., 1989; Al-Izzi, 2002). In Iraq, C. bezziana has attacked livestock in almost two thirds of the country, (IAEA, 1998). Compared to C. bezziana, C. megacephala has a larger distribution, throughout the Oriental, Australian and Oceanian regions, and the species has been introduced into Africa, South America and Central America (Kurahashi and Magpayo, 2000; Tomberlin et al., 2001) and, most recently, in Europe (Castro and Garcia, 2009) (Figure 1.4).

Even though C.  bezziana has never established in Australia, the habitat conditions over much of tropical and subtropical Australia are favourable for the fly, which is endemic in the neighbouring countries of Indonesia, and Papua New Guinea (Sutherst et al., 1989; Lee, 2002; Wardhaugh and Mahon, 2002). The fear of C. bezziana incursion has been justified by the fact that, in 1988, several adult OWS flies were trapped in an empty livestock vessel in Darwin harbour, which had just returned from delivering cattle to Brunei (Rajapaksa and Spradbery, 1989). Another observation conducted in April 1998 found nine live C. bezziana adults in an electrocution trap on a vessel, Lady Geraldin, after berthing in Darwin. These events caused renewed interest in the possibility that screwworm may be introduced in Australia via returning livestock vessels (Thomson, 1992).

Based on a climate matching model (CLIMEX analysis), Sutherst et al. (1989) deduced that C. bezziana is most successful in hot, wet conditions and that it is sensitive to prolonged cold or dryness. The parameter values reflect the highly favourable conditions in areas further north of the Transvaal and Zimbabwe (Norval et al., 1988). This hypothesis is supported by Al-Izzi et al. (1999) who states that C. bezziana cannot be found in Lebanon, Syria and Jordan due to extreme weather conditions that are unfavourable for insect survival and development. The winter in Lebanon is very cold with a minimum temperature of <0^oC during November to February in the Albiqa’a area, <10^oC from November to April in the Ash-Shemal area, and <10^oC January to February in Beirut.  In contrast, the temperature rapidly rises to >30^oC from June to September in Ash-Sehmal, from April to October in Albiqa’a and from March to September in Beirut.

Factors affecting C. bezziana distribution in Indonesia

There are several factors affecting why myiasis cases still remain a major problem in Indonesia e.g. the country is an archipelago with a tropical climate and many rainforests. In terms of C. bezziana distribution, Ready et al. (2009) investigated a genetic diversity within the species within a small geographic area (Sulawesi and East Sumba), indicating that they could be further genetic diversity throughout the rest of the archipelago.

Climate and weather in Indonesia are characterised by two seasons, dry and wet. In addition, Indonesia has many rainforests and large plantation regions. Generally, the monsoon-type of climate of Indonesia causes a change in season approximately every six months; even though its pattern has been now somewhat disrupted as part of global warming. The dry season is from June to September and the rainy season from December to March. Intervening periods are transition months in which the weather will be mixed. Average temperature is 23-26^oC with the average relative humidity between 70-90% RH (DPII, 2003). This atmosphere is very suitable for C. bezziana to develop, thus myiasis has become established as an endemic disease (Partoutomo, 2000, Sutherst et al., 1989, Wardhana, 2006).