Sydney University Department of Medical Entomology Westmead Hospital
Exotic Myiasis

Dermatobia
modified from: K.G.V. Smith (ed). 1973. Insects and Other Arthropods of Medical Importance. British Museum, London.

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Introduction

Myiasis is the invasion of living tissue or organs by the immature stage (maggots) of flies. Every year several species of exotic diptera are imported into Australia in the subdermal layers of the skin of overseas travellers. Patients returning from overseas present to their doctor with painful furuncular lesions, often unaware that the cause is a maggot.


Natural History

In Australia, the exotic species of fly isolated from patients with travel history include the human bot fly (Dermatobia hominis), the Tumbu fly (Cordylobia anthropophaga), the Lund's fly (Cordylobia rodhaini) and the New World screw worm fly (Cochliomyia hominivorax) (Table 1). Each species has an interesting life cycle, but the human bot fly D. hominis is especially unique. The eggs of D.hominis are couriered to the host by a blood sucking insect, such as a mosquito. After the young larvae penetrate the host's skin, they feed continuously for 5-12 weeks from a dermal pocket, causing severe pain and intense pruritus to the host. At the prepupal stage, they fall from the host to pupate

Click on image to enlarge!
Fig. 1 Larval Cordylobia rodhaini, note the scattered spines.
Bar = 1mm.

in the ground. In its native home of Central and South America, D.hominis regularly infests humans and a wide range of animal hosts, especially cattle. This fly's distribution extends from Mexico through Central and South America down to northern Argentina and Trinidad.

Both species of Cordylobia are found only within the African continent. The Tumbu fly is a common blowfly of tropical Africa, south of the Sahara Desert. The Lund's Fly is a blowfly rarely involved in human myiasis (Zumpt, 1965); it is found in rainforest areas of tropical Africa, from Senegal to Central Africa and south to Angola and Rhodesia, usually associated with rodents. Females of these two Cordylobia species deposit their eggs below the surface of sandy soil and occasionally on clothing that has been tainted with traces of faeces or urine. Any disturbance of the soil surface is met with an immediate response from the young larvae that wriggle to the surface in order to penetrate the skin of the host. The life cycle of the two Cordylobia species takes 10-12 days to develop to the prepupal stage, when it leaves the host to pupate. Within this time the patient will endure pain and intense itching from the developing lesion.

Click on image to enlarge!
Fig 2. Larval Dermatobia hominis, note the rows of spines. Bar = 1mm.

The New World screw worm (C.hominivorax), is a species of maggot that can burrow into the flesh of humans, wild and domestic animals. Where it was once more widespread in its distribution it is now generally limited to Central and South America (Kettle, 1995). Females of this fly species deposit their eggs on the edges of wounds, sores and healthy mucous membranes. After burrowing into the host’s tissue, the young larvae feed on living tissue for 4-8 days. This causes extreme pain and disfunction to the host from the festering wounds before the maggot matures to the prepupal stage when it leaves the host.


Myiatic flies - Diagnosis

Species of diptera

Country of origin

Incubation period

Appearance of mature maggot

Cochliomyia hominivorax

Central & South America

4-8 days

Typical maggot shape, 15-17mm long, bands of spines encircling anterior margin of each body segment.

Cordylobia anthropophaga

Central & Tropical Africa

10-12 days

Oval, 11-15mm, 3 curved slits in spiracles, numerous small black spines.

Cordylobia rodhaini
(Fig. 1)

Central & Tropical Africa

10-12 days

Up to 23mm long, scattered spines, 3 sinuous slits in each posterior spiracle.

Dermatobia hominis
(Fig. 2)

Central & South America

5-12 weeks

18-25mm long, pair of flower like anterior spiracles, spines in rows.


Clinical Presentation & Treatment

Infections with myiatic flies start out as itchy sores that then develop into painful boil-like lesions, which often ooze. Most of these cases of human myiasis are uneventful, but patients should be monitored for additional and subsequent lesions as the development of the maggots is not synchronous or isolated, and their growth phase may be prolonged. Treatment of this condition can be undertaken by forcing the maggots to the skin's surface by cutting off their air supply. An application of Vaseline or similar material will encourage the maggot to move towards the surface exposing more of the maggot's body that can then be extracted. A local anaesthetic and incision to extract the maggot is another common method of treatment.


Quarantine Considerations

Although imported cases of exotic myiasis are not notifiable in Australia, they demonstrate the ease at which these flies cross geological boundaries with air travel. As there are no obligatory fly parasites of humans in Australia, these cases of exotic myiasis are a constant reminder of the problems in other countries. Awareness of exotic myiasis by health professionals would assist quarantine, agriculture and other departments in monitoring the different species entering Australia. Establishment of any one of these pest species of fly would have disastrous effects on our health and agricultural industries and economy.


References

Kettle, D.S., 1995, Medical and Veterinary Entomology. CAB International, Wallingford, UK.

Zumpt, F., 1965, Myiasis in Man and Animals in the Old World. London, UK.
 

Confirmation and Enquiries

Identification of myiatic flies, and all other medically important insects, is performed through the Medical Entomology Department at ICPMR, Westmead Hospital.

See 'Contacts' for further information.

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