Myiasis

Myiasis is an infestation of the skin by developing larvae (maggots) of a variety of fly species (myia is Greek for fly) within the arthropod order Diptera. Worldwide, the most common flies that cause the human infestation are Dermatobia hominis (human botfly) and Cordylobia anthropophaga (tumbu fly).

Larva of Dermatobia hominis is shown below.

In cutaneous myiasis, the two main clinical types are wound myiasis and furuncular (follicular) myiasis. ^[1] Other forms include creeping/migratory myiasis and cavitary myiasis of body organs. ^[2] In nasopharyngeal myiasis, the nose, sinuses, and pharynx are involved. Ophthalmomyiasis affects the eyes, orbits, and periorbital tissue, and intestinal and urogenital myiasis involves invasion of the alimentary tract or urogenital system. ^[3]

A rare type of myiasis, hematophagous myiasis, is common in infants younger than 9 months, especially in those living in rural and endemic areas, and the furuncular lesions are usually on the face. ^[4]

The pathophysiology of the human infection differs depending on the type of fly and its mode of infestation.

Dermatobia hominis (human botfly) - Furuncular myiasis

This type is endemic to tropical southeast Mexico, South America, Central America, and Trinidad. The adult fly resembles a bumblebee (see image below); it is short lived and survives for little more than a week. It does not feed and is infrequently seen. The life cycle of the botfly is unique, as the female, egg-bearing fly catches a blood-sucking arthropod, usually a mosquito (although 40 other species of insects and ticks have been reported), midflight and attaches her eggs to its abdomen (means of transportation known as phoresy). When the mosquito takes a blood meal from a warm-blooded animal, the local heat induces the eggs to hatch and drop to the skin of the host and enter painlessly through the bite of the carrier or some other small trauma.

Once deposited in the skin, the larvae start out as small and fusiform and later become pyriform to ovoid as they reach full development at lengths of 15-20 mm. They are encircled by several rings of spines. Eventually, if the cycle is unperturbed, fully-developed larvae emerge from the host in 5-10 weeks and drop to the ground, where they pupate to form flies in 2-4 weeks. ^[5]

Cordylobia anthropophaga (tumbu fly) - Furuncular myiasis

This type is endemic to sub-Saharan Africa. The adult fly is about the size of a housefly but stockier. It prefers shade and is most active in the early morning and afternoon. It is attracted by the odor of urine and feces. The females lay their eggs on dry, sandy soil or on damp clothing hung out to dry. The eggs hatch in 1-3 days and can survive near the soil surface or on clothes for up to 15 days waiting for contact with a suitable host. Activated by heat, such as the body heat of the potential host, they are capable of penetrating the unbroken skin with sharp mandibles. ^[5] They become fusiform to ovoid and reach a length of 13-15 mm. Their larval stage is shorter than that of the human botfly and is completed in 9-14 days.

Hypoderma bovis/Gasterophilus intestinalis - Creeping/migratory myiasis

The adult fly of the Hypoderma genus is large and hairy and resembles a bumblebee. Normal hosts for the larvae of this fly are deer, cattle, and horses. Humans are abnormal hosts, in which the parasite is unable to complete its development. Human infections usually occur in rural areas where cattle and horses are raised. In animals, the fly attaches the eggs to the hairs. The larvae hatch, penetrate the skin, and wander extensively through the subcutaneous tissues, eventually locating under the skin of the back, where they produce the furuncular lesions. In humans, the larvae migrate rapidly (as much as 1 cm/h) and erratically through the subcutaneous tissues, producing intermittent, painful swelling over months. The larvae may emerge spontaneously from the furuncles or die within the tissues. In the rare case, the larvae are seen invading the orbit, pharyngeal region, and spinal canal.

The larvae of the Gasterophilus genus are usually gastrointestinal (Gasterophilus intestinalis) or nasal (Gasterophilus nasalis) parasites of horses. In humans, the young larvae burrow in the skin and wander intradermally, creating narrow, tortuous, erythematous, and linear lesions with intense pruritus. Lesions usually advance 1-30 cm/d. ^[6] Death of the larvae terminates the infection in 1-2 weeks without sequelae.

Cochliomyia hominivorax/Phaenicia sericata (blow fly) - Wound myiasis

The adult flies are rather stocky flies and metallic blue-green to purplish black in color. The larvae are pinkish, fusiform, and strongly segmented. Female flies deposit the eggs near poorly managed wounds and the larvae feed on necrotic tissue. ^[6] Flies may be dispersed by prevailing winds, and infection is often acquired while resting outside during the day or may result from trauma. ^{[4, 7]}

As mentioned above, the cause for myiasis is the infestation of humans with the larvae of the Diptera order of fly species. More than a hundred species of Diptera have been reported to cause human myiasis. Some of the most common are as follows:

• Dermatobia hominis (human botfly) causes furuncular myiasis.

• Cordylobia anthropophaga (tumbu fly) also causes furuncular myiasis.

• Cochliomyia hominivorax (America) and Chrysomyia bezziana (Africa, Australia, Asia) both cause wound myiasis.

• Hypoderma bovis (infested cattle) and Gasterophilus intestinalis (infested horses) both cause creeping (migratory) myiasis.

• Oestrus ovis (sheep botfly) causes ophthalmomyiasis.

Frequency

United States

Myiasis is uncommon in the United States, and any cases reported are usually imported cases of myiasis from travelers returning from tropical destinations. However, reported incidence rates are increasing among individuals from nonendemic countries who have traveled to tropical destinations or engage in outdoor activities. ^[8] A study in urban and suburban United States found an association of homelessness, alcoholism, and peripheral vascular disease with cutaneous myiasis; the most common fly identified in that study was Phaenicia sericata (green blowfly). ^[1]

International

Myiasis is a worldwide infestation with seasonal variation, the prevalence of which is related to the latitude and life cycle of the various species of flies. Its incidence is higher in the tropics and subtropics of Africa and the Americas. The flies responsible prefer a warm and humid environment and so are restricted to the summer months in the temperate zones, while living year-round in the tropics. ^[1]  Dermatobia hominis, also known as human or tropical botfly, is endemic to tropical Mexico, South America, Central America, and Trinidad, ^[7] while Cordylobia anthropophaga (tumbu fly) is endemic to sub-Saharan Africa.

Race

Myiasis is not prevalent in any particular race.

Sex

No sex predilection exists for myiasis.

Age

Myiasis may occur at any age.

Myiasis is a self-limited infestation with minimal morbidity in the vast majority of cases. ^[1] The major reasons for treatment are reduction of pain, cosmesis, and psychologic relief. Once the larva has emerged or has been removed, the lesions rapidly resolve. ^[3] However, larvae such as C hominivorax (cause of wound myiasis) can infest around orifices of the head and may burrow into brain tissue. ^[1]

Complications include infections such as cellulitis. ^[5]

Cases of neonatal fatal cerebral myiasis, caused by the penetration of larva through the fibrous portion of the fontanel, have been reported. ^[8]

Patients should be educated about preventive measures (see Prevention) to avoid exposure to or being bitten by Diptera flies.