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.

Often, a history of traveling to a tropical country or existence of a previous wound is noted. In one study, the average time from exposure to diagnosis was 1.5 months.[9]

Patients complain of boil-like lesions usually on exposed areas of the body, like the scalp, face, forearms, and legs.

Lesions can be painful, pruritic, and tender, and patients often have the sense of something moving under the skin. Sometimes, patients also complain of fever or swollen glands.

In the cases of ophthalmomyiasis, patients complain of severe eye irritation, redness, foreign body sensation, pain, lacrimation, and swelling of the eyelids.[10]

In the cases of nasal myiasis, patients present with epistaxis, foul smell, passage of worms, facial pain, nasal obstruction, nasal discharge, headache, dysphagia, and sensation of foreign body in the nose.[11, 12]

Furuncular myiasis

This type of myiasis , caused by both the human botfly and the tumbu fly, causes nonhealing boil-like lesions (see images below). Whereas myiasis from the tumbu fly occurs on the trunk, thigh, and buttocks, botfly lesions are on the exposed areas of the body, including the scalp, face, forearms, and legs. A pruritic erythematous papule develops within 24 hours of penetration, enlarging to 1-3 cm in diameter and almost 1 cm in height. These lesions can be painful and tender. Each has a central punctum (see images below) from which serosanguineous fluid may be discharged. Lesions may become purulent and crusted; the movement of the larva may be noticed by the patient.[1] The tip of the larva may protrude from the central opening (punctum), or bubbles produced by its respiration may be seen.[4] The inflammatory reaction around the lesions may be accompanied by lymphangitis and regional lymphadenopathy.[3]

Wound myiasis

In wound myiasis , the larvae are deposited in a suppurating wound or on decomposing flesh. The diagnosis is obvious when larvae are visible on the surface within or around the wound and more difficult when they have burrowed beneath the surface.[1]

Creeping cutaneous myiasis

Creeping (or migratory) cutaneous myiasis may be caused when there is exposure to infested cattle or in those who work with horses. This form of myiasis resembles cutaneous larva migrans, with an apparent tortuous, thread-like red line that ends in a terminal vesicle marking the passage of the larva through the skin. The larva lies ahead of the vesicle in apparently normal skin.[3]

Ophthalmomyiasis externa

Ophthalmomyiasis externa, caused by Oestrus ovis, is characterized by conjunctivitis, lid edema, and superficial punctate keratopathy in response to movement of larvae across the external surface of the globe. Larvae may appear within the cornea, lens, anterior chamber, or vitreous body but rarely undergo development once the globe has been entered.[7]

Posttraumatic myiasis

Patients who are victims of facial trauma or extensive scalp injury can develop extensive intracranial maggot infestation causing meningitis and encephalitis if not properly managed within a reasonable period of time.[13]

Nasal myiasis

In nasal myiasis, examination of the nose (rhinoscopy) reveals an edematous, ulcerated mucous membrane filled with necrotic material and crawling maggots. Patients may have septal perforation or palatal perforation or both. Erosion of the bridge of the nose and adjacent area of the face can also be seen as well as orbital cellulitis and diffuse cellulitis of the face. In a smaller number of patients, examination reveals extensive ulceration of the tonsils and the posterior pharyngeal wall due to maggots.[12]

Diagnosis is typically made by identification of fly larvae or maggots. The exact type of species can be difficult to determine from examination.[5] However, CBC count may show leukocytosis and eosinophilia.

MRI has been used in a number of cases of cerebral myiasis; breast myiasis; and facial, orbital, and furuncular myiasis. Ultrasonography can also be very useful in establishing the diagnosis and in determining the size of the larvae. CT scan has also been suggested.[15]

In one study, Doppler ultrasonography (DUSG) was performed using a high-resolution (10-MHz) soft-tissue transducer. Using the standard mode, hypoechogenic masses underneath the skin were first located, and then DUSG was used, which demonstrated circulation of fluid within the parasite as well as the number of parasites, their size, and their situation within the lesion. This is especially useful when lesions are still small and look like insect bites. Secretion and pain are minimal or absent, and the punctum is almost always absent. In these cases, high-resolution DUSG proved to be 100% effective in diagnosis.[16]

Biopsies are not necessary, but if performed, histopathologic findings include an ulcerated epidermis with an inflammatory infiltrate of neutrophils, lymphocytes, giant cells, mast cells, plasma cells, and eosinophils that occur in stages. The larvae can be seen in cross-section.[1, 16]

Occlusion/suffocation approaches

This noninvasive approach includes placing petroleum jelly, liquid paraffin, beeswax or heavy oil, or bacon strips over the central punctum and has been used to coax the larvae to emerge spontaneously head-first over the course of several hours, at which time, tweezers (or forceps) aid in the capture. Enlarging the punctum beforehand may make it easier to remove the larvae later as it emerges and is usually needed for botflies.[17] Anecdotal evidence has shown the larvae to emerge within 3-24 hours after application of suffocating material.[18, 19]

These approaches take advantage of the larva's oxygen requirements, encouraging it to exit on its own. However, the covering should not be restrictive (eg, nail polish) because this may asphyxiate the larva without causing it to migrate out of the skin. If the larva does asphyxiate, then surgical removal is necessary.

Surgical removal with local anesthesia

The skin lesion is locally anesthetized with lidocaine and excised surgically followed by primary wound closure. Alternatively, lidocaine can be injected forcibly into the base of the lesion in an attempt to create enough fluid pressure to extrude the larvae out of the punctum.[20] The larvae are anchored deeply to the subcutaneous by anterior hooklets and it is important to remove all parts from the site to prevent a foreign body reaction.[2] Another surgical approach would be to perform a 4- to 5-mm punch excision of the overlying punctum and surrounding skin to gain better access to and visibility of the larva. The larva can then be removed carefully using toothed forceps (experience of B.B. Wilson, MD).

The larva should not be forcibly removed through the central punctum because its tapered shape with rows of spines and hooks prevents simple extrusion.

Furthermore, care should be taken to avoid lacerating the larva because retained larval parts may precipitate foreign body reaction. After removal of the larvae, antiseptic dressings, thorough cleansing, and debridement are indicated, as well as antibiotics if secondary infection is present.

Systemic/topical ivermectin

An alternative treatment for all types of myiasis is oral ivermectin or topical ivermectin (1% solution), proven especially helpful with oral and orbital myiasis.

Wound myiasis

Wound myiasis requires debridement with irrigation to eliminate the larvae from the wound or surgical removal. Application of chloroform, chloroform in light vegetable oil, or ether, with removal of the larvae under local anesthesia, has been advocated for wound myiasis.[1, 20]

Surgical removal is not required unless requested by the patient, as the larvae are naturally sloughed within 5-7 weeks.

Depending on the location of the larval infestation, dermatologists (wound and furuncular myiasis), ophthalmologists (ophthalmomyiasis), or otorhinolaryngologists (oral, facial, nasal myiasis) may need to be consulted.

Care must be taken to extract the larva whole, otherwise a considerable foreign body reaction may ensue. Also, in the case of secondary pyogenic infection, appropriate antibiotics should be administered.

Myiasis can be a portal of entry for Clostridium tetani; therefore, vaccination should be considered in affected individuals.[1]

Individuals traveling to rural endemic areas should be covered at all times with long-sleeved shirts, pants, and hats. At night, sleeping on raised beds, in screened rooms, or under a mosquito net is appropriate. Insect repellents are also recommended. Clothing should be hot-ironed and dried appropriately to remove any residual eggs in areas endemic to tumbu flies.[2]

To prevent wound myiasis, simple antisepsis is usually adequate. Wounds should be cleaned and irrigated intermittently, and proper dressings should be applied. Patients with any type of wound should not be permitted to sleep outside and, if in an indoor or hospital environment, the windows should never be opened, unless properly screened.[20]

Although the common approach for either furuncular or wound myiasis is occlusion/suffocation techniques that have been mentioned above or surgical debridement and irrigation, oral ivermectin has been proven especially helpful with oral, orbital, and nasal involvement.[1] Ivermectin has decreased the associated inflammation and the destructive process prior to debridement.[21] Thus, it has been suggested that oral ivermectin should be considered as an option for treatment of human cavitary myiasis.[22] A case has been reported of facial furuncular myiasis in a HIV-infected patient who was treated with applications of topical solution that killed the larvae and facilitated their extraction.[23]

Class Summary

Neoadjuvant ivermectin therapy prior to surgical debridement has been recommended to prevent enucleation in patients with massive orbital involvement or to avoid the difficulties associated with mechanical removal of the larvae.

Ivermectin (Mectizan, Stromectol)

Ivermectin binds selectively with glutamate-gated chloride ion channels in invertebrate nerve and muscle cells, causing cell death. Its half-life is 16 hours; it is metabolized in the liver.