Are You Confident of the Diagnosis?

  • What you should be alert for in the history

Myiasis (from the Greek for fly, ‘myia’) is the infestation of human or animal tissue by the larvae (maggots) of Diptera two-winged flies. Common Diptera families involved in myiasis include Calliphoridae, Oestridae, and Sarcophagidae. Larvae (otherwise known as bots or maggots) can be obligatory (i.e., lifecycle requires living tissue), facultive (i.e., larvae can use living tissue, carrion, and vegetable matter for sustenance), and accidental (e.g., inadvertent ingestion) parasites.

Various forms of myiasis exist. These include:

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  • Cutaneous myiasis (the focus of this chapter).

  • Nasal myiasis (invasion of any portion of the upper respiratory tract).

  • Ophthalmomyiasis externa (invasion of the cornea, anterior chamber, vitreous body, or lens).

  • Enteric (i.e., intestinal).

  • Urogenital.

  • Characteristic findings on physical examination

Three clinical variants of cutaneous myiasis are:

1. Furuncular

Variable-size inflamed furuncle (otherwise known as “warble”) with a central punctum for larva respiration. The end of the larva may be visible in the center. Patients often note a prickly burrowing (moving sensation) and painful sensation as the larvae penetrate the skin. Serosanguinous or purulent discharge from the punctum is not uncommon.

2. Migratory (creeping)

Mobile inflammatory epidermal or subcutaneous papules, nodules, or tracts. Patients often note moderate to extreme pruritus or a burning sensation. Larvae usually die within subcutaneous tissue or exit via an opening in the skin prior to pupation.

3. Wounds (traumatic) myiasis

Occurs in open wounds or decaying tissue. Flies are attracted to the odor of wounds and chronic mucocutaneous infections (e.g., sinusitis), and lay hundreds of eggs in the area. Risk factors include chronic draining wounds, chronic infections, ulcerated cutaneous malignancies, malnutrition, and physical and mental debilitation.

The clinical variant is dependent on the species of fly (discussed in detail under ‘Pathophysiology’).

Myiasis occurs in tropical and subtropical climates, or summer months in temperate climates. Eggs and/or larvae from these flies are transmitted to humans either via direct environmental contact (e.g., water and soil) or insect vectors (e.g., adult female fly, mosquitoes, ticks, etc.). Larvae can penetrate intact skin, open wounds, and external body orifices. Travel history to endemic regions is important to obtain. Fly species vary around the world.

  • Expected results of diagnostic studies

Laboratory investigations are not necessary. Complete blood count may demonstrate elevated eosinophils and immunoglobulin (Ig) E titers, as well as an elevated white blood cell count if there is secondary infection. Bacterial cultures from the wound site can be used to guide systemic antimicrobial therapy.

Histopathology demonstrates a mixed cellular infiltrate with abundant eosinophils and a larva within a tract surrounded by dermal fibrosis. Morphological analysis of the larvae is helpful in determining the species of fly (Figure 1).

Figure 1.

Dermatobia hominis larva extraction (Courtesy of Benjamin Barankin, MD, Toronto, Ontario, Canada)

Localized ultrasound of furuncles or tracts may also demonstrate larvae.

  • Diagnosis confirmation

Differential diagnosis for furuncular myiasis includes:

1. Epidermal cyst

A common benign cutaneous cyst containing keratin or calcification that usually originates from follicular infundibulum. Most are noninflamed smooth nodules with a tiny central pore. These lesions can become inflamed and secondarily infected.

2. Abscess

A cavity containing pus (i.e., dead neutrophils) from either a primary infectious or inflammatory cause. It is usually associated with surrounding erythema and edema. Furuncular myiasis may evolve into an abscess following secondary bacterial infection.

3. Cutaneous leishmaniasis

A single-cell parasitic skin infection transmitted by the sand fly. Patients present with red papules and plaques that can occur spontaneously, with atrophic scarring. Mucocutaneous leishmaniasis can lead to significant tissue destruction and disfigurement. High-risk areas include Africa, India, and Sudan.

The differential diagnosis for migratory (creeping) myiasis includes:

1. Cutaneous larva migrans

A self-limiting nematode infection from hookworms (e.g., Ancylostoma, Uncinaria) excreted in fecal matter by hosts (usually infected dogs and cats, less commonly humans). The larval form is able to penetrate the epidermis, where the infection is usually confined. It is common in travelers who walk barefoot on beaches in the southeastern United States, Mexico, Caribbean, Central and South America, Southeast Asia, Africa, and other tropical countries.

2. Larva currens (Strongyloidiasis)

A self-limiting nematode infection caused by Strongyloides stercoralis. The skin lesions are pruritic when they appear, and disappear within hours. Gastrointestinal and pulmonary symptoms, and peripheral eosinophilia, are common.

3. Tinea corporis

Ask about a history of contact with animals or affected individuals. Look for annular (round) lesions with fine scaling. A potassium hydroxide test can be performed to visualize microscopic fungal elements or fungal cultures may be performed.

4. Scabies (Sarcoptes scabiei parasite)

Ask about a history of close contact with someone who has had a similar acute onset of pruritus. Look for burrows, particularly in the finger and toe webspaces. Pruritic papules are often located within the intertriginous zones and on the scrotum in men.

5. Contact dermatitis

Ask about a history of potential contact allergens such as new cleansing or cosmetic products, sunscreens, clothing, or shoes. Contact dermatitis is usually localized unless the patient develops a hypersensitivity response (Id reaction). It almost never has a serpiginous appearance.

6. Erythema chronicum migrans (associated with Lyme disease)

A pathognomonic sign for infection with Borrelia burgdorferi, which can appear immediately or weeks to months after infection. Look for an asymptomatic enlarging annular (approximately 5cm) erythematous plaque with central erythema or clearing. Borrelia serology should be obtained, and if positive, the patient should be treated with appropriate systemic antibiotics.

Who is at Risk for Developing this Disease?

Myiasis is considered the fourth most common infestation encountered by travelers. Cutaneous myiasis is the most common of all forms of myiasis.

Risk factors include travel to or inhabitation of endemic areas, particularly in persons who spend significant amounts of time outdoors or those with have prolonged contact with the ground (e.g., sleeping, kneeling, cultural rituals, etc.,) or contact with infected host animals (e.g., livestock, wildlife, and domestic animals), compromised skin barrier (e.g., open wounds), poor hygiene, diabetes, and physical or mental debilitation.

What is the Cause of the Disease?

  • Etiology

  • Pathophysiology

The pathophysiology varies with the species involved.

1. Furuncular

Appear as boils with central punctum for larvae respiration. Examples are Dermatobia hominis (the human botfly, Oestridae family, endemic to Central and South America, the most common cause of myiasis in North America), Cordylobia anthropophaga (the tumbu fly, Calliphoridae or blowfly family, endemic to sub-Saharan Africa), Cuterebra species (the rabbit botfly, common in the United States), and Wohlfahrtia vigil (Sarcophagidae family, common in the northern United States). These larvae have spines on the body to prevent accidental removal from the host.

Dermatobia hominis female adults are blue-gray colored and measure 1.5cm long. Eggs are attached to the abdomen of mosquitoes, smaller flies, and ticks (a process known as phoresis). Larvae hatch upon contact with warm-blooded hosts and enter via hair follicles or the carrier insect bite within 10 minutes. Dermatobia myiasis most often affects the head (e.g., scalp, face) and exposed extremities.

Cordylobia anthropophaga female adults are yellow-brown colored and measure 7-12 mm. Eggs are deposited in sand and soil or moist clothing, where they can survive for over 1 week. Flies are attracted by the odor of urine or feces. Larvae hatch almost immediately upon contact with a warm human host and burrow into intact skin within seconds to minutes. People in endemic areas iron clothing or blankets to destroy eggs and avoid sleeping on the ground. C. anthropophaga myiasis most often affects clothed areas such as the buttock, trunk, and arms.

Cuterebra myiasis usually affects rabbits (natural hosts), rodents, and humans (mostly in the United States during the fall months). Eggs are deposited near rabbit and rodent habitats.

In animals, the eggs hatch upon contact with the host, and larvae enter an orifice or penetrate the skin. If the eggs hatch near the rabbit’s mouth or nose, the larvae enter the upper respiratory tract, followed by the pleural cavity, diaphragm, and abdominal cavity. Finally, they migrate to the dermis. Human infection most likely results from percutaneous larval penetration following contact with eggs in grass and bushes. The lifecycle of this species in humans is uncertain. Cuterebra myiasis more often affects children on the head, neck, and chest.

Wohlfahrtia vigil female adults are gray-colored, with a distinctive pattern of three rows of black dots on the abdomen. Humans and mammals are hosts for this species, and the infestation is similar to Dermatobia hominis and Cordylobia anthropophaga, presenting as dozens of furuncles (plaque-like appearance). Manual expression is somewhat easier with this species, since the spines are much smaller. Young children are more often affected than adults.

2. Migratory (creeping)

Larvae form subcutaneous inflammatory tracts or tunnels, and either exit the skin via a furuncle (or abscess) or die in subcutaneous tissue. Examples are Gasterophilus intestinalis (the horse botfly, Oestridae family) and Hypoderma bovis (the cattle grub, Oestridae family). They are present worldwide.

Gastrophilus species are common in horses and humans who have contact with horses. Female flies are yellow-brown-colored and measure up to 17 mm. Eggs are deposited on horsehair. Larvae burrow into the epidermis, creating pruritic serpiginous inflammatory tracts for months until the larvae die or exit the skin through a perforation in the skin barrier.

These larvae are typically larger and slower moving than cutaneous nematode infestations. Certain Gasterophilus species (e.g., G. intestinalis, G. nigricornis) can enter the circulation by surface migration or ingestion. Visceral invasion and death can occur from vital organ involvement.

Hypoderma bovis and Hypoderma lineatum (the cattle grub) affect cattle and humans who have contact with cattle in temperate climates. Female flies resemble bees with yellow-black-white bands. Eggs are deposited on cattle hair. In cattle, larvae enter the subcutaneous tissues via hair follicles or the buccal mucosa. In humans, larvae enter only via the latter.

Over several months, larvae travel through connective tissue and migrate to epidural fat (H. bovis), or the esophagus (H. lineatum), and finally move toward the skin surface, before exiting the host via a perforation in the skin barrier. Myalgias, arthralgias, and parathesias are not uncommon. Visceral and central nervous system involvement is associated with high morbidity and even death. This intensely pruritic and painful infestation can last for several months.

3. Wound (or decaying tissue) infestation

Larvae inhabit the surface of open wounds or subcutaneous tissue. Examples are Cochliomyia hominivorax (New World screwworm in the United States and South America, Calliphoridae family), Chrysomyia bezziana (Old World screwworm in sub-Saharan Africa, Asia, India, and Australia; Calliphoridae family) and Wohlfahrtia magnifica (sarcophagid fly in northern Africa, Europe, Middle East, and Russia; Sarcophagidae family).

Screwworm larvae are the most common cause of wound myiasis. Cordylobia flies (as discussed previously) can also cause wound myiasis.

Cochliomyia and Chrysomyia screwworms cause human infestation via contact with eggs or larvae of Cochliomyia (on domestic animals) or Chrysomyia (on sheep). Female flies are attracted by the odor of wounds or mucous membranes (e.g. chronic sinusitis), where they lay hundreds of eggs that hatch in less than 24 hours. W. magnifica is a sarcophagid fly that infests humans and mammals.

The W. magnifica fly deposits larvae in their first stage of development. Larvae from screwworms, as well as sarcophagid flies, can burrow deep into tissue (i.e., centimeters), invade deeply into tissue down to and including cartilage and bone, and may enter vital structures, causing death.

Larvae from the Calliphoridae (blowfly) family are used for wound debridement (i.e., maggot debridement therapy), since they only ingest necrotic dead tissue.

Systemic Implications and Complications

1. Secondary infection

2. Invasion of normal tissue or vital structures

3. Cosmetic disfigurement

4. Death (uncommon, but can occur with wound myiasis)

Cutaneous myiasis can be complicated by secondary bacterial superinfection and tetanus. Wound and blood cultures may be required if the patient demonstrates any infectious symptoms (e.g., fever, sweats, rigors, progression of localized pain, and erythema, etc.).

Invasive wound or mucocutaneous myiasis can cause significant tissue destruction, cosmetic disfigurement, and involve vital structures. Cerebral myiasis can be fatal (has been reported in infants with infestation of the fontanelles). Detailed clinical exam with appropriate radiographic investigations (e.g., ultrasonography) may be indicated.

Treatment Options

Treatment options are summarized in Table I.

Table I.
Medical Management Surgical Management Other Modalities Prevention
Topical or systemic antibiotic for prophylaxis of active treatment of secondary skin infection (e.g., cephalexin 500mg QID for 7-10 days). Injection of lidocaine hydrochloride anesthetic to paralyze and/or flush out larvae. Bacon strips to lure the larvae to the surface. Diethyltoluamide (DEET)-containing insect repellents.
Topical ivermectin 10% in propylene glycol for 2 hours to kill larvae. Manual expression in early larval stage. Occlusion with mineral oils, animal fat, petroleum jelly, glue, nail polish, to suffocate the larvae. Cover skin with protective clothing.
Ivermectin 200 microgram/kg (single or multiple doses). Surgical removal in latter larval stages (larvae develop anchoring spines). Surveillance (not recommended) since larvae usually leave host in 5-10 weeks for pupation. Window screens and mosquito netting at night.
Tetanus booster Irrigation and debridement under local or general anesthetic for wound myiasis. Sleeping above ground and other behaviors that prevent direct contact with the ground.
Ironing clothes to kill eggs (C. anthropophaga).
Good hygiene.
Wearing footwear.

Optimal Therapeutic Approach for this Disease

Removal of the larvae and irrigation of the wound is curative and the recommended therapy.

It is important to prevent secondary infection, but if it is present, antimicrobial therapy is required. Cephalexin 500 mg orally, twice daily for 7 days is an excellent first choice. Bacterial cultures should be checked to look for resistant organisms.

Occlusive treatments are generally not considered highly effective. Dead larvae can cause an inflammatory or granulomatous response in tissue.

Surveillance is not recommended, as it can increase the risk of secondary infestation and adversely affect psychological well-being.

Many larval species contain backward-orientated spines to resist extraction. Incomplete removal of the larvae or anchoring spines often leads to an inflammatory response and granuloma formation.

Wound debridement may be necessary, particularly for the hundreds to thousands of larvae that can be present in wound myiasis.

Prevention is important for travelers or inhabitants of endemic regions.

Patient Management

1. Ensure the patient does not develop a secondary infection. Prophylaxis with oral antibiotics is prudent. Patients should be instructed to return immediately if they develop infectious symptomatology and have not improved with antimicrobial therapy.

2. Appropriate surgical follow-up should be arranged, particularly for wound myiasis.

3. Psychological impact of cutaneous myiasis should be considered.

Unusual Clinical Scenarios to Consider in Patient Management

Myiasis in debilitated patients with large open wounds or those created by incurable cutaneous malignancies carries a poor prognosis.

Travelers who frequent endemic regions for myiasis should be educated about preventative measures.

Ophthalmomyiasis should be treated by an ophthalmologist promptly. An immediate consult with an ophthalmologist is required.

What is the Evidence?

Caissie, R, Beaulieu, F, Giroux, M, Berthod, F, Landry, PE. “Cutaneous myiasis: diagnosis, treatment, and prevention”. J Oral Maxillofac Surg. vol. 66. 2008. pp. 560-8. (Contains a comprehensive table of cases and responsible fly species of myiasis in Canada.)

McGraw, TA, Turiansky, GW. “Cutaneous myiasis”. J Am Acad Dermatol. vol. 58. 2008. pp. 907-26. (An excellent and detailed review of the most common flies responsible for cutaneous myiasis and management strategies.)

Patel, S, Sethi, A. “Imported tropical diseases”. Dermatol Ther. vol. 22. 2009. pp. 538-49. (A good review of common tropical skin infections, including cutaneous larva migrans, myiasis, swimmer's itch, mycetoma, Chagas disease, and leishmaniasis.)

Quintanilla-Cedillo, MR, León-Ureña, H, Contreras-Ruiz, J, Arenas, R. “The value of Doppler ultrasound in diagnosis in 25 cases of furunculoid myiasis”. Int J Dermatol. vol. 44. 2005. pp. 34-7. (Ultrasound has the appropriate resolution to aid in diagnosis and accurately determine the number of larvae in each skin lesion. Most lesions were identified on the head, followed by the trunk and extremities.)

Robbins, K, Khachemoune, A. “Cutaneous myiasis: a review of the common types of myiasis”. Int J Dermatol. vol. 49. 2010. pp. 1092-8.

Sesterhenn, AM, Pfützner, W, Braulke, DM, Wiegand, S. “Cutaneous manifestation of myiasis in malignant wounds of the head and neck”. Eur J Dermatol. vol. 19. 2009. pp. 64-8. (Contains a summary table of all reported cases of malignant head and neck wounds affected by myiasis, the species of fly involved, and management strategies used in these patients.)

Ting, PT, Barankin, B. “Cutaneous myiasis from Panama, South America: case report and review”. J Cutan Med Surg. vol. 12. 2008. pp. 133-8.