Myiasis refers to the infestation of living vertebrate animals with dipterous fly larvae, commonly known as maggots. These larvae feed on the host’s tissues, body fluids, or ingested substances. When this infestation occurs on the skin, it is specifically termed cutaneous myiasis. This condition can manifest in various forms depending on the fly species involved and the nature of the infestation. Clinically, myiasis is categorized by the affected body area, including cutaneous, ophthalmic (eye), auricular (ear), and urogenital locations. Within cutaneous myiasis, further classifications are made based on the larval behavior, leading to furuncular, migratory, and wound myiasis.
Furuncular Myiasis
Furuncular myiasis is characterized by boil-like lesions on the skin, each typically containing a single larva. Several fly species are known to cause this type of myiasis.
Dermatobia hominis Infestation
Dermatobia hominis, commonly known as the human botfly, is prevalent in Central and South America. Uniquely, the female D. hominis fly doesn’t directly lay eggs on humans. Instead, it deposits them on foliage or carrier insects, often mosquitoes. Transmission to humans occurs through contact with contaminated foliage or during a bite from an infected carrier insect.
Once hatched, the larvae painlessly burrow into the skin, initially forming a small, red papule. This papule evolves into a furuncle-like nodule with a central pore, essential for the larva’s respiration. In some instances, the larva’s posterior end may be visible through this pore.
Over 5 to 10 weeks, the larva develops further, burrowing deeper and creating a dome-shaped cavity. Patients may experience itching, a sensation of movement, sharp, stabbing pain (often nocturnal), and a serosanguinous discharge.
Eventually, the mature larva migrates back to the skin surface, detaches, and pupates on the ground to become an adult fly. Lesions typically resolve with minimal scarring after larval removal or emergence. Rare but significant complications of D. hominis infestation include secondary bacterial infections and tetanus. Fatal cerebral myiasis has been reported in infants due to scalp infestations through fontanelles.
Cordylobia Myiasis
Cordylobia species are found in tropical Africa. While three species can cause furuncular myiasis, Cordylobia anthropophaga, known as the Tumbu fly, is the most common culprit. These flies favor shaded environments and typically lay eggs on urine or feces-contaminated items like sandy soil or damp clothing left to dry on the ground.
Eggs hatch within 1 to 3 days, and the larvae can survive for up to two weeks while awaiting host contact. They painlessly penetrate unbroken skin and develop over 8 to 12 days before emerging, falling to the ground, and pupating.
Symptoms appear within two days of infestation, ranging from a ‘prickly heat’ sensation to intense pain, potentially accompanied by agitation and insomnia. Furuncular lesions with surrounding inflammation rapidly develop within six days of symptom onset. In later stages, the larva’s tail end may be visible in the central pore and retract upon touch.
Multiple infestation sites can lead to enlarged lymph nodes and fever. Numerous lesions may coalesce, forming large plaques with serous discharge.
Cuterebra Myiasis
Cuterebra species are found in parts of North America. Human infestation is rare, as these flies primarily target rodents, rabbits, and squirrels. Cuterebra eggs are laid near these animal hosts on grass or brush. Human infections are likely accidental, occurring when individuals inadvertently contact eggs, which hatch and larvae penetrate the skin or mucous membranes (nose, eyes, mouth, anus).
Most human cases occur between August and October. Lesions typically present as 2-20mm red papules or nodules with a central pore for larval breathing. The larva might be occasionally visible through this pore. Serous, serosanguinous, or purulent discharge is possible. Lesions can be itchy or painful, and patients may feel movement within the lesion.
Wohlfahrtia vigil and Wohlfahrtia opaca Myiasis
Wohlfahrtia vigil is found in parts of North America, Europe, Russia, and Pakistan, while Wohlfahrtia opaca is present in parts of North America. Larvae of both species cause furuncular myiasis in various animals including cats, dogs, rabbits, and humans. Infestations predominantly affect the very young in most host species as larvae struggle to penetrate adult skin.
Female Wohlfahrtia flies are most active in shaded areas during late afternoon. Larvae are deposited on host skin and penetrate it, with furuncles developing within 24 hours. Larval development lasts 4-12 days, after which they exit the skin, fall to the ground, and pupate. Most cases occur from June to September.
Migratory Myiasis
Migratory myiasis, also known as creeping myiasis, is characterized by larvae migrating through the subcutaneous tissue, creating linear or serpentine lesions.
Gasterophilus intestinalis Myiasis
Gasterophilus intestinalis, the horse botfly, is the most frequent cause of human migratory myiasis and is found worldwide. Typically, it is an intestinal parasite of horses and related animals. Humans are accidental hosts, becoming infested through direct contact with eggs on horse coats, or eggs may be directly laid on human skin.
Initially, the larva produces a papule similar to furuncular myiasis. It then burrows into the lower epidermis layers, causing an intensely itchy, snake-like, raised red linear lesion. This lesion advances at one end and fades at the other as the larva searches for a development site. Lesions can extend up to 30 cm per day and persist for months. Infestation may resolve spontaneously, with or without suppuration.
Hypoderma bovis and H. lineatum Myiasis
Hypoderma species, typically infesting cattle, are found widely in the Northern Hemisphere. Human infections are rare, mainly occurring in rural areas where cattle are raised. Eggs are laid on body hairs, and larvae enter through the skin or oral mucosa.
The larva migrates subcutaneously, causing a slightly red, tender, ill-defined, raised area (1-5cm). Patients report a ‘prickly’ sensation, and less often, burning and itching. Redness subsides after hours to days, leaving a yellow patch as the larva moves. A faint, irregular, palpable line may connect previous and newer areas of redness. Larval migration can be 2-30 cm per day. Most often, the larva dies in subcutaneous tissue.
In about 1 in 15 human cases, the subcutaneous larva penetrates the dermis forming a slowly enlarging, tender, red nodule (warble). A central pore develops, potentially showing the larva. Intermittent serosanguinous discharge may occur, later becoming purulent. Itching intensifies, and the larva eventually grows, exits, and pupates.
Human Hypoderma myiasis is generally mild but can cause fever, muscle and joint pain, scrotal swelling, ascites, pericardial effusion, and, rarely, brain and spinal cord invasion.
Wound Myiasis
Wound myiasis occurs when fly larvae infest open wounds in a living host. Mucous membranes (oral, nasal, vaginal) and body cavity openings (ears, eye sockets) can also be affected. Severe cases can present with fever, chills, pain, bleeding, secondary infection, elevated neutrophils and eosinophils on blood tests. Massive tissue destruction, loss of eyes or ears, bone and sinus erosion, and even death can occur in extreme cases.
Risk factors for wound myiasis in humans include poor social conditions, inadequate hygiene, extremes of age, psychiatric illness, alcoholism, diabetes, peripheral vascular disease, poor dental hygiene, and physical disabilities that hinder fly deterrence.
Cochliomyia hominivorax Myiasis
Cochliomyia hominivorax, the New World screwworm fly, is a significant cause of wound myiasis in the Americas. These flies are attracted to open wounds and lay eggs at the wound edges. The hatched larvae are voracious tissue feeders, burrowing deeply into the wound.
Chrysomya bezziana Myiasis
Chrysomya bezziana, the Old World screwworm fly, is found in Africa, India, and Southeast Asia. Its life cycle and activity are similar to C. hominivorax. Larvae burrow deeply, often leaving only their black posterior ends visible. C. bezziana infests wounds, soft skin areas, and mucous membranes. Nasal sinus infestation may present with facial swelling, headache, fever, burning nasal pain, and nasal discharge.
Wohlfahrtia magnifica Myiasis
Wohlfahrtia magnifica is found in parts of Europe, Russia, North Africa, and the Middle East. Adult flies are active during warmer parts of summer days. In humans, wounds, ears, eyes, and nasal passages are commonly infested. W. magnifica larvae are generally less destructive than C. bezziana and C. hominivorax.
Diagnosis of Cutaneous Myiasis
Diagnosis of cutaneous myiasis primarily relies on the clinical appearance of lesions, associated symptoms, and patient travel history. Dermoscopy, biopsy, and ultrasound can be valuable adjuncts. Submerging the lesion in water can aid diagnosis; larval bubbling indicates viability.
Gasterophilus intestinalis larval diagnosis can be aided by massaging mineral oil over the lesion. Under magnification, black transverse bands (larval spines) may be visible.
Treatment for Cutaneous Myiasis
Treatment strategies for cutaneous myiasis include occlusion, manual larval removal, and larvicides.
Occlusion Therapy
Larvae require air for respiration. Occlusion aims to kill the larva or encourage upward migration for removal. Various occlusive substances like petrolatum, animal fat, beeswax, paraffin, hair gel, mineral oil, or even bacon can be applied over the furuncle pore or wound area for up to 24 hours.
Once larvae reach the surface, forceps can be used for removal. This can be challenging as larvae use spines to anchor themselves. D. hominis larvae are particularly difficult to extract due to their tapered shape. Sometimes, larvae asphyxiate without emerging, potentially causing an inflammatory response and foreign body granuloma formation, which can calcify.
Manual Larval Removal
Furuncular Myiasis
Surgical incision followed by forceps extraction is a common method. Care is crucial to avoid larval damage, as retained fragments can trigger severe inflammation. Local anesthetic injection into the lesion base can help anesthetize the larva and potentially create hydraulic pressure to expel it. Commercial vacuum snake venom extractors can also be used for suction removal. Traditional methods involve squeezing the skin around the furuncle using fingers or spatulas. These techniques are also applicable to Hypoderma warble removal.
Migratory Myiasis
Hypoderma larvae without warble formation can be surgically excised, though capture can be challenging. Gasterophilus larvae can be removed by a small incision at the advancing lesion edge and using a sterile needle tip for extraction.
Wound Myiasis
Manual removal and irrigation are the mainstays of wound myiasis treatment. Surgical debridement to remove necrotic tissue may be necessary.
Larvicides
Ivermectin, a broad-spectrum antiparasitic agent, can kill larvae or induce migration. It can be administered topically or orally. Mineral turpentine can be effective against Chrysomya larvae, aiding removal in wound myiasis. Topical ethanol spray and betel leaf oil can be used for C. hominivorax myiasis.
Prevention of Myiasis
Preventive measures against myiasis include:
- Using window screens and mosquito nets.
- Applying insect repellents and insecticides.
- Wearing adequate protective clothing.
- Maintaining good skin and wound hygiene to deter flies, mosquitoes, and ticks.
- Covering open wounds and changing dressings daily.
- For C. anthropophaga prevention, drying clothes in direct sunlight and/or ironing them to destroy eggs and larvae.
- Improving hygiene and sanitation, including removing waste from living areas.
