What is cutaneous myiasis?
Myiasis is infestation by the larvae (maggots) of fly species within the arthropod order Diptera (two-winged adult flies). The larvae feed on the host's dead or living tissue, body substances, or ingested food. Cutaneous myiasis is myiasis affecting the skin.
Myiasis can be categorised clinically based on the area of the body infested, for example cutaneous, ophthalmic, auricular, and urogenital. Cutaneous presentations include furuncular, migratory, and wound myiasis, depending on the type of infesting larvae.
- Dermatobia hominis is found in Central and South America. The D. hominis female fly lays her eggs onto foliage or carrier insects, most commonly mosquitoes. The eggs are passed to humans by direct contact with foliage, or during a bite from the carrier.
- Once the eggs hatch, the larvae burrow painlessly into the host's skin producing a small red papule (bump). The papule later becomes a furuncular-like (boil-like) nodule with a central pore through which the organism breathes. Occasionally the tail end of the larva can be seen through this pore.
- Over the following 5 to 10 weeks the larvae further develop and burrow deeper into the host's skin, forming a dome-shaped cavity. Symptoms include itching, a sensation of movement, stabbing pain (often at night), and a serosanguinous (thin, yellow or bloody) discharge.
- The larvae eventually work their way back to the skin surface, then drop to the ground where they pupate to form flies.
- The lesions generally resolve with minimal scarring after the larvae emerge or are removed. The most important complications of D. hominis are bacterial super-infection (rare) and tetanus. D. hominis has caused fatal cerebral myiasis in infants due to infestation of the skin covering the fontanelles.
- Cordylobia is found in tropical Africa. All three species of Cordylobia can cause furuncular myiasis, however C. anthropophaga is most commonly responsible.
- These flies prefer shade and usually lay their eggs on objects contaminated with urine or faeces, such as sandy soil or damp clothing laid to dry on the ground.
- The eggs hatch in 1-3 days and the larvae can survive for up to 2 weeks while waiting to come into contact with a host. The larvae then painlessly penetrate the unbroken skin of the host. They develop over 8-12 days. Following this they emerge from the skin, fall to the ground, and pupate.
- Symptoms develop within the first 2 days of infestation and can range from a ‘prickly heat' sensation to severe pain. Agitation and insomnia can also occur. Furuncular lesions with surrounding inflammation rapidly develop over a period of 6 days after symptoms begin. In the late stages, the tail end of the larvae may sometimes be seen in the central pore, and may withdraw when touched.
- If there are multiple sites of infestation, hosts may develop enlarged lymph nodes and fever. If lesions are numerous, they may coalesce forming large plaques with a serous (thin, yellow) discharge.
- Cuterebra is found in parts of North America. Human Cuterebra infestation is rare as the usual hosts are rodents, rabbits, and squirrels.
- Cuterebra eggs are laid near their usual hosts on grass or brush. Humans likely inadvertently contact the eggs, which then hatch, and larvae enter the host through the skin or mucous membranes of the nose, eyes, mouth, or anus.
- Almost all human cases present in August, September, or October.
- The typical lesion is a 2-20mm red papule or nodule with a central pore, through which the organism breathes. The larva is occasionally visible through this pore. A serous, serosanguinous, or purulent (consisting of pus) discharge may occur. The lesions may be itchy or painful, and some patients experience a sensation of movement within the lesion.
Wohlfahrtia vigil and Wohlfahrtia opaca
- W. vigil is found in parts of North America, Europe, Russia, and Pakistan. W. opaca is found in parts of North America. Larvae from W. vigil and W. opaca cause furuncular myiasis in cats, dogs, rabbits, ferrets, mink, foxes, and humans. In nearly all hosts, infestations mostly occur in the very young, because the larvae are unable to penetrate adult skin.
- Females from both Wohlfahrtia species are most active in shaded areas, during the late afternoon hours. The larvae are dropped onto host skin, which they then penetrate. Within 24 hours furuncles form. The larvae develop over 4-12 days, then leave the skin, fall to the ground, and pupate.
- Most cases occur during the months of June to September.
- G. intestinalis is the most frequent cause of human migratory (or creeping) myiasis and is found worldwide. G. intestinalis is usually an intestinal parasite of horses and other equids.
- Humans are an accidental host and become infested by direct contact with eggs on the horse's coat or eggs may be directly laid onto human skin. The larva initially produces a papule similar to furuncular myiasis. Then the larva burrows to the lower layers of the epidermis, causing an intensely itchy, snake-like, and raised red linear lesion that advances at one end and fades at the other as it searches for a place to develop. The lesion can extend up to 30cm per day and can continue for several months. The infestation may end spontaneously with or without suppuration (formation of a purulent sore).
Hypoderma bovis and H. lineatum
- Hypoderma species usually infest cattle and are found in most locations in the northern hemisphere.
- Human infections are rare and usually occur in rural areas where cattle are raised. The eggs are laid on the hairs of the body, and larvae enter through the skin or the mucosa of the mouth. The larva migrates in the subcutaneous tissue, causing a slightly red, tender, and ill-defined 1-5 cm raised area. A ‘prickly’ sensation and, less frequently, burning and itch, are reported. After several hours to several days the redness subsides, leaving a yellow-pigmented patch, as the larva wanders to another location. A faint, irregular, palpable line connects the old area of redness with the newer one. The larva can migrate 2 to 30 cm per day. Most often, the larva eventually dies in the subcutaneous tissue.
- In around 1 out of 15 human cases, the subcutaneous larva penetrates the dermis and forms a slowly enlarging tender red nodule (warble). A central pore develops, through which the larva may be visible. The pore intermittently drains a serosanguinous discharge, that later becomes purulent. Itching becomes intense, and the larva then grows, exits, and falls to the ground to pupate.
- Human Hypoderma myiasis is usually a mild disease, but can cause fever, muscle pain, joint pain, scrotal swelling, ascites (fluid in the peritoneal cavity of the abdomen), fluid around the heart, and invasion of the brain and spinal cord.
Wound myiasis occurs when fly larvae infest open wounds in a living host. Mucous membranes (e.g. oral, nasal, and vaginal membranes) and body cavity openings (e.g. in or around the ears and eye socket) can also be affected. Severe cases may be accompanied by fever, chills, pain, bleeding from the infested site, and secondary infection. Blood tests may show raised neutrophils and eosinophils. Massive tissue destruction, the loss of eyes and ears, erosion of bones and nasal sinuses, and death can occur.
Factors that make humans susceptible to wound myiasis include poor social conditions, poor hygiene, advanced or very young age, psychiatric illness, alcoholism, diabetes, peripheral vascular disease, poor dental hygiene, and physical disabilities that restrict ability to discourage flies.
- Cochliomyia hominivorax is found in Central and South America. In humans, infestations of C. hominivorax usually occur in or around the ears, nose and eye socket. Even tiny wounds such as a tick bite or an ingrown toenail can attract C. hominivorax. The female lays her eggs on the edges of wounds or healthy mucous membranes. Within one day the eggs hatch and the larvae feed on tissue causing massive tissue destruction and large deep lesions. An odour is produced which attracts more female flies to lay additional batches of eggs. A single wound can contain up to 3000 larvae, which eventually fall to the ground to pupate.
- Chrysomya bezziana is found in Africa, India, and Southeast Asia. The life cycle and biologic activity of C. bezziana is similar to that of C. hominivorax. As these larvae burrow deeper into host tissue, only the black tail ends are seen. C. bezziana infests wounds, areas of soft skin, and mucous membranes. The only presenting features of a nasal sinus infestation may be a swollen face associated with headaches, fever, burning nasal pain, and a nasal discharge.
- Wohlfahrtia magnifica is found in parts of Europe, Russia, North Africa, and the Middle East. Adult W. magnifica flies are active in the summer months during the warmest part of the day. In humans, wounds, ears, eyes, and nasal passages are commonly infested. W. magnifica larvae are usually less destructive than C. bezziana and C. hominivorax.
How is cutaneous myiasis diagnosed?
- Diagnosis of cutaneous myiasis is made primarily on the clinical appearance of the lesions, associated symptoms, and travel history. Dermoscopy and ultrasound may be helpful.
- Submerging the lesion under water may confirm the diagnosis – if the larva is alive, bubbling will occur.
- G. intestinalis larva can be diagnosed by massaging a thin layer of mineral oil over the red lesion. Under magnification, black transverse bands can be seen that represent spines on the larva's body segments.
What is the treatment for cutaneous myiasis?
Occlusion, manual removal of the larva, and larvicides may be used.
- The larvae require contact with air to breathe. Occlusion either kills the larva or induces it to move upwards, where it can be removed.
- A variety of occlusive substances have been used, including petrolatum, animal fat, beeswax, paraffin, hair gel, mineral oil, and bacon. The occlusive substance is placed over the pore of the furuncle, or over the area of wound myiasis, for up to 24 hours.
- Once the larvae have migrated to the skin surface, they can be removed with forceps. This can be difficult as the larvae resist extraction using their spines to anchor themselves to the host. D. hominis is the most difficult larva to extract due to its tapered shape.
- Occasionally the larva is asphyxiated without emerging. The retained larva can cause an inflammatory response, leading to foreign body granuloma formation (a clump of inflammatory tissues) that may progress to calcification.
Manual removal of larvae
- A surgical incision is made. The larva is then removed with forceps. Care is taken to avoid damaging the larva, as retained parts can lead to a severe inflammatory reaction. Anaesthetising the larva with local anaesthetic may prevent it from anchoring its spines.
- Alternatively, local anaesthetic is injected forcibly into the base of the lesion in an attempt to create enough fluid pressure to push the larva out of the pore.
- A commercial vacuum snake venom extractor may also be used to suck the larva out.
- Traditional methods of larvae removal involve squeezing the skin surrounding the furuncle with fingers or with wooden spatulas.
- Hypoderma myiasis can be treated using these methods if a warble has formed.
- Hypoderma larvae can be extracted through a surgical incision if there is no warble formation, but can be difficult to capture.
- Gasterophilus can be extracted by making a small surgical incision over the leading edge of the advancing lesion and using the tip of a sterile needle to remove.
- Manual removal followed by irrigation is used to treat wound myiasis. Surgery may be necessary to remove dead host tissue.
- Ivermectin is a broad-spectrum antiparasitic agent that may kill larvae, or at least cause them to migrate out of the skin. Ivermectin can be administered topically or as an oral dose.
- Mineral turpentine can be effective against Chrysomya larvae and may aid their removal in cases of wound myiasis.
- Ethanol spray and oil of betel leaf can be used topically to treat C. hominivorax myiasis.
How can myiasis be prevented?
- Use window screens and mosquito netting, insect repellents and insecticides, adequate protective clothing, and good skin and wound hygiene to keep flies, mosquitoes, and ticks from reaching the skin
- Cover open wounds and change dressings daily
- In the case of C. anthropophaga, hang clothes to dry in bright sunlight and/or iron them (the heat destroys both the eggs and larvae)
- Improve hygiene and sanitation (e.g. remove rubbish from around living areas)