Myasis

Myiasis is the infestation with the larval stage (maggots) of various flies. Flies in several genera may cause myiasis in humans.

Myiasis occurs most frequently in tropical and subtropical areas.People with untreated and open wounds are more at risk for getting myiasis.

LIFE CYCLE

There are several ways for flies to transmit their larvae to people.

Some flies (e.g. Dermatobia hominis) attach their eggs to mosquitoes and wait for mosquitoes to bite people. Their larvae then enter these bites. The larvae feed in a subdermal cavity for 5-10 weeks, breathing through a hole in the host's skin. Mature larvae drop to the ground and pupate in the environment.

Other flies' larvae burrow into skin. These fly larvae are known as screwworms. They can enter skin through people's bare feet when they walk through soil containing fly eggs or attach themselves to people's clothes and then burrow into their skin.

Some flies deposit their larvae on or near a wound or sore, depositing eggs in sloughing-off dead tissues.

PATHOPHYSIOLOGY

The pathophysiology of myiasis is dependent on the type of fly involved. 

Worldwide, the most common flies responsible for myiasis are Dermatobia hominis, or the human botfly, and Cordylobia anthropophaga, or the tumbu fly. Both are responsible for furuncular myiasis. 

Cochliomyia hominivorax in America and Chrysomyia bezziana in Africa, Australia, and Asia cause wound myiasis.

Oestrus ovis, or sheep botfly, causes ophthalmomyiasis (ocular infestation).

The tumbu fly is endemic in sub-Saharan Africa. Adult flies resemble stocky houseflies and are active during the early morning and afternoon. They are attracted by the odour of urine and faeces. Females lay eggs on sandy soil or clothing: therefore they cause lesions on areas of the body normally covered with clothing. Eggs hatch in 1-3 days and can survive up to 2 weeks while awaiting contact with a host. When exposed to local heat, they migrate and penetrate local skin. Within the host they grow up to 13-15 mm, usually completing their larval stage in 9-14 days. Children are most commonly affected.

SIGNS AND SYMPTOMS

Furuncular myiasis, the most common manifestation of myiasis, produces typically boil-like lesions that may be painful, pruritic, and tender. An erythematous papule, 1-3 cm in diameter and up to 1 cm in height, develops with 24 hours of infection. There may be the sensation of something moving under the skin. The lesion has a central puctum with serosanguinous discharge. Larvae rely on the central punctum to provide airflow to breathe, and bubbles may be seen exuding from the punctum. The primary reaction or secondary infection may produce lymphangitis and regional lymphadenopathy (fever, swollen glands, swollen extremities). If not removed, the larvae of most species will spontaneously emerge, leaving behind an exit wound.

In cases of nasal myiasis, patients may report epistaxis, foul odour, pain, obstruction, discharge, headache, dysphagia, and foreign body sensation. 

It is characterized by oedematous ulcerated mucous membranes filled with necrotic material and crawling maggots. There may be septal or palatal perforation, nasal bridge erosion, or orbital and facial cellulitis. 

Wound myiasis occurs when larvae are deposited on non-viable flesh. It is typically caused by C hominivorax and C bezziana. These larvae usually do not invade healthy tissue. This preference for dead tissue is the reasoning behind maggot debridement therapy. A large number of small maggots consume necrotic tissue far more precisely than it would be possible in a normal surgical operation, and can debride a wound in a day or two. In Western countries, sterile, medical-grade larvae of the green bottle fly are available for use, most commonly for patients with chronically nonhealing wounds, like diabetic foot ulcers.

DIAGNOSIS

Diagnosis of myiasis is made by direct visualization of the larvae. 

Full Hemogram may show leukocytosis and eosinophilia. 

TREATMENT

Treatment of myiasis typically involves direct surgical extraction of the larvae under local anesthesia. 

Second-line options involve suffocation techniques to encourage the larvae to migrate out of the skin through the use of petroleum jelly, liquid paraffin, beeswax, or meat strips.

Third-line treatment with topical or systemic ivermectin may be very helpful with orbital myiasis.

Proper hygiene of wounds is very important when treating myiasis and is fundamental for prevention. Essential is debridement with removal of dead and necrotic tissue which constitutes the pabulum for the maggots.

Dr Giuseppe Gaido

Medical Officer in charge

Cottolengo Mission Hospital. Chaaria

PO BOX 1426

60200 MERU - KENYA