Myiasis: fly larvae living in the skin or a wound

The CDC defines myiasis as a parasitic infestation of human tissue by fly larvae. The larvae feed and grow in the tissue and, in the skin form, produce a visible lump as they enlarge. Unlike many parasites, the larvae usually stay in one place beneath the skin instead of travelling through the body.

Different flies are responsible and they reach people in different ways. Some lay eggs directly on wounds or in body openings. Others attach their eggs to a mosquito or tick, and the larvae hatch and enter the skin when the carrier bites. A few African species lay eggs on the ground or on damp clothing.

CDC laboratory material names several species. The human botfly, Dermatobia hominis, occurs from Mexico through Central and South America. Its larvae develop in a pocket under the skin for about 5 to 10 weeks and are delivered by a mosquito carrier. The New World screwworm, Cochliomyia hominivorax, has larvae that feed on living flesh and can cause extensive tissue damage. The tumbu fly, Cordylobia anthropophaga, occurs in Africa south of the Sahara and causes furuncle-like lesions.

The CDC describes a lump in the affected tissue. CDC laboratory material lists forms by location and behaviour: cutaneous swellings that are often painful, wound myiasis where larvae colonise an existing wound, furuncular lesions that look like boils, and eye involvement. Many people feel movement or a sharp pain within the lump and notice a small central opening that may discharge fluid.

A boil that does not behave like a boil, particularly several weeks after tropical travel, should prompt the clinician to ask about insect exposure and to consider myiasis. Antibiotics alone do not clear an infestation, so a lesion that persists despite treatment for a skin infection deserves another look.

According to the CDC, healthcare providers diagnose myiasis clinically. Travel history is the key clue. When larvae are removed they can be examined for features such as spiracles, mouthparts and body spines to identify the species, which helps predict how the lesion will behave and whether it might involve deeper tissue.

Lesions near the eye, nose, ears or mouth, or lesions in an open wound that is enlarging, need prompt specialist assessment because some species invade deeper tissue.

The CDC states that treatment involves removing the larvae, which may require a minor surgical procedure, followed by daily wound cleaning to allow healing. Removal should be done by a clinician. Trying to squeeze out a botfly larva at home can crush it, leave fragments and provoke inflammation or infection.

Because the opening can be contaminated, clinicians also consider tetanus protection and look for secondary bacterial infection. A GP, an urgent treatment centre, a dermatologist or an infectious diseases or tropical medicine service may see the problem depending on location and severity.

The CDC recommends wearing long-sleeved clothing, keeping wounds covered, using an insect repellent and treating gear with permethrin, along with following general travel health advice for the destination. Mosquito avoidance is relevant to botfly because mosquitoes can carry the eggs.

Where laundry is dried outdoors in tumbu fly areas, ironing clothes or tumble-drying them is a commonly advised precaution, although that detail comes from general travel medicine practice and not from the pages cited here.

• Keep any cut, graze or insect bite clean and covered.

• Use repellent and consider treating clothing with permethrin.

• Check skin after trips for lumps, boils or crawling sensations and seek review if one does not heal.

See a GP or contact NHS 111 if a lump appears after travel and does not settle, or if you see larvae in a wound. Seek urgent care if the eye, nose or a deep wound is involved, if redness spreads quickly, or if you have a fever or feel unwell, because bacterial infection can follow.