Pest that feeds and grows on human body | The Daily Star
12:00 AM, April 13, 2010 / LAST MODIFIED: 12:00 AM, April 13, 2010

Beware of Botfly

Pest that feeds and grows on human body

Botflies (Order Diptera, Family Cuterebridae) are large, stout bodied, hairy flies that resemble bumblebees. Larvae of the botfly (inset)

FOUR decades ago I visited a distant relative who was a cancer patient at the Dhaka Medical College Hospital. A chain smoker, he was always chewing betel leaves containing all the available spices. His cancer was first detected in the tongue, which soon spread to the esophagus. I saw his throat was open from beneath the chin. In those days you could see flies inside a hospital ward, and I saw maggots crawling down his open wounds. Then several years ago, I watched a documentary on a young girl's struggle to get out of the Amazon jungle while a large larva was oozing out of her open wounds. I thought humans were good habitats for flies. I was wrong! Only recently, I learned flies are rarely true parasites for human. I became curious.
The only well characterized fly parasite for human are several species of botfly. They belong to the Order Diptera, and Family Cuterebridae. It is not very common, but their case reports have been documented in patients from Mexico and Central America, and in defense personnel and visitors requiring jungle dwelling in such fly-infested areas. With a yellowish head, blue-black upper body and metallic lower body, the adult fly can be 12-19 mm long. Dermatobia hominis, commonly known as torsalo, is a human bot parasite species. After mating with an adult male, the female fly lays 15-30 eggs on the abdomen of other arthropods such as tick or mosquito. The latter biting insects while sucking bloodmeal from human or other animals, help hatching of the eggs into the first stage larva and deposit it on the human skin. Ticks or mosquitoes are thus intermediate hosts. The larva digs into the skin through hair follicles or damaged skin. The larva has two oral hooks to help feed and curved spines to anchor onto skin. It breathes through the spiracles located at its posterior end facing the skin opening. After going through two molts, the larva attains its third instar stage. At this stage, it emerges from the skin and drops to the ground. Here it undergoes metamorphosis into an adult in 14-30 days.
Initial diagnosis is by combining the patient's travel history and clinical manifestations that include 2-3 mm insect bite-like bump on the skin after 24 hours of infestation. The bump gradually becomes bigger to 10-35 mm in diameter and 5-10 mm in height with a visible pin-head sized breathing hole at the center. Development of the larva leads to pus like discharge from the lesion accompanied by the feeling of a stabbing pain. Confirmatory diagnosis is by removal of the larva and its identification by a trained medical entomologist who looks for spiracles, mouth hooks and pattern of spines on the body.
Patients are treated by removal of the larva by grasping with forceps applying pressure or by surgical excision. The larva will also emerge out of the skin if its breathing is blocked by applying nail polish, wax, tape or mineral oil on the skin opening. Insect repellants should help minimize human infestation.
I did not look hard but could not find a report of natural fly infestation of humans in the Indian subcontinent. The one I saw decades ago in Dhaka must have had been an opportunistic phenomenon. But we certainly should get rid of flies from the immediate vicinity of our dwellings.

The author, a former Dhaka University teacher, is a biomedical scientist working in the USA.

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