Bot fly: Is this what is starting to bug you?

Costa Rica got a bad rap last week when a California mom went public with her 7 year old’s bot fly problem.

The family had returned from a Costa Rica vacation, according to local news outlets. The mother was the source of a lengthy news article about the fly larva in her son’s scalp.

Most expats in Costa Rica know about bot flies. But apparently not the California’s family’s physician, who prescribed antibiotics, according to the news report.

The bot fly, Dermatobia hominis, is a parasite of mammals not restricted to humans despite its scientific name. The mechanism of infection is referred to as myiasis, where larva of a parasite burrow into the tissues of the host.

This fly is restricted to the lowlands in Costa Rica, generally around domestic animals. Locally it is referred to as tórsalo, though country people rarely are infected. Some student researchers at the La Selva biological station in Sarapiquí seem to consider it a badge of honor to have one and allow it to complete its larval development in their skin.

The bot fly has an unusual reproductive cycle. The adult is a substantial fly looking more like a bumblebee and would likely be repelled by the prospective host, so a vector is used. This is usually a mosquito, which is captured by the female fly. She lays her eggs on the mosquito. When it lands to bite, the larvae of 

the fly which have hatched in the meantime drop onto the warm host. After completion of
development while the larva feeds on tissue fluids, it drops off to pupate in soil. Secondary infection is rare.

Presence is usually detected by a boil in the skin with a hole that is the larva’s breathing tube. The most common remedy is to suffocate it with the application of petroleum jelly, nail polish, or tree sap. Then the worm can be carefully squeezed out or pulled out with tweezers. It is recommended that this part be done by a medical professional since if the entire worm is not removed infection may result.

The larva in the scalp of the California boy was removed with tweezers.

Another fly that uses myiasis is the screwworm, which does much more damage since it moves around thorough the host’s tissues. It causes major losses in livestock but has been mostly eliminated from north and Central America by the sterile insect technique where large numbers of male flies sterilized by radiation are released into the wild. These mate with local females which then produce sterile eggs.

Adult bot fly (Dermatobia hominis)

Adult bot fly (Dermatobia hominis)

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