The bot fly, Dermatobia hominis, is a parasitic insect whose larvae can infest humans and other mammals. The adult fly itself does not bite or feed, but its reproductive cycle involves placing larvae beneath the skin of a host, a condition known as myiasis. While myiasis is not typically fatal, it represents a significant health concern due to the pain, discomfort, and risk of secondary infection it causes. Travelers returning from endemic areas, such as Central and South America, are the most common cases seen in non-endemic regions.
The Danger: Understanding Myiasis
Myiasis is the infestation of a living host by fly larvae, and the bot fly is the most common species to cause this in people. The female bot fly uses an indirect method called phoresis: she captures a blood-feeding arthropod, often a mosquito, and glues 10 to 50 eggs to its body.
When the carrier insect lands on a host, the body heat triggers the eggs to hatch. The larvae penetrate the skin, usually through a bite wound or hair follicle. Once beneath the skin, the larva begins its developmental stage, known as furuncular myiasis because it resembles a boil.
The larva lives in the subdermal layer for five to ten weeks, feeding on tissue fluid. It develops tiny spines for anchoring and maintains an open pore in the skin, called a spiracle, for breathing. This mechanism causes the wound to continuously discharge a clear or bloody fluid. Once mature, the larva exits the host and drops to the ground to pupate.
Recognizing Symptoms and Assessing Severity
A bot fly infestation presents as a boil-like lump at the site of larval penetration. This lesion, or warble, is painful and firm to the touch, and it may be mistaken for a bacterial infection or an infected cyst. A defining characteristic is the presence of a central opening, which is the larva’s breathing pore.
Intermittent sharp pain or a tingling sensation is common, often described as a feeling of movement beneath the skin. The area around the lump is swollen and red, with the wound discharging fluid as the larva keeps the breathing pore clear. While most infestations involve a single, localized larva, complications can make them serious.
Secondary bacterial infection is a concern, particularly if the larva is damaged or the wound is improperly managed. Although rare, infestation near the eye, ear, or nose can be serious. Cases of the larva migrating to deep tissue or the brain have been reported, especially in infants. Medical attention should be sought immediately if the lesion is on the face or near an orifice, or if there are signs of systemic infection, such as fever or spreading redness.
Prevention and Safe Removal Methods
Prevention should focus on minimizing contact with carrier insects in endemic areas. Using insect repellents containing DEET or picaridin on exposed skin is an effective strategy to deter mosquitoes and other flying arthropods that carry bot fly eggs. Wearing protective clothing, such as long-sleeved shirts and pants, provides a physical barrier against bites.
Travelers should be mindful of clothing line-dried outdoors, as the adult fly may lay eggs on the fabric. Ironing clothes dried outside before wearing them can destroy any eggs present. Using window screens and sleeping under permethrin-treated mosquito nets further reduces the risk of exposure while indoors.
If an infestation is suspected, professional medical extraction is recommended. Attempting to squeeze the larva out can be dangerous; rupturing the larva may trigger a severe foreign body reaction or anaphylactic shock, and its anchoring spines can tear surrounding tissue. Healthcare providers use non-surgical methods, such as occlusion, where petroleum jelly is applied over the breathing pore to suffocate the larva. This forces the larva to protrude its posterior end, allowing a doctor to safely remove it with forceps or a small incision.