The human botfly (Dermatobia hominis) is a species of fly whose larvae are obligate parasites of mammals, including humans. The developing larva causes furuncular myiasis, appearing as a painful, boil-like lesion on the skin. This infestation is most commonly seen in travelers returning from endemic areas in Central and South America.
The Larva’s Appearance Beneath the Surface
The lesion caused by the human botfly larva typically appears as a firm, raised, erythematous nodule, often described as a boil or “warble.” This swelling is the body’s inflammatory response to the larva residing beneath the skin’s surface. At the center is a small opening called a punctum or breathing pore.
The larva positions its posterior end directly at this punctum to access air for respiration. Occasionally, the larva’s breathing apparatus, known as the spiracles, may be visible within this central pore as two dark dots. The larva itself is creamy white or yellowish and has a distinctly segmented, tapered body.
To anchor itself securely within the subcutaneous tissue, the larva is covered in concentric rings of tiny, backward-pointing spines that prevent the host’s body from easily expelling the parasite and make manual extraction more difficult. As the larva matures, it can grow up to 1 inch (25 mm) in length and 7 mm in diameter before it exits the host.
Stages of Infestation and Growth
The infestation begins when the first-stage larva, or instar, hatches upon contact with the host’s warm skin, often transferred by a mosquito or other carrier insect. This larva rapidly burrows into the skin, usually through the carrier’s bite wound, and establishes itself in the subcutaneous layer. Initially, the lesion is small and may be mistaken for a common insect bite or pimple.
The larva undergoes two molts, developing through three distinct instars as it feeds on the surrounding tissue exudates. The second and third instars represent the rapid growth phase, which is when the surrounding nodule becomes most pronounced and noticeable. The entire larval development period within the human host typically lasts between five and ten weeks.
This continual growth of the larva directly corresponds to the progressive enlargement of the visible lump on the skin. Once the larva reaches its full size as a mature third instar, it is prepared to leave the host, usually dropping to the ground to pupate and complete its life cycle.
Symptoms and Sensations
The growing larva triggers a localized inflammatory reaction, resulting in swelling and tenderness of the surrounding tissue. A common complaint is intermittent, sharp, or shooting pain, often reported when the larva shifts position or begins to molt. This movement can sometimes be felt as a distinct crawling or wriggling sensation beneath the skin’s surface.
The host may also experience intense itching (pruritus) around the lesion site. The punctum, necessary for the larva’s breathing, often discharges a thin, serous, or bloody fluid. This discharge is a combination of larval excretions and the body’s reaction to the parasite.
Common Removal Procedures
The primary goal of treatment is the complete extraction of the larva to prevent secondary bacterial infection. One common non-surgical method involves occluding the central breathing pore with a substance such as petroleum jelly, thick oil, or bacon fat. This action restricts the larva’s air supply, forcing it to emerge in search of oxygen. After several hours, the suffocated or weakened larva can often be removed using forceps.
However, caution is necessary, as the larva’s posterior breathing tube is fragile and can easily break off during removal, leaving remnants behind. If the larva is not successfully coaxed out, a medical professional may need to perform a minor surgical procedure. This involves making a small incision to enlarge the punctum, allowing the larva to be extracted entirely with surgical forceps.