Botflies are flies whose larvae are obligate internal parasites of mammals, generally classified within the family Oestridae. This life history strategy, where the larval stage develops inside the living tissue of a host, is known as myiasis. The parasitic nature of their young often sparks concern, though the adult flies are harmless. Botflies represent a highly specialized lineage within the insect world, with approximately 150 known species worldwide.
Taxonomy and Adult Characteristics
Botflies belong to the Order Diptera and are grouped into the family Oestridae, often referred to as warble flies, heel flies, or gadflies. Adult botflies are typically large, robust, and possess a dense coat of hair, making them visually similar to bees or bumblebees. This bee-like appearance may serve as mimicry to deter predators.
A defining characteristic of the adult stage is the absence of functional mouthparts; they cannot feed. The sole purpose of the adult life, which is short, is reproduction. All the energy required is stored during the prolonged larval development phase inside the host, underscoring the species’ dependency on the parasitic larval stage.
The Parasitic Life Cycle
The botfly life cycle involves the infestation of living vertebrate tissue by fly larvae. The initial stage involves specialized methods of egg delivery that vary significantly by species. Some female botflies attach their eggs directly to the host’s hair or skin, relying on the host licking or scratching the area to ingest or transfer the larvae.
A more complex strategy is seen in species like the human botfly, which uses an intermediate carrier, such as a mosquito or tick, in a process called phoresis. The female fly glues its eggs onto the vector. When the vector lands on a warm-blooded host, the heat triggers the eggs to hatch, and the tiny larvae then burrow into the skin.
Once inside, the larvae, or grubs, feed on the host’s subcutaneous tissue, often creating a lump or “warble” with a small breathing hole, or punctum, open to the air. The larva develops through three stages, or instars, feeding and growing for several weeks or months, depending on the species. To feed, the larvae use specialized hook-like mandibles and secrete digestive enzymes that break down the host tissue. Once fully mature, the larva exits the host, drops to the ground, and burrows into the soil to begin the pupal stage.
Notable Species and Host Specificity
Most botfly species exhibit a high degree of host specificity, specializing in parasitizing only a single or small group of hosts. The Human Botfly, Dermatobia hominis, is the species most often associated with human infestation, though it targets a wide range of mammals across Central and South America. Other economically significant species primarily target livestock.
The Horse Botfly, Gasterophilus intestinalis, lays its eggs on the hair of horses, which the horse then ingests, leading to larval development in the digestive tract. The Cattle Botfly, Hypoderma bovis, is a “warble fly” whose larvae migrate extensively before creating characteristic bumps, known as warbles, under the skin of the back.
Larval Removal and Medical Context
Infestation often presents as a painful, boil-like lesion, or furuncle, which may discharge fluid and is characterized by a central breathing hole. Patients frequently report feeling sharp pain or movement within the lesion, particularly when the larva shifts position. The spines on the larval body anchor it firmly within the host tissue, making forceful removal difficult and potentially dangerous.
Medical professionals advise against attempting to squeeze the larva out, as rupture can lead to localized infection, severe inflammation, or an anaphylactic reaction to the larval contents. The safest non-surgical removal methods focus on asphyxiating the larva to encourage it to emerge. This is commonly done by covering the breathing hole with an occlusive substance like petroleum jelly, thick grease, or nail polish.
Blocking the air hole forces the larva to extend its posterior end to search for air, at which point it can be safely grasped and removed with forceps. If non-surgical methods fail, a small surgical incision under local anesthesia may be performed to safely extract the entire grub, which is necessary for the wound to heal properly.