The botfly belongs to the insect family Oestridae, a group of flies whose entire larval stage is spent as an obligate parasite inside mammals. These insects are commonly known by various names, including warble flies, heel flies, and gadflies, depending on the specific species and the host they target. The relationship between the botfly and its host is defined by the larva’s need to feed on living tissue or body fluids to complete its development. Examining their unique life cycle reveals the specialized ways they interact with other species.
The Botfly Life Cycle and Larval Stages
The botfly life cycle includes four stages: egg, larva, pupa, and adult. The adult stage is short-lived, as adults lack functional mouthparts and cannot feed, surviving only long enough to mate and deposit eggs. Their survival depends on the energy reserves accumulated during the larval stage.
The larval stage, also known as the maggot or grub, is the only stage that poses a risk to the host, as it is an internal parasite. Larvae typically complete three growth phases, called instars, while residing within the host’s body. After development, which can last from a few weeks to several months depending on the species, the mature third-instar larva must exit the host. It drops to the ground, burrows into the soil, and begins the pupal stage, transforming into a non-parasitic adult fly.
Diverse Methods of Host Infestation
Botflies utilize various strategies to ensure their eggs or newly hatched larvae reach a suitable mammalian host. One unusual method is phoresis, where the female botfly captures an intermediate arthropod, such as a mosquito or tick, and glues her eggs to its body. When this vector lands on a warm-blooded host, the temperature change causes the eggs to hatch rapidly. The tiny larvae then penetrate the host’s skin through the bite wound or a hair follicle.
Another common method involves ingestion, exemplified by the horse botfly (Gasterophilus intestinalis). The female deposits eggs onto the hair of the horse’s legs or shoulders, where they remain until the host accidentally licks them. The moisture and friction of the tongue trigger the eggs to hatch. The larvae migrate to the mouth, eventually burrowing into the stomach lining to continue their parasitic development.
Other species, such as the cattle grubs (Hypoderma spp.), employ direct laying by attaching eggs to the host’s hair, usually around the legs or belly. The hatched larvae burrow directly through the skin and begin a migratory path through the host’s internal tissues. This diversity in entry strategy—by vector, ingestion, or direct penetration—determines the larva’s initial location and subsequent path within the host.
Pathology and Clinical Impact on Hosts
The presence of botfly larvae within a host causes a condition known as myiasis, which is the infestation of living tissue by fly larvae. The clinical impact varies based on the botfly species and the part of the body the larva inhabits. In cases of dermal myiasis, such as with the human botfly (Dermatobia hominis), the larva creates a painful, dome-shaped nodule resembling a boil, often called a warble.
This lesion features a small, central opening the larva uses as a breathing hole. The host’s immune system reacts by forming a fibrous capsule around the parasite. The larva feeds on exudates and surrounding tissue for several weeks, causing localized pain, irritation, and a sensation of movement beneath the skin. If the larva is not removed, it eventually exits the host through the opening to pupate, leaving a wound that risks secondary bacterial infection.
In livestock, the pathology often extends beyond the skin and can cause significant economic losses. Cattle grubs migrate internally for months, sometimes damaging the esophageal wall or spinal cord before settling under the skin to form warbles, which damages hide quality. Horse botfly larvae attach to the lining of the stomach or intestine, causing ulceration and erosion that can lead to digestive issues or blockages in severe infestations. The sheep nose botfly (Oestrus ovis) deposits larvae directly into the nostrils. These larvae then migrate into the nasal passages and sinuses, causing intense irritation, discharge, and sometimes neurological symptoms in the host.