Botfly infestations are caused by parasitic fly larvae burrowing into the skin of mammals, where they feed and grow for weeks. Unlike most parasitic insects, the human botfly doesn’t land on you directly. Instead, it hijacks mosquitoes and other biting insects to deliver its eggs, making the transmission method one of the strangest in the insect world.
How Human Botflies Use Mosquitoes to Reach You
The human botfly (Dermatobia hominis) has evolved an unusual strategy called phoresy, essentially using another insect as a taxi service. A female botfly captures a mosquito midair and glues up to 24 eggs onto the mosquito’s abdomen. When that mosquito later lands on a person to feed, the sudden rise in skin temperature triggers the botfly eggs to hatch. The newly hatched larvae then crawl onto the skin and enter through hair follicles or tiny openings created by the mosquito bite itself.
Mosquitoes are the most common carriers, but ticks and other flies can serve the same role. This indirect delivery system is unique among all flies that cause myiasis (the medical term for larvae living inside a host). It also means you won’t see a botfly coming. You’ll feel a mosquito bite, and only days later notice something unusual developing beneath your skin.
Where Botflies Are Found
Human botflies are endemic to warm, humid regions stretching from southern Mexico through Central America and into South America, as far south as northern Argentina. Belize is a particularly common hotspot. Most cases in the United States, Canada, and Europe are travel-related, showing up in people who’ve recently returned from these regions. If you haven’t traveled to Central or South America, a botfly infestation is extremely unlikely.
What Happens Inside the Skin
Once a larva enters the skin, it settles into a pocket just beneath the surface and begins feeding on tissue. It maintains a small breathing hole, called a punctum, that stays open to the outside air. Over 5 to 10 weeks, the larva grows steadily, and you’ll typically notice a raised, boil-like bump that doesn’t heal the way a normal wound would. Many people report feeling intermittent sharp or crawling sensations, which is the larva moving inside the cavity.
When the larva matures, it backs out through the same breathing hole, drops to the ground, and burrows into soil to pupate. About a month later, an adult botfly emerges to mate and start the cycle over. If the larva isn’t removed, it will eventually leave on its own, but the process takes weeks and carries a risk of secondary bacterial infection in the wound it leaves behind.
How Botflies Affect Dogs, Cats, and Horses
Pets and livestock face botfly species of their own. In North America, Cuterebra flies target rabbits and rodents, but dogs and cats frequently become accidental hosts. Female Cuterebra flies lay eggs near the openings of animal burrows, along runways, or on rocks and vegetation in those areas. When a curious dog or cat passes through, body heat triggers the eggs to hatch. The larvae typically enter through the mouth or nose during grooming, or through open wounds, then migrate under the skin and form the same type of breathing-hole cyst seen in human cases.
Horses deal with a different group of botflies entirely. Adult flies glue tiny eggs directly onto the horse’s leg hairs, neck, and withers. When the horse licks or grooms itself, it ingests the eggs. The larvae then spend up to 21 days migrating through the tissues of the tongue before relocating to the stomach wall, where they attach and feed for months. This internal migration can cause inflammatory reactions in the mouth and stomach lining.
How Botfly Larvae Are Removed
The most common home method involves covering the breathing hole with an occlusive substance like petroleum jelly. This cuts off the larva’s air supply and theoretically forces it to wriggle toward the surface, where it can be grasped and pulled out. One published case report in BMJ Case Reports documented successfully extracting a larva this way using Vaseline over the punctum. Some travelers in endemic areas use strips of raw bacon or tape with the same goal of suffocation.
This approach doesn’t always work cleanly. If the larva breaks apart during removal, retained fragments can trigger infection or a strong inflammatory response. Medical extraction, where a small incision is made to widen the opening and the larva is pulled out intact, tends to be more reliable. Either way, the wound typically heals well once the larva is fully out and the site is kept clean.
Preventing Botfly Infestations
Because the eggs arrive via mosquitoes, preventing mosquito bites is effectively the same as preventing botfly exposure. DEET-based repellents are the standard recommendation. Products range from 5% to 99% DEET concentration, and higher concentrations last longer, repelling mosquitoes for anywhere from 2 to 12 hours depending on the formula. Apply repellent to all exposed skin when spending time outdoors in endemic regions.
Long sleeves, long pants, and permethrin-treated clothing add another layer of protection, particularly in rural or jungle settings where mosquito density is high. Sleeping under mosquito nets is worth the effort, since mosquitoes carrying botfly eggs are most active at dawn and dusk. For pet owners, keeping dogs and cats away from rodent burrows and checking them regularly for unusual lumps reduces the risk of Cuterebra infestation.