Fascinatingly Gross Botfly Facts

Common botfly or warble fly: Hypoderma lineatum male.
Common botfly or warble fly: Hypoderma lineatum male. London Scientific Films / Getty Images

The botfly is a type of parasitic fly, best-known for disturbing images of its larval stage in skin and horror stories of infested people. The botfly is any fly from the family Oestridae. The flies are obligate internal mammalian parasites, which means they can't complete their life cycle unless the larvae have a suitable host. The only species of botfly that parasitizes humans is Dermatobia hominis. Like many species of botfly, Dermatobia grows within the skin. However, there are other species that grow within the host's gut.

Fast Facts: Botfly

  • Common Name: Botfly or Bot Fly
  • Scientific Name: Family Oestridae
  • Also Known As: Warble flies, gadflies, heel flies
  • Distinguishing Features: Hairy fly with a metallic "bot" appearance. Infestation is characterized by an irritated bump, with a hole in the center for the larval breathing tube. Movement may sometimes be felt within the lump.
  • Size: 12 to 19 mm (Dermatobia hominis)
  • Diet: Larvae require mammalian flesh. Adults do not eat.
  • Lifespan: 20 to 60 days after hatching (Dermatobia hominis)
  • Habitat: The human bot fly lives primarily in Central and South America. Other botfly species are found worldwide.
  • Conservation Status: Not evaluated
  • Kingdom: Animalia
  • Phylum: Arthropoda
  • Class: Insecta
  • Order: Diptera
  • Family: Oestroidae
  • Fun Fact: Botfly larvae are edible and are said to taste like milk.

How to Recognize a Botfly

With its hairy, striped body, you could say a botfly looks like a cross between a bumblebee and a house fly. Othersv liken a botfly to a living "bot" or miniature flying robot, because the reflective hairs give the fly a metallic appearance. The human botfly, Dermatobia, has yellow and black bands, but other species have different coloration. The human botfly is about 12-19 mm in length, with hair and spines on its body. The adult lacks biting mouthparts and does not feed.

In some species, botfly eggs are easily identified. For example, equine botflies lay eggs that resemble tiny drops of yellow paint on the horse's coat.

The fly is best known for its larval stage or maggot. Larvae that infest skin grow under the surface, but leave a small opening through which the maggot breathes. The larvae irritate skin, producing a swelling or "warble." Dermatobia larvae have spines, which worsen the irritation.

Bot fly larvae cause a lump with an open center, through which the breathing tube may be visible.
Bot fly larvae cause a lump with an open center, through which the breathing tube may be visible. Petruss

Where Do Botflies Live?

The human botfly lives in Mexico, Central America, and South America. People who live in other areas generally get infected while traveling. Other species of botfly are found across the globe, primarily (but not exclusively) in warm tropical and subtropical regions. These species infest pets, livestock, and wild animals.

The Botfly Life Cycle

Cuterebra sp. botfly larva extracted from the neck of a dead rabbit.
Cuterebra sp. botfly larva extracted from the neck of a dead rabbit. Katja Schulz

The bot fly life cycle always involves a mammalian host. Adult flies mate and then the female deposits up to 300 eggs. She may lay eggs directly on the host, but some animals are wary of botflies, so the flies have evolved to use intermediate vectors, including mosquitoes, houseflies, and ticks. If an intermediate is used, the female grasps it, rotates it, and attaches her eggs (under the wings, for flies and mosquitoes). When the botfly or its vector lands on a warm-blooded host, the increased temperature stimulates the eggs to drop onto the skin and burrow into it. The eggs hatch into larvae, which extend a breathing tube up through the skin to exchange oxygen and carbon dioxide. The larvae (instars) grow and molt, finally dropping from the host into the soil to form pupae and molt into adult flies.

Some species do not develop in skin, but are ingested and burrow into the host's intestine. This happens in animals that lick themselves or rub their nose on body parts. After several months to a year, the larvae pass through the feces to complete its maturation process.

In most cases, bot flies do not kill their host. However, sometimes the irritation caused by the larvae leads to skin ulceration, which can result in infection and death.

How to Remove Botfly Larvae

Botfly larvae Hypoderma diana under a deer's skin.
Botfly larvae Hypoderma diana under a deer's skin. Avalon_Studio / Getty Images

Infestation with larval flies is termed myiasis. While it's a characteristic of the botfly life cycle, it actually occurs with other types of flies, too. There are several methods used to remove fly larvae. The preferred method is to apply a topical anesthetic, slightly enlarge the opening for the mouthparts, and use forceps to remove the larvae. Other methods include:

  • Using a venom extractor syringe from a first aid kit to suck the larvae from the skin.
  • Oral dosing with the antiparasitic avermectin, which leads to spontaneous emergence of the larvae.
  • Flooding the opening with iodine, which causes the fly to poke out of the hole, facilitating its removal.
  • Applying the sap of the matatorsalo tree (found in Costa Rica), which kills the larvae, but does not remove it.
  • Sealing the breathing hole with petroleum jelly, white glue mixed with insecticide, or nail polish, which suffocates the larvae. The hole is enlarged and the carcass is removed with forceps or tweezers.
  • Applying adhesive tape to the breathing hole, which sticks to the mouthparts and pulls out the larvae when the tape is removed.
  • Forcefully squeezing the warble from the base to push the larvae through the opening.

Killing the larvae before removal, squeezing them out, or pulling them out with tape is not recommended because rupturing the larvae body can cause anaphylactic shock, make removal of the entire body more difficult, and increase chance of infection.

How to Avoid a Botfly Infestation

The easiest way to avoid getting infested with botflies is to avoid where they live. Since that isn't always practical, the next best tactic is to apply insect repellent to deter flies, as well as mosquitoes, wasps, and ticks that can carry fly eggs. Wearing a hat and clothing with long sleeves and pants helps to minimize exposed skin.

Sources

  • Felt, E.P. (1918). "Caribou Warble Grubs Edible". Journal of Economic Entomology. 11: 482.
  • "Human Bot Fly Myiasis" (2010). U.S. Army Public Health Command (provisional), formerly U.S. Army Center for Health Promotion and Preventive Medicine.
  • Mullen, Gary; Durden, Lance, eds. (2009). Medical and Veterinary Entomology. Amsterdam, NL: Academic. ISBN 978-0-12-372500-4.
  • Pape, Thomas (April 2001). "Phylogeny of Oestridae (Insecta: Diptera)". Systematic Entomology. 26 (2): 133–171. doi:10.1046/j.1365-3113.2001.00143.x
  • Piper, Ross (2007). "Human Botfly". Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Westport, Connecticut: Greenwood Publishing Group. pp. 192–194. ISBN 0-313-33922-8.