Flies and Maggots

Causative Agent
  • Mammals, birds and livestock are prone both to the bites and larvae of several species of flies that feed on flesh, blood or secretions.
  • The main groups that are important for wildlife are:
    • Calliphorid flies - common examples include bluebottle and greenbottle flies, and bird blow flies;
    • Bot and warble flies (Cuterebrinae and Hypodermatinae) - common examples include bot flies of rodents;
    • Nasal and pharyngeal bot flies (Oestrinae) - common examples include the nasal bot fly of caribou (Rangifer tarandus).
Click on images to enlarge.
Warble Warble Bird Blow Fly
Larval warble fly underneath the skin of a caribou. Close-up of a larval warble fly. Larval blow flies that parasitize nestling birds often cling to featherless areas of the bird, while some species embed in tissues.
  • In most areas of North America. 
  • Dependent on species; throughout the year in southern regions. 
Hosts and Life Cycle
Calliphorid flies:
  • Eggs are laid on wounded skin, infected skin, or skin soiled with feces.
  • Larvae feed within the wounds and then drop to the ground to develop into adults and continue the cycle.
  • Female bird blow flies (Protocalliphora) lay eggs in nests of birds, and the larvae live within the nesting material, feeding intermittently on the blood of nestlings. One species (Trypocalliphora braueri) burrows under the skin of nestling birds, often with detrimental consequences.
Bot flies:
  • In North America, bot flies typically parasitize rodents and rabbits.
  • Female flies deposit eggs at entrances to burrows and other areas frequented by hosts.
  • Eggs hatch in response to cues generated by the host (e.g., increased environmental temperature, carbon dioxide and moisture from the host).
  • Hatched larvae gain entrance to the host through the mouth, nasal openings or cuts in the skin. In infected rodents, larvae are commonly located in the region of the groin but may also occur beneath the skin on other parts of the body such as the neck.
  • In cervids such as caribou, the adult fly lays eggs on the hairs of the caribou’s legs and lower body. Hatched larvae penetrate the skin, and travel under the skin to the caribou’s back where they mature.
  • Larvae develop under the skin until late summer at which time they emerge from the host through a breathing pore in the skin called a warble.
  • Newly hatched larvae measure from 2 to 4 mm long and are grayish-white; larvae mature to a dark brown color just prior to emergence from the host. Fully grown larvae will measure from 20 to 42 mm in length and up to 7 to 10 mm in width.
  • Pupation (development into the adult stage) occurs in the soil and the fly over-winters in the pupal stage.
  • Adults emerge in summer and will mate and lay eggs within a few days.
Nasal bot flies:
  • In North America, the larvae of nasal bot flies are found in the nasal passages and throat pouches of cervids such as caribou, moose, elk, white-tailed and mule deer as well as bighorn sheep.
  • Female nasal bot flies deposit larvae into the nostrils of the host during hot days of summer where early stages of larval growth take place.
  • After a short period, larvae move to pouches that lie on either side of the throat at the base of the tongue where further growth takes place.
  • Fully developed larvae are about 25-36 mm in length and, at maturity, are expelled from the throat to pupate in the soil. Adults emerge from the soil 2-3 weeks later.
  • Nasal bot flies overwinter in the larval stage within the cervid host.
Signs and Symptoms
Calliphorid flies:
  • Fly larvae can cause direct damage to tissues which may also lead to the development of secondary infections from bacteria.
Bot flies:
  • The early stages of infection by bot flies are rarely evident from the outside, and often are only detected by touch or after sufficient growth has occurred so that they are visible.
  • In smaller mammals, such as chipmunks, the larvae often produce an obvious awkwardness in locomotion, which may render them more susceptible to predation.
  • As with Calliphorid flies, wounds made from bot fly larvae may become infected with bacteria.
  • Paralysis and death may occur in rodents or rabbits if the larvae migrate through the brain or spinal cord.
  • Animals infected with warbles usually appear healthy, although those with heavy infections may appear weak and there may be damage to the hide quality. While laying their eggs, warble flies harass animals and interfere with feeding.
Nasal bot flies:
  • Larvae or "bots" can cause inflammation and edema, occasionally leading to local damage or to pneumonia. Difficulty with breathing may result when the infestation is particularly heavy.
  • Nasal bot flies cause annoyance to cervids that can lead to loss of body condition due to inadequate feeding. Animals under attack generally cluster, or may become nervous or panic-stricken, even stampeding.
  • Larvae often cause local inflammation in the infected pouches.
Meat Edible?
  • The meat of animals infected with larvae is considered safe for human consumption following removal of any affected tissue. Animals in poor condition are likely to have poor-quality meat.
Human Health Concerns and Risk Reduction
  • Some Calliphorid flies can be annoying pests of humans, and may occasionally invade human tissues.
  • There are cases reported worldwide of nasal bot fly larvae infecting the eyes of humans.
  • There are occasional reports of bot flies infecting humans. These cases tend to occur during late summer, incidental to outdoor activities, when adult flies are most numerous.
Samples for Diagnosis
  • Collection of the flies or pupae is sufficient for diagnosis.
Further Reading
Return to Manual Home Page Disease List - Body Region Affected Disease List - Causative Agent or Risk Factor Disease Surveillance Form Glossary Contact Information
Return to Manual Home Page Return to Disease List - Body Region Affected Return to Disease List - Causative Agent or Risk Factor Disease Surveillance Form Download Glossary Contact Information