Screwworm – Parasitic Fly
Screw-worm, Cochliomyia hominivorax, Produces Carnivourous Maggots
The New World screwworm fly, Cochliomyia hominivorax has parasitic maggots that infest animals and people and cause terrible tissue destruction.
Cochliomyia hominivorax is commonly known as the screwworm (often incorrectly spelled screw-worm) fly. Both the scientific name and the common name are indicative of the horrific nature of the insect: "hominivorax" literally means “man eater,” and "screwworm" doubtless refers to the way in which the feeding maggots position themselves mouthparts-down in an open wound. The activities of the voracious maggots explain why people fear the screwworm fly wherever it occurs.
Life cycle of C. hominivorax
Screw-worm flies pupate (the stage where the larva, or maggot, develops into a fly) several inches down in warm soil and work their way up to the soil surface when they have completed their metamorphosis. They mature quickly and then search for mates. Males linger near flowering shrubs and other plants, where they feed on flower nectar, while females often fly long distances until they encounter a male. The female screw-worm fly mates only once, but produces several clusters of eggs.
Female screwworm flies are carnivorous – they feed on the seepage and drainage produced by wounds on warm blooded animals. They also lay their eggs on an animal near the site of a wound or abrasion. Tick bites and the umbilical cords of newborn animals are typical choices—even a lesion as small as a fly bite, or the moist environment of a human nasal sinus will suffice for C. hominivorax. As soon as they hatch, the maggots crawl into the wound and begin to feed.
Feeding screwworm maggots enlarge the wound and make it vulnerable to bacterial infection. They also make the wound even more attractive to mated female screwworm flies, and subsequent clusters of eggs are often deposited there, causing an increasing number of feeding maggots and increasing tissue destruction. Terrible wounds and scarring result. Untreated animals and humans often die.
When the maggots have fed sufficiently, they drop out of the wound, bury themselves in the soil and pupate—to emerge shortly as adult flies and start the cycle over again. In warm weather, the entire life cycle takes only about three weeks; therefore, during the summer season, C. hominivorax can fly very long distances and spread far outside its tropical winter range.
Where are screwworm flies found?
Screwworm flies were once widely distributed in northern South America, Central America, and the American South. The flies are able to survive the winter in southern Texas and Florida, and spread north each summer, inflicting great suffering on wild animals, agricultural livestock, and humans. Eradication efforts have pushed the fly southward so that it is now contained in South America.
Read about the Cochliomyia hominivorax eradication program.
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Sources:
Food and Agriculture Organization of the United Nations. The New World Screwworm Eradication Programme: North Africa 1988-1992. Rome: FAO, 1992.
Schmidt, Gerald D. and Larry S. Roberts. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
