Screwworms are fly larvae (maggots) that feed on living flesh. These parasites infest all mammals and, rarely, birds. Two different species of flies cause screwworm myiasis: New World screwworms (Cochliomyia hominivorax) occur in the Western Hemisphere, and Old World screwworms (Chrysomya bezziana) are found in the Eastern Hemisphere. However, the climatic requirements for these two species are similar, and they could become established in either hemisphere. New World and Old World screwworms have adapted to fill the same niche, and their life cycles are nearly identical. Female flies lay their eggs at the edges of wounds or on mucous membranes. When they hatch, the larvae enter the body, grow and feed, progressively enlarging the wound. Eventually, they drop to the ground to pupate and develop into adults. Screwworms can enter wounds as small as a tick bite. Left untreated, infestations can be fatal. Screwworms have been eradicated from some parts of the world, including the southern United States, Mexico and Central America, but infested animals are occasionally imported into screwworm-free countries. These infestations must be recognized and treated promptly; if the larvae are allowed to leave the wound, they can introduce these parasites into the area.
Iowa State University
Iowa State University Center for Food Security and Public Health, "Screwworm Myiasis" (2016). Center for Food Security and Public Health Technical Factsheets. 120.