The Primary or New World Screwworm, Cochliomyia hominivorax (Coquerel), is an obligate parasite (meaning it must pass part of its life cycle in a living host) of warm blooded animals, including humans. The species name, hominivorax, in fact means "man-eater" and was given by the French physician, Dr. Charles Coquerel, in 1858 describing the species that he identified as responsible for the deaths of hundreds of prisoners of the Devil's Island penal colony in French Guiana. Once an animal becomes infested with screwworm, death almost inevitably results unless the wound is cured.
 

The female fly typically lays an egg mass containing 200-300 eggs on or near an open wound on a living animal. The eggs hatch in 10-12 hours and the first stage larvae (also known as maggots or worms, each approximately 0.04" or 1 mm in length) crawl into the wound and begin tearing at the host's tissue with a pair of sharp mouth hooks. The larvae feed on the resulting liquid that oozes into the wound. Once a wound has become infested with larvae, it also becomes more attractive to female screwworm flies ready to lay eggs. As a result, a wound may become infested with hundreds to thousands of larvae. The larvae feed in the wound for about 5 days and pass through two additional stages before they crawl out of the wound and drop to the ground. Once on the ground, the larvae (now approximately 0.2-0.7" or 6-17 mm in length) burrow into the soil to a depth of about 1-2 inches and form the pupal stage. After about 7 days, the adults (flies) exit the pupae and make their way to the surface of the soil. Both sexes range in color from dark metallic blue to metallic bluish green to metallic green and have three dark longitudinal stripes (the middle stripe slightly shorter than the outer stripes) on their backs between their wings. Both sexes are larger than house flies with males being about 0.4-0.5" (10-12 mm) and females about 0.3-0.4" (8-10 mm) in length. They then expand and dry their wings and fly away. After about 3 days the males and females mate and females begin the cycle of laying eggs on the living hosts. Females may be capable of laying a mass of eggs once every three days for up to 10 or 11 times during their approximately month long adult life span.
 

Previous to the Screwworm Eradication Program, screwworm was present in every country of the Western Hemisphere. Screwworm was limited in its distribution in both the Northern and Southern Hemisphere by freezing temperatures during the winter. Also, screwworm is limited by the cold temperatures at higher altitudes (about 7,874 ft or 2,400 m above mean sea level). Studies by our scientists in the tropics suggest that forests are preferred habitats of the screwworm fly. More eggs are laid on animals located in forests than pastures or other non-forested areas and more flies are collected in forested areas versus non-forested areas. Recent studies suggest that more flies are collected in tropical forests that are 15-20 years old and along the edges of forests than other forested areas. More flies are collected during the rainy season than in the dry season and more flies are collected from wet or humid habitats than dry habitats.