Screw-worm flies as pests

The screwworm is an insect pest. The larvae of the fly feed on the flesh of warm-blooded animals, including man, and can cause serious losses to livestock production in countries where it is endemic. The female screwworm fly lays 200-300 eggs in animal or human wounds - a tiny tick bite is a sufficient environment - and after about 12 hours the larvae hatch an eat into the wound, tearing at the flesh to produce a fluid on which they feed. After five to seven days the larvae drop to the ground and burrow into the soil to pupate. The damage done by the feeding larvae is considerable, and because they enlarge the wound they make it more attractive to further infestation. In most cases, unless the animal is treated it will slowly die. Large outbreaks occur during calving season when the screwworm lays eggs in fresh umbilical wounds. In areas of Texas up to 80% of newborn infested fawns were killed.

Screwworm is the name for both Cochliomyia hominivorax (the true screwworm) and C. macellaria, and the confusion accounted for the failure of control programmes until the distinction was made in the 1930s. Indirect evidence of the New World Screwworm dates back to the early 1500s when prisoners captured by Hernando Cortez fell victim to el gusano de muerto (the worm of death) which infested wounds left by branding. In the mid-19th century, the alarming death rate among prisoners at the penal colony on Devil's Island in French Guiana was caused in part by screwworm which lay eggs in prisoners' nostrils; the hatched larvae would then bore their way into the nasal sinuses.

In 1960, the cost of the screwworm infestation in the USA was estimated at $100 million a year. Clearing Mexico and southern USA of the pest took more than 20 years and cost nearly $700 million. The means was by selective release of male flies sterilized with radiation and quarantine and treatment of animals with wounds. The arrival of the New World Screwworm fly in Libya in 1988-9 represented a grave threat to the African continent, and from there into south-eastern Europe, the Middle East and Asia. In the Americas, the screwworm is primarily a livestock pest, but in Africa it could have claimed a great many human lives and vast numbers of wildlife as well. The outbreak was controlled with a $25 million programme.