Tufts Veterinary Medicine

A flesh-eating parasite that has reemerged in the United States after more than a half-century has killed a fifth of the endangered Key deer on the Florida Keys since last fall. In January, a stray dog near Miami was successfully treated for these New World screwworm parasites, fly larvae that feast on open wounds in warm-blooded animals.

Cynthia Duerr, V99, led field operations for the U.S. Panama Commission for Eradication and Prevention of Screwworm from 2008 to 2011 as an agricultural science officer with the U.S. Department of Agriculture. Now she’s working with 24 countries in Africa to help protect livestock from Old World screwworm, a close cousin of the pest identified in Florida, and from other contagious diseases. The last reported case of screwworms in the United States occurred in 1982 in Star County, Texas.

Duerr talked with us about the history of screwworms in the United States and the implications of the Florida infestation.

A screwworm fly.

What is screwworm?

Screwworm is the larval state of the screwworm fly, known as Cochliomyia hominivorax. These flies feed on flowers and plants, but lay eggs at the edges of wounds on warm-blooded animals, including people. The flies can detect a wound from a long distance, and they release pheromones that attract even more flies. Hundreds and thousands of eggs can hatch into larvae that burrow into the wound. The wound can grow quite large because of this infestation and can be fatal if not treated—hence the fly’s Latin name hominivorax, which roughly translates as “man-eating.” Human cases of screwworm are rare, but they have occurred.

Why is the arrival of screwworms in Florida such big news?

This was immediately worrisome, because we have not seen screwworms there for more than 50 years. They were found regularly in Florida and the southern United States until the mid-1960s. During the summer months, screwworms could be found as far north as Nebraska and even Canada. It caused great expense to the cattle industry. Cowboys did more than move animals from place to place in those days; they treated wounds to prevent screwworms.

After World War II, two USDA researchers, Raymond Bushland and Edward Knipling, developed a method called the sterile insect technique, which used radiation to sterilize hordes of screwworms. Female screwworms mate only once in their lifetimes, so when they mated with the sterile males, no larvae were produced. The continued release of sterile flies into the wild essentially meant that females were more likely to find a sterile mate, and over time, there were fewer and fewer fertile flies until they were eradicated.

By 1966, the United States was largely screwworm free, saving $900 million annually in costs associated with the production of livestock.

Why has screwworm shown up in the United States again?

While adult screwworm flies can travel long distances, they do not generally travel over open water. It is therefore likely that screwworms hitched a ride.

Is this a serious conservation issue for the Key deer?

I am afraid it is. Because wildlife are not monitored for wounds, these animals are more vulnerable. So far, 135 Key deer, from an estimated population of 700 to 800, have died from wounds associated with this parasite. Efforts are being made to treat the remaining animals with an oral antiparasitic medicine.