What is screwworm?

In late July the Screwworm Coalition of Texas, a partnership of over a dozen farming and wildlife groups, launched a website to provide public resources on an emerging and serious threat to Texas cattle: the New World screwworm. Economists worry a resurgence of the flesh-eating parasite in the United States decades after being eradicated may lead to a spike in beef prices.

What is the New World screwworm? Known scientifically as Cochliomyia hominivorax, the pest is a tropical fly that lays eggs in the open wounds of warm-blooded animals. Once hatched, the larvae use hook-like mouths to burrow into and feed on its host. A female fly may lay more than 2,000 eggs during her weeks­long lifespan, according to the Centers for Disease Control and Preven­tion. As more larvae hatch, wound sites grow deeper and threaten the health of the host animal. After about a week of feeding, the larvae drop to the ground and burrow into the soil before emerging as mature flies.

Gross. Which animals are at risk? Any warm-blooded animal may be infected, and any open wound site is vulnerable. The flies may take advantage of a simple tick bite, a buck’s annual antler shedding, or the umbilical cord site on a newborn mammal. Historically, farmers intentionally timed livestock procedures like branding, castration, and birthing to avoid fly season.

Can humans be infected? Yes, although human cases are very rare. According to COPEG, the U.S.-Panama commission established to monitor and eradicate the fly, fewer than 1% of known infections in the current outbreak have occurred in humans.

How can we combat them? The most effective method of eradication is by sterilization: Scientists sterilize male flies and then release them to mate with females, which then lay unfertilized eggs. Because the female screwworm fly will only mate once in her lifespan, the population in the area ultimately dies out. Treating infected animals, quarantining livestock, and using pesticides may also slow the fly population’s spread.

Why is screwworm suddenly a risk? Officials eradicated the fly from the United States in the late 1960s, working with foreign leaders to exterminate the pests as far south as the Darién Gap near the Panama-Colombia border. Mexico was declared screwworm-free by 1991, and all of Central America by 2006, with a sterile fly barrier between Panama and Columbia. The Darién Gap boundary held for years despite endemic infestations in South America and the Caribbean. In 2023, average screwworm detections along the Panama boundary jumped from 25 to over 6,500. Since then, cases have been reported across Central America and this summer as far north as Veracruz, Mexico—only 370 miles south of Texas.

What is the U.S. government doing about it? Agriculture Secretary Brooke L. Rollins stopped all shipments of live horses, cattle, and bison from Mexico in May. By June, Rollins announced plans for an $8.5 million sterile screwworm fly dispersal facility in the Moore Air Base in South Texas. The facility, expected to take about six months to complete, will complement sterile fly production already underway in Panama and Mexico, according to Rollins’ readiness plan. Meanwhile, Texas Agriculture Commissioner Sid Miller plans to deploy traps baited with a synthetic scent of rotting flesh to trap and kill adult screwworm flies in possible hot-spot areas.