First Case of Flesh-Eating Screwworm Parasite Raises Questions Over Spread

The United States has confirmed its first human case of the New World screwworm, a
flesh-eating parasite previously eradicated from North America more than half a century
ago. Federal health officials stressed that the infection, detected in a traveler returning
from El Salvador, does not pose a significant risk to the wider public, but its
reappearance has reignited concerns over whether the fly could re-establish itself.

The screwworm, Cochliomyia hominivorax, is a species of blowfly whose larvae feed on
the living tissue of warm-blooded animals. Female flies lay eggs around wounds, bites
or surgical openings. Within hours, the larvae hatch and burrow into the flesh, using
sharp mouth hooks to create spiral-like tunnels that enlarge the wound. Untreated
infestations can be fatal in livestock and, in rare cases, humans.

Unlike ordinary maggots, which consume decaying flesh, screwworms thrive on fresh,
living tissue, making them uniquely destructive. A single female can deposit hundreds of
eggs, and the life cycle of the fly completes in little more than two weeks.

The parasite was once a major scourge of American agriculture, costing cattle
producers millions of dollars annually. A pioneering eradication campaign in the 1950s
and 1960s, based on the mass release of sterilized male flies, successfully eliminated
screwworms from the U.S. and, later, from Mexico and much of Central America. Until
now, no human infections had been reported in the U.S. since eradication.

Recent outbreaks in Central America, however, have altered the risk landscape.
Panama reported thousands of livestock cases in 2023, and the parasite has since
advanced north into southern Mexico. U.S. authorities are responding with renewed
vigilance. The Department of Agriculture has launched a five-point strategy that includes
ramping up sterile fly production at a facility in Texas, deploying border patrol “tick
riders” and detector dogs, and coordinating closely with Mexican counterparts.

While the confirmed human case remains isolated, officials warn that the greatest threat
is to livestock. An established outbreak in Texas alone could cause losses estimated at
nearly $2bn. For humans, screwworm infections remain exceedingly rare, non-
contagious and treatable if caught early.

Scientists say the appearance of a travel-associated case is a reminder that globalised
movement and climate shifts can reintroduce old threats. For now, the U.S. response
aims to prevent the screwworm from gaining any foothold, drawing on the hard-won
lessons of the past.

Photo – Larvae of the screwworm fly, collected from infected cows in Panama on June 11.Enea Lebrun / Reuters