Researchers in California are preparing for the possible return of a parasitic flesh-eating fly which once decimated livestock.
The New World screwworm is the larval (or maggot) stage of a blowfly known as Cochliomyia hominivorax. While most blowflies are harmless and feed on decaying carcasses, the screwworm feeds on living tissue.
- These deadly parasites can create suicidal rats, pulsating snails, dancing fish – and can even infect humans
- Parasites: what they are, where they live and how parasites are transmitted
It lays its eggs in open wounds on mammals – including humans, if wounds are left untreated. After the eggs hatch, the larvae burrow into the flesh in a corkscrew motion.
Once common in California and much of the southern United States, the screwworm was wiped out around 60 years ago due to a large-scale programme of releasing male sterile flies. As female screwworms only mate once, this caused the population to collapse and push the species south to Panama.
But the parasite has reappeared in parts of Central America and Southern Mexico, with experts believing that it is being spread in the movement of infested animals.
In August 2025, the BBC reported that the first travel-associated human case of screwworm infestation had been confirmed in the US. It was found in a patient who had returned to the US from El Salvador.
Tackling screwworm
“It hasn’t been found in California yet, but it’s within 70 miles of the Texas border,” said Amy Murillo, an assistant professor of entomology at the University of California Riverside (UCR). “We need to be prepared.”
Murillo is the principal investigator of a new project led by the university, which aims to stop the fly before it can reestablish itself. The project has been funded by the California Department of Food and Agriculture.
Part of the project involves deploying traps across the state that use a special lure (developed by the UCR) that mimics the smell of rotting flesh. The traps will be regularly checked by the researchers for any signs of the screwworm.
The project also includes outreach efforts aimed at veterinarians, livestock handlers and entomologists.
“You don’t have to worry that they’re going to start eating your flesh,” Murillo said. “But we do hope that this project will help people to be more vigilant in recognising and preventing screwworm infestations in animals should they return to California.”
Top image: the New World screwworm. Credit: John Kucharski, Public domain, via Wikimedia Commons
