This fly’s flesh-eating maggot is making a comeback. Here’s what to know 

A type of flesh-eating maggot is making a comeback — and it’s creeping toward the United States.

The new world screwworm is a blowfly whose larvae burrow into animal wounds and gorge on living tissue. This parasitic behavior poses a threat to livestock, pets, wildlife and, in rare cases, humans. For decades, the United States, Mexico and Central America have successfully collaborated to eradicate screwworms from the region. But these gash-guzzling maggots are having a surprising resurgence. 

In 2023, the number of new world screwworm cases in Panama leapt from 25 to more than 6,500. The maggots have since been spotted in eight countries, wriggling closer to the United States. By late September, the nearest screwworm case was reported only about 110 kilometers from the U.S.–Mexico border. The threat has heightened screwworm surveillance in the United States. And the U.S. Food and Drug Administration has approved the first screwworm prevention drugs for cattle and dogs. 

“The screwworm is incredibly damaging to our livestock, our companion animals, our wildlife and even ourselves,” says Phillip Kaufman, a veterinary entomologist at Texas A&M University in College Station. “It’s a very nasty fly, and it’s why we want to make sure that we can re-eradicate it like we did before.”

As the United States ramps up defenses against screwworms and the creature creeps closer to vulnerable border states, it raises the question: How concerned should we be about screwworms? 

To learn more, Science News spoke with Kaufman and Yan Zhang, a veterinary microbiologist at the University of Arizona in Tucson, about what’s going on to slow the spread of the pest and the damage it can cause.

What is a new world screwworm? 

Cochliomyia hominivorax is a blowfly that feeds on living tissue. Female screwworms, about the size of an average housefly, are attracted to the scent of open wounds. They can lay up to 3,000 eggs, depositing a few hundred at a time in cuts as small as tick bites or in orifices such as ears, noses and mouths. Maggots emerge from the eggs in less than 24 hours and promptly start screwing deeper into the animal’s flesh with their hooked mouths, Kaufman says. 

The burrowing process is painful. Larvae tear through living tissue, leaving dead, rotting flesh in their wake. Foul-smelling wounds that do not heal are telltale signs of a screwworm infestation, and untreated wounds often develop into fatal infections. 

Livestock are the most commonly reported animals infected with screwworms, but the flies aren’t picky about their hosts. Any warm-blooded animal, including deer, dogs, wildlife or even humans, will do. 

“The fly doesn’t care what type of mammal that it parasitizes,” says Kaufman, who was involved with responding to a 2016 screwworm outbreak in Key deer on Florida’s Big Pine Key. “It just really likes mammals.”

After feasting for about a week, the maggots fall to the ground, dig shallow burrows and transform into adults. Full-grown flies have bulging orange eyes and metallic blue bodies with three black stripes on their backs. 

Why are these pests on the rise? 

More than 50 years ago, the U.S. Department of Agriculture squashed the spread of screwworms in the United States. The USDA, along with agriculture departments in Mexico and Panama, created fly factories — facilities that produced sterile male screwworm flies. Each week, millions of sterile flies were loaded onto airplanes and released in infested areas. After encountering barren males, wild females — which mate only once in their lifetime — produced eggs that would never hatch, Kaufman says. 

This strategy successfully shrank screwworm populations. By 2006, eradication programs had controlled infestations from the United States to the Darién Gap, a natural barrier in Panama that suppressed screwworms from traveling north from South America, where they are endemic. 

More than 20 years later, cases are climbing in Central and North America again. The Mexican government has reported more than 9,000 screwworm infections since last November. The cause of this resurgence is unknown. “It was actually surprising to us,” Zhang says. “Within two years, they have moved very fast, and quite far.”

Experts suspect that a combination of factors led to the sudden uptick. One possibility is that the COVID-19 pandemic disrupted production in the fly factories. Lax animal inspections and deforestation in the Darién Gap may have also helped screwworms travel to different countries. The flies on their own can travel only about 20 kilometers a day, Kaufman says. They probably gained more distance by hitching rides in cattle transported across international borders.

“How we go from just a couple of cases to lots of them is when people are not checking animals,” Kaufman says. “This creates a lot of opportunities for the fly while we aren’t trying to control them.”

Other possible causes include climate change, underreported wildlife cases or livestock management practices such as dehorning or tail docking that produce wounds.

What’s at stake? 

Officials have yet to detect screwworms in the United States. But recent cattle infections in Mexico, located about 110 and 270 kilometers south of the U.S. border, indicate the pest may be expanding northward. The threat to human lives, livestock, pets and the economy follows close behind.

Screwworm infections in humans are rare, but they do happen. Since the current outbreak started in 2023, Mexico and Central America have reported more than 830 human cases as of September 26. The United States saw its first human infection in a Maryland resident, who encountered the pest while traveling in El Salvador last August. Treatment typically involves health care providers removing the maggots from the wound.

The pest also threatens local economies in border states that rely on livestock. The USDA projected that a hypothetical outbreak in 2024 would have cost Texas $1.8 billion in losses.

Kaufman says that the recent cattle infections found near the U.S. border were isolated in one or two animals. The incidents were discovered in routine inspections and were quickly resolved. The event triggered a five-prong protection from the USDA, which included building a new screwworm factory in Mexico and setting nearly 8,000 traps along the border.

“We should be concerned, but not overly worried,” Kaufman says. “We have a good handle on what we’re doing. We have surveillance happening all along the [Texas-Mexico] border, from Brownsville to Del Rio.” 

While the United States is closely monitoring the situation and is prepared for a potential outbreak, Kaufman says, it may take several years to fully eradicate the fly in the region.