The United States spent more than half a century and hundreds of millions of dollars driving the flesh-eating New World screwworm as far from its borders as possible. Now, it’s back.
The species can eat the tissue of any warm-blooded animal, but it is a particular threat to livestock and often fatal for cattle. Some environmentally minded bioethicists have debated whether it would be moral to deliberately drive the screwworm into extinction.
“There are some species that it’s worth considering wiping out altogether and I do think the screwworm is one,” said Gregory Kaebnick, a senior research scholar at the Hastings Center for Bioethics.
The Agriculture Department announced last week that the New World screwworm had been found in a calf in Texas , then it reported a second case Friday, discovered about 6 miles from the first . Two more cases were reported Monday , including one in a dog. The discovery represents a worrisome comeback for the species and a failure in containment for the U.S., reprising a decadeslong battle the country waged once already.
Experts said the U.S. will run much the same playbook as it did starting in the late 1950s, when the government embarked on an aggressive, multinational fight against the screwworm. Because female screwworms only mate once, the strategy is to mass-produce sterile males and release them into the wild, where they serve as reproductive dead ends.
“It is a tremendous strategy. It has worked and will continue to work moving forward,” said Chad Cross, a professor of parasitology at the Texas Tech University School of Veterinary Medicine.
He added that the first case found in Texas last week was “a stark reminder of how quickly we need to act to ensure that it doesn’t spread further.”
The screwworm is not a worm at all, but a species of blowfly native to the southern U.S. The flies are attracted to rotting, unkept wounds. Females can lay 200 to 300 eggs, which grow into larvae that look like screws and have special mouth hooks to tear into an animal’s flesh and burrow deeper.
“The larvae that emerge from the eggs consume the flesh of warm-blooded animals,” said Phillip Kaufman, a professor of entomology at Texas A&M University. “It’s a pest of all of our livestock, most of our wildlife and our companion animals, our cats, our dogs and ourselves.”
As screwworm larvae — or maggots — multiply, a wound becomes an open, rotting sore, and the smell sometimes attracts other fly species. Unless the larvae are removed and an animal is given larvicide and antibiotics, the infections are typically deadly.
A cow grazes in Quemado, Texas, on June 2. Brandon Bell / Getty Images For humans, infections are extremely painful but uncommon.
“It’s eating your tissue, whether that be muscle or fat or skin,” Kaufman said. “There is little to no way that you would not know that you have this problem.”
Kaufman said screwworms are native to only the southernmost parts of the U.S., though they can range into more temperate climates when it’s warm enough.
“When weather conditions are good, it will survive in the Midwest, but it can’t survive the winters,” Kaufman said. “South Texas and South Florida never get cold enough to kill it off.”
Those are the main places the species lived in the U.S. until the 1960s, when the U.S. ramped up its war on the screwworm. Over the next four decades, factories and dispersal sites were built in Florida, Texas and Central America that produced and released hundreds of millions of sterile flies each week.
The sterile flies are irradiated and released en masse, designed to blanket a region experiencing an outbreak. Their presence makes it nearly mathematically impossible for female screwworms to find and select a nonsterile mate. Without viable mates, the flies can’t lay their eggs and reproduce.
The strategy worked: Once cases reached zero in 1982, the U.S. continued its campaign in Mexico and other Central American countries, driving the screwworm farther and farther south.
“It took us until about 2004 to eradicate it all the way down past the Panama Canal,” Kaufman said.
But in time, he added, the U.S. and its partners stopped investing in facilities that produced and dispersed the sterile insects in locations where the screwworms had been eradicated.
“As they open the new plants further south, they close the northern plants, and so the Texas plant closed, and then Mexico, and then the Nicaragua plant, leaving us only with the plant in Panama,” Kaufman said. “That plant is showing its age.”
For about two decades, the Darién Gap, a forbidding, roadless rainforest on the border of Panama and Colombia, represented the geographic border of the screwworms’ reach. However, in 2023, an outbreak of screwworm began spreading north, first to Panama and Costa Rica, then to Mexico and now to the U.S.
“Why did it get out?” is the golden question, Kaufman said. “No one really knows.”
Regardless of the answer, the U.S. is investing once again. The USDA is spending $750 mill…
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