The New World screwworm has returned to the U.S. Now what?
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Flesh-eating, hooked-mouth maggots have wriggled their way back into the United States.
As of June 21, 15 cases of New World screwworm have been confirmed in Texas and New Mexico in cows, goats, sheep and even a dog. The screwworm (Cochliomyia hominivorax) threatens the health of animals and humans as well as the beef industry. Females of this subtropical blowfly, which is native to South America, deposit their eggs into the open wounds of mammals. The larvae burrow into and feed on the living flesh as they grow. The burrowing process can be painful for the host and, if left untreated, can cause secondary infections and even death.
The flies like mammals, and they aren’t selective. They parasitize wildlife, pets, livestock — like cattle, sheep and goats — and occasionally humans. After eradicating the screwworm in the United States in 1982, the pest has been gradually creeping north again.
case was detected June 3 in a three-week-old calf in Zavala County, Texas, according to the United States Department of Agriculture. Science News talked to experts about how worried we should be about this new invasion and what is being done to fight it. In humans, the risk is low, experts say. There are currently no reported human cases of locally acquired screwworm in the United States, though in 2025, an infection was reported in a person returning to the country after visiting El Salvador. However, since Panama and Costa Rica first identified an outbreak in 2023, there have been 2,100 reported human cases in Mexico and Central America.
Still, human cases are exceedingly rare, says Edwin Burgess, a veterinary entomologist at the University of Florida in Gainesville. “You can look online and find horror stories about the number of human cases, but you have to think about the total population over that time period,” Burgess says, “and that number is itty-bitty in terms of a risk.”
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