A flesh-burrowing parasite that can terrorize humans and animals alike has made its dreaded return to the United States.
Late Wednesday afternoon, officials from the United States Department of Agriculture reported a case of New World screwworm detected in a 3-week-old calf in Texas. It’s the first known case of the parasite among U.S. livestock reported in decades, following a concerted eradication program. Officials say they are implementing containment measures to ensure the screwworm does not reestablish itself within the country as it has lately in many areas of the Americas.
The ‘man-eater’ maggot returns
The New World screwworm is actually a species of fly formally called Cochilomyia hominivorax. Its nickname comes from the fly’s larval form: a maggot shaped like a screw.
Adult flies will lay eggs into the open wounds or mucous membranes (eyes, for instance) of their warm-blooded hosts, after which the maggots will use their screw-like bodies to burrow ever deeper into the flesh of their hosts. The maggots burrow into tissue and feed using sharp mouth hooks, causing extensive damage to the host and increasing the risk of secondary infections; severe enough infestations can be outright deadly.
Though its formal Latin name actually means “man-eater,” these maggots only rarely sicken humans, and they don’t directly infest our meat or other foods. That said, they are a serious problem for livestock. Large, widespread outbreaks of screwworms can especially devastate cattle populations and eventually lead to rising beef prices.
In the 1930s, screwworms were identified as a major economic threat to U.S. farms, particularly in the southwest, and the USDA launched a program aimed at eradicating it from the country. By the 1960s, researchers had developed the method that would then serve as the cornerstone of the program: the Sterile Insect Technique. This involves releasing large amounts of infertile males (sterilized via radiation) into the local population. Fertile females will mate with these sterile males and fail to produce offspring, leading to a population crash.
The sterile insect technique helped eliminate the screwworm from the U.S. in 1966, and other countries in the Americas started working together to expand its use. By the early 2000s, the coalition had succeeded in pushing back the screwworm as far down as Panama.
Constant vigilance is required to maintain this barrier, however, and by 2023, the barrier had begun to fail, allowing the screwworm to resurge with a vengeance. It’s not entirely clear why this happened, though pandemic-related disruptions to the program, illegal cattle trafficking, and the inadvertent use of less fit sterile males (meaning less likely to mate) may all have contributed to it.
In any case, outbreaks have steadily cropped back up throughout Central America and Mexico, and it was only a matter of time before the fly made its return to the U.S. Last August, officials detected a human case of New World screwworm in a traveler who had recently visited El Salvador.
What to do now
Last summer, USDA officials began ramping up screwworm prevention efforts (though only after the Trump administration had initially cut funding and staff from screwworm-related programs). And now they’re taking things further.
The agency is working with local officials to set up a 12-mile (20-kilometer) quarantine zone around the detected case. It will immediately release batches of sterile flies into the zone, in addition to the four million flies already deployed weekly in the region. The Food and Drug Administration also recently approved the use of several drugs intended to treat dogs for screwworm.
“USDA invested heavily in the tools needed to eliminate NWS ever since cases started increasing in Central America and Mexico,” said Dudley Hoskins, undersecretary for marketing and regulatory programs, in a statement from the USDA. “The United States has defeated this pest before, and we will do it again.”
This isn’t the first time that the screwworm has threatened to make a comeback in the states. In 2016, an outbreak affected deer living in the Florida Keys, but officials were able to contain it by the following spring.
