U.S. Department of Agriculture (USDA) Secretary Brooke Rollins ordered a complete shutdown of livestock trade across the Southern border on Wednesday in an effort to stop the spread of the deadly New World Screwworm (NWS) that burrows into mammals and eats them alive.
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“I have ordered an immediate shutdown of live cattle, bison, and horse trade through the southern U.S.–Mexico border. This decisive action comes after Mexico confirmed another case of New World Screwworm in Veracruz. As promised, @USDA remains vigilant to ensure the protection of America’s livestock and food supply,” said Secretary Rollins.
The decision came in response to a Northward expansion of a growing screwworm outbreak in Mexico, which threatens to spread into the U.S.
“New World screwworm (NWS, Cochliomyia hominivorax) is a devastating pest. When NWS fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage to the animal. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people,” says the USDA’s Animal and Plant Health Inspection Service.
Screwworm flies typically lay their eggs in livestock animals, such as cattle, though they can infest other animals and even humans if there are any open wounds.
The eggs hatch within 12-14 hours, and the larvae begin to feed on the host. The damage done by the feeding larvae attracts more flies to lay their eggs, and the host typically dies within 5-10 days if nothing is done to stop the Screwworms.
In the 20th Century, Screwworms preyed upon livestock across much of the U.S. but, in 1966, eradication efforts succeeded in exterminating all Screwworms across the country.
By distributing sterile male flies, which mate with females but produce no offspring, Screwworms were eradicated through North and Central America. The Darien Gap in Panama, a region of dense jungle separating Central and South America, served as a barrier, between North America, where the flies had been eradicated, and South America, where they remain endemic.
The USDA intends to reinvigorate eradication efforts, with the ultimate goal of pushing the flies back to the Darien Gap.


