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The US has closed its ports of entry to Mexican cattle for fear of the parasitic, flesh-eating worm spreading north.
Published On 10 Jul 2025
Mexican President Claudia Sheinbaum has denounced a decision by the United States to once again suspend imports of her country’s cattle over a flesh-eating parasite called the screwworm.
On Thursday, Sheinbaum used her morning news conference to call fears of the worm overblown. She pointed out that a single case in the eastern state of Veracruz had prompted the import pause.
“From our point of view, it is a totally exaggerated decision to close the border again,” Sheinbaum said.
At the centre of the cross-border debate is the New World screwworm, a species endemic to the Caribbean and parts of South America. It had previously been eradicated from the northernmost part of its range, in Central and North America.
The US, for instance, declared it eliminated from the country in 1966.
But the parasite may be making a comeback, leaving the US government alarmed about its potential impact on its cattle and beef sector, a $515bn industry.
The New World screwworms appear when a variety of parasitic flies, Cochliomyia hominivorax, lay their eggs near wounds or sores on warm-blooded animals. Most commonly, its hosts are livestock like horses or cattle, but even household pets or humans can be infested.
Each female fly is capable of laying hundreds of eggs. When the eggs hatch, they release larvae that tunnel into the flesh of their hosts, often causing incredible pain.
Unlike maggots from other species, they do not feed on dead flesh, only living tissue. If left untreated, infestations can sometimes be deadly.
Animal health worker Eduardo Lugo treats the wounds of a cow in Nuevo Palomas, Mexico, on May 16 [Jose Luis Gonzalez/Reuters]The fear of New World screwworms expanding northwards has caused the US to halt shipments of Mexican cattle several times over the past year.
In late November, it put in place a ban that lasted until February. Then, on May 11, Secretary of Agriculture Brooke Rollins announced the US would once again bar entry to Mexican cattle after the “unacceptable northward advancement” of the bug.
A port of entry in Arizona was slated to reopen to Mexican cattle starting on Monday. But that plan was suspended under a new announcement on Wednesday, which implemented the cattle ban once more, effective immediately.
“The United States has promised to be vigilant — and after detecting this new NWS [New World screwworm] case, we are pausing the planned port reopening’s to further quarantine and target this deadly pest in Mexico,” Rollins said in a statement.
The statement explained that the US hopes to eradicate the parasite, pushing its encroachment no further than the Darien Gap, the land bridge in Panama that connects South and Central America.
It also asserted that the US Department of Agriculture (USDA) was “holding Mexico accountable by ensuring proactive measures are being taken”.
A worker drops New World screwworm fly larvae into a tray at a facility that breeds sterile flies in Pacora, Panama [Handout/COPEG via AP Photo]Part of its strategy will be to release male flies — lab-raised and sterilised through radiation — from airplanes in Mexico and the southern US. Female flies can mate only once, so if they pair with a sterile fly, they will be unable to reproduce.
The same strategy has been deployed in the past to control the New World screwworm, as an alternative to more hazardous methods like pesticides that could affect other animals.
In a social media post on June 30, Rollins touted gains in recent weeks, including “over 100 million sterile flies dispersed weekly” and “no notable increase” in screwworm cases in eight weeks.
She thanked her Mexican counterpart, Julio Berdegue, for his help.
“He and his team have worked hand in hand with our @USDA team since May 11 to get these ports reopened. We are grateful,” she wrote.
Source:
Al Jazeera and news agencies
English (US) · 