screwworm

The US has closed its ports of entry to Mexican cattle for fear of the parasitic, flesh-eating worm spreading north.

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.

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”.

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.

June 30 (UPI) — Imports of livestock from Mexico will resume in phases starting next week after a ban in May at ports of entry because of fear of the spread of New World screwworm, the U.S. Department of Agriculture announced Monday.

Ports of entry will allow certain cattle, horses and bison to go into Arizona, New Mexico and Texas after they were banned on May 11, the agency said in a news release.

The screwworm has been eradicated from the United States for decades. But it has been detected in Mexico as far north as Oaxaca and Veracruz, about 700 miles away from the U.S. border.

When NWS maggots burrow into the flesh of a living animal, they cause serious and often deadly damage to the animal, USDA said. Mature screwworm larvae can grow up to two-thirds of an inch.

The USDA Animal and Plant Health Inspection Service experts and their counterparts in Mexico worked to increase New World Screwworm surveillance, detection and eradication.

A phased reopening of the southern ports will start Monday in Douglas, Ariz., based on the lowest risk because of the geography of Sonora and an effective collaboration between APHIS and Sonora.

The other openings are set for July 14 in Columbus, N.M.; July 21 in Santa Teresa, N.M.; Aug. 18 in Del Rio, Texas; and Sept. 15 in Laredo, Texas.

In the past eight weeks, there hasn’t been a notable increase in reported NWS cases in Mexico or any northward movement, the agency said.

USDA has been conducting sterile NWS fly dispersal seven days each week, including the dispersal of more than 100 million flies each week.

Five teams of APHIS staff were sent to observe and gain a deeper understanding of Mexico’s NWS response.

“At USDA we are focused on fighting the New World Screwworm’s advancement in Mexico,” USDA Secretary Brooke L. Rollins said. “We have made good progress with our counterparts in Mexico to increase vital pest surveillance efforts and have boosted sterile fly dispersal efforts. These quick actions by the Trump Administration have improved the conditions to allow the phased reopening of select ports on the Southern Border to livestock trade.

“We are continuing our posture of increased vigilance and will not rest until we are sure this devastating pest will not harm American ranchers.”

On June 18, she met with cattle fever tick riders along the Rio Grande River. If the NWS advances northward into the United States, these tick riders “will play a crucial role in spotting and combating this pest,” an agency news release said.

USDSA is building a fly-production center at Moore Air Base in Edinburg, Texas, that could boost domestic sterile fly production by up to 300 million flies per week. Another one is planned at Moore Air Base, which alo could boost domestic sterile fly production by up to 300 million flies per week.

This week, Mexico will begin renovation of its sterile fruit fly facility in Metapa with completion by July 2026. The changes will allow for production of between 60 and 100 million sterile NWS flies each week.

The goal is produce an estimated 400 to 500 million flies each week to re-establish the NWS barrier at the Darien Gap, which is the border between Panama and Colombia.

Only cattle and bison, born and raised in Sonora or Chihuahua, in Mexico, or are treated according to cattle and bison NWS protocol when entering the U.S. will be eligible for import.

Equines may import from anywhere in Mexico though there is a seven-day quarantine at the port of entry.

USDA plans to remove any federal regulatory hurdles for sufficient treatments and work with state officials on emergency management plans in states.

The Texas and Southwestern Cattle Raisers Association and the National Cattlemen’s Beef Association back the agency’s efforts.