New Mexico cattle industry prepares and cattle imports paused as screwworm travels north

A small, pale maggot about a half-inch long could cost New Mexico cattle ranchers greatly if it gets back into the country. One of the state’s senators introduced new legislation to combat the New World screwworm fly, which has recently spread across Central America toward the U.S.

At the U.S.-Mexico crossing in Santa Teresa, the pens supporting the nation’s largest livestock entry point sat empty and eerily quiet Friday, with workers performing maintenance projects and catching up on paperwork at facilities normally bustling with thousands of cattle passing through daily.

The U.S. Department of Agriculture closed the border to livestock imports after the screwworm was detected in the southern Mexico state of Chiapas in November. Imports resumed in February once the two countries established protocols for inspection and treatment.

However, when screwworm was detected in Oaxaca and Veracruz, just 700 miles from the U.S., the USDA closed the border to imports once again on May 11.

Unlike typical maggots, which feed on the dead, screwworm larvae feast on the living, meaning the fly larvae can do significant damage to cattle, even causing death. While New World screwworms overwhelmingly affect cattle, any warm-blooded animal can be susceptible to infestation, including livestock, pets, wild animals and people.

The U.S. eradicated the parasites in 1966 and has worked with the Panama government to keep screwworms there in check, maintaining a buffer zone of several countries between U.S. ranches and the insects.

But the New World screwworm population in Panama exploded in 2023, and last year’s infestation in Chiapas prompted the U.S. to release $165 million in emergency funding to protect livestock, pets and wildlife.

Some experts see a new U.S. infestation as inevitable.

“It’s not an ‘if we’re going to see it;’ it’s a ‘when we’re going to see it,’” said Bronson Corn, president of the New Mexico Cattle Growers’ Association. “And unfortunately, I think we’re going to see it sooner, much rather than later. Probably within the next three months, I would say, we’re probably going to have it here in the United States.”

New Mexico has more than 10,000 cattle farms and ranches. The U.S. livestock industry benefits by more than $900 million a year because there aren’t screwworms in the country, USDA estimates.

Dr. Samantha Holeck, state veterinarian with the New Mexico Livestock Board, is preparing for the worst and hoping for the best by working to educate livestock producers on the New World screwworm and developing protocols for sample collection to make sure flies can be rapidly identified.

“The challenge is, we’ve got very warm weather. The flies are able to move on their own, as well as with animal movement and things like wildlife that are going to move independently. We don’t have control over how wildlife move. There are risk factors that still make it an imminent threat to the United States, even with a lot of good preventive strategies and surveillance in place,” Holeck said.

In the 1960s, New Mexico’s livestock industry was heavily affected by the screwworm, she said.

“A lot of our common management practices, even ear tagging potentially, can put an animal at risk, because any sort of fresh wound as small as a tick bite for example, can attract those female flies to lay eggs,” Holeck said. “So once they have an opportunity to lay eggs in, those larvae begin to burrow in, because the larvae feed on live, healthy flesh.”

Left untreated, screwworms can cause traumatic injuries, are able to enlarge a wound and capable of even eating through bone. An untreated screwworm infestation could kill an animal within one to two weeks. Treatment with a dewormer like ivermectin can clear an infestation in a cow within two to three days.

Livestock producers can usually identify an infestation by observing a rapidly expanding wound, signs of distress or pain in their animal, or by the foul smell of the wound. If screwworms are identified in New Mexico, the infested livestock will be thoroughly inspected and treated, Holeck said. They also won’t be transported until free of screwworm larvae.

Fly sterilization

Female screwworms only mate once, so the strategy for managing the screwworm population has long been to produce sterile male screwworms and then release them into the wild. But, when the screwworm population was eradicated in the U.S., the country was producing sterile flies to release across a broad area. At present, the only sterilization facility is in Panama and is capable of producing hundreds of millions of sterile flies in a week.

“If that facility were to go down, we’d be in a real big wreck,” Holeck said.

On Tuesday, Agriculture Secretary Brooke Rollins announced that the USDA is investing $21 million to renovate an existing fruit fly production facility in Mexico to produce 60 to 100 million additional sterile New World screwworm flies weekly.

Sens. Ben Ray Luján, D-N.M., and John Cornyn, R-Texas, introduced a bill to authorize money for a new sterile fly production facility in the U.S. to combat the growing screwworm outbreak. The STOP Screwworms Act is cosponsored by Martin Heinrich, D-N.M., Ted Cruz, R-Texas, and Cindy Hyde-Smith, R-Miss. With bipartisan support, Luján is hopeful the bill will move quickly through Congress because of the urgent situation.

“The last time that there was an outbreak, it devastated the cattle industry, beef prices and availability in the United States,” Luján said. “This little thing is quite devastating.”

At the border, empty cattle pens

The orange theater-style seating at the Santa Teresa Livestock Auction was empty Friday. Hay and feed were neatly piled and only a few head of cattle were penned together.

General Manager Hector Alberto “Teto” Medina estimated that the pause on imports will affect producers in Mexico, who exported over $1 billion in livestock to the U.S. in 2024, but would have ripple effects throughout the U.S. food system and related business sectors — down to what consumers pay for groceries or dining out.

“This is the hardest we’ve ever been hit on the U.S.-Mexico border,” he said. “We’re talking about employment for farmers that feed these cattle, ranchers, transportation companies, brokerage companies, federal workers, feedlots, processors, for plants that process the cattle into meat — all the way down to restaurants and all the way down to our plates.”

So far, he said he had no plans to lay off any of his staff, and was upbeat about a solution.

“What’s being done at the USDA is the correct thing to do,” he said. “They are ensuring that our food supply is safe and clean. … I wish it didn’t have to affect us as much, but that’s the way life is sometimes.”