The border with Mexico has been closed to cattle imports for more than a year in hopes of slowing the spread of New World Screwworm.

But the United States Department of Agriculture (USDA) has announced a phased reopening on Aug. 24, starting with the port in Douglas.

The move earned praise from the state’s feedlot industry, which has been hurt hard by the border closure, and caution from cow-calf operators struggling to monitor vast terrain for the destructive pest.

“The threat of New World Screwworm along our southern border is a serious concern for our members, for wildlife and anyone who manages livestock or has pets,” Arizona Farm Bureau president John Boelts said.

“Cross-border movement of livestock will require ongoing vigilance and additional resources, including a sterile fly distribution facility in Arizona, to confront this threat head-on.”

Sonora has zero screwworm cases

The screwworm, unlike most other maggots, thrives on living flesh. It bores deep into scrapes, cuts and other openings, causing painful wounds that can be fatal if left untreated.

The pest had been eradicated from the United States and Mexico for decades — until 2024, when the first case was confirmed in southern Mexico.

The flies quickly spread north, with the first American infestation confirmed in Texas in early June.  

More than 35,000 cases have been confirmed in Mexico over two years, with nearly 1,700 active cases now.

Sonora — which borders Douglas — is one of the few remaining Mexican states without any cases. 

“While the rest of Mexico has not been able to stop the spread of the screwworm or stop legal and illegal movement, Sonora and Chihuahua do have a track record of success in these spaces, giving USDA confidence that they can do the job,” the agency noted in a news release.

USDA plans to reopen the Douglas port to Sonoran cattle within weeks, though that timeline depends on Mexico meeting goals in an action plan adopted last year to control livestock movement and monitor the pest.

A previous phased border reopening lasted only two days last year before changing screwworm conditions forced another closure.

The federal government will keep close tabs on Douglas and if all goes well, it will then consider reopening ports in Santa Teresa and Columbus, N.M. — which border Chihuahua — to cattle, bison and horses.

Arizona is working closely with Sonora

The Arizona Department of Agriculture has been working with officials in Sonora for months to monitor and slow movement of the pest.

Sonora rigorously inspects cattle coming into the state and does not import cattle from affected screwworm zones.

It has deployed surveillance traps at its borders with other Mexican states and along the border with Arizona.

Arizona has sent dozens of samples from those traps for analysis at the national lab in Iowa, but none have been confirmed as screwworms.

Sonora has distributed detection kits and educated ranchers on quickly reporting and treating affected animals.

Officials in the Mexican state also recently engaged in a planning exercise with the U.S. to ensure a coordinated response, should a case emerge on either side of the border.

“Sonora is known for their excellent disease protocols, and we have implemented new requirements to assure the safe importation of livestock,” Arizona agriculture department director Paul Brierley said in a statement.

“When trade resumes, we will implement all necessary measures to protect Arizona livestock and wildlife. We are ready.”

All cattle must be inspected, pest-free

Every head of cattle must be inspected before it can enter Arizona.

Before the border closed last year, the United States imported about a million head of cattle a year — with more than 300,000 head typically moving through Arizona.

Brierley has issued an order requiring each imported animal to be fitted with an electronic identification tag instead of the previously required blue metal tag.

Electronic tags cause less trauma on ears than the traditional metal tags, reducing the risk of screwworm infestation.

They also can quickly be scanned for required proof of origin and animal health inspection documents.

Anyone importing cattle from Sonora must ensure that each animal has an electronic tag before reaching the port.

Brierley had previously issued an order in June requiring that any animals originating from an infested zone be inspected, treated and certified as screwworm-free.

Douglas fly distribution center is needed

Meanwhile, Arizona is building its case with the federal government to reopen a sterile fly distribution center in Douglas.

Female screwworm flies only mate once in their lifetime. If they mate with a sterile fly, their eggs won’t develop, crashing the population over time.

An estimated 500 million sterile flies must be distributed weekly to push the pest back south. And while efforts are underway to ramp up that production, additional distribution facilities will be needed to disperse them efficiently.

Though the Douglas facility would not produce flies, it would be ideally located to distribute them 400 nautical miles — covering border regions from west Texas to San Diego, including Arizona and Sonora.

“We are confident that through our continued work with USDA, the state agriculture department and other government agencies, we can mitigate this threat and continue to protect Arizona’s livestock industry,” Boelts said.

Got questions?

What do you want to know about New World Screwworm or the border reopening? Email questions to advocacy@azfb.org.


Joanna Allhands writes about water, land use and other issues important to the Arizona Farm Bureau. Reach her at joannaallhands@azfb.org.