Screwworm scrutiny continues as Mexican border set to reopen
The United States is preparing to reopen its border to Mexican cattle imports next month, ending a chapter in the battle against the New World Screwworm [NWS]. The border has been closed for over two years for cattle, and since May of 2025 for horses – with a financial impact to those selling to buyers in the USA.
The phased reopening for livestock beginning August 24 at the Douglas, Arizona port of entry is positioned as a trial and follows months of deliberation between US and Mexican authorities.
United States Department of Agriculture [USDA] inspections of animals have always been scrupulous where border crossings are concerned. “There’s a very strict protocol and there always has been a very strict protocol for any live animal going into the United States,” said dressage rider Susan Thomas, who commutes between Guadalajara and Wellington, for her horse brokerage business and has experienced first hand the challenges of importing horses to the States even when the borders are open.
“We wash our horses in insecticide before we load them on the trailer and extensive blood tests and proof of vaccination records are mandatory before travel – the blood tests are also repeated in quarantine. Once in transit, the intense scrutiny does not end there,” she said. “The animals are gone over with a fine tooth comb. If inspectors find so much as a tick on an animal entering the US, the whole shipment is barred entry. So it makes no sense for horse owners nor for transit companies – who then lose their entire revenue on that shipment as a result – to take any risks at all when it comes to stringent border regulations.”
The borders being closed made horse imports for business impractical. “The only practical way we can send horses has been by air from Mexico City to Miami,” said Susan. “That adds around $15,000 to the cost of every horse, making many sales impossible.”
Cattle producers have been further affected as the prolonged shutdown disrupted supply chains and squeezed ranchers on both sides of the border contributing to rising beef prices.
Borders reopening does not signal the end of the screwworm threat. The flesh-eating parasite – which is the larval stage of the fly after hatching from eggs laid in an animal’s open wound – has not disappeared. If untreated, the larvae feed on living tissue creating deep, painful wounds that can quickly become fatal. Horses, cattle, wildlife, pets and even humans can be affected. Diligence is the order of the day – and closing borders or setting boundaries cannot prevent a fly from migrating through the air. Typically screwworm flies travel less than 10 miles, research has shown that they are capable of flying over a hundred miles.
Dr Paul Wollenman, Founder of Palm Beach Equine Clinic, recalled the last screwworm outbreak in the late 60s and early 70s, which required drastic measures to overcome. “It’s a surprising story about how [USDA scientists] started growing screwworms and then irradiating the larvae so they turned into sterile flies,” he said. “Then they released literally millions of them from air drops with small little boxes that broke open when they hit the ground.” As a result there were less fertile males to breed with females resulting in a progressive population reduction and eventual eradication.
Dr Wollenman was also honored by the USDA after identifying an isolated case of NWS on a horse arriving in Wellington from Argentina in 2000. “By immediately reporting the situation to us, we were able to take the actions necessary to ensure that screwworms did not get a foothold in Florida,” a USDA representative said at the time. “For that we are very grateful.”
This incident prompted a precautionary response from the USDA and changes in quarantine rules. “They sprayed both sides of the Turnpike from Miami to Palm Beach,” Dr Wollenman said. “That was the end of it, but it changed the way they quarantined horses in Miami from three to seven days.”
Recent outbreaks in Mexico and detections in Texas and New Mexico have forced officials to remain vigilant. The reopening of the border is coupled with an ongoing commitment to eradicate NWS. Public awareness is also a factor as the sterile flies when released are distinctive in their slightly luminescent appearance and should not be destroyed. “We tell everyone it is important not to kill the green ones,” Susan said.
The USDA is partnering with the US Army Corps of Engineers to build a domestic sterile fly production facility in Southern Texas, with a projected capacity of producing 100 million sterile flies a week by November 2027. Last week the USDA announced a ‘commitment’ to invest $25 million in a new sterile fly dispersal facility in Arizona, saying online that it ‘would complement existing investments in Texas, Mexico, and Panama while improving the Department’s ability to respond rapidly to NWS threats along the Southwest border.’
US Secretary of Agriculture Brooke L. Rollins reflected on the initial decision to close the borders in a recent press release. “Closing the southern border ports over the past year was a difficult but necessary step to control the spread of New World screwworm in Mexico and protect the U.S. livestock industry,” she said. “Thanks to the work carried out by the federal government, as well as state and local governments and industry stakeholders, it is now safe to reopen the Douglas, Arizona, port in 30 days and resume the movement of livestock that has taken place for hundreds of years.”