America’s fight against the New World screwworm was expected to be a multiyear effort, yet the number of new cases has dropped to a surprisingly low level.
Only four cases of screwworm were detected in the U.S. in August and none of them in cattle, according to a dashboard maintained by the Agriculture Department. In June, there had been 30 cases detected, and an expectation that the fight against the screwworm had a long way to go.
That August number baffles experts, given how far the screwworm had advanced into Texas and the high number of cases in Mexico and Central America.
“It’s very strange,” a former longtime USDA official who worked for the department’s plant and animal health department told NOTUS. “It doesn’t make a lot of sense, biologically.”
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On top of that, the USDA reports it hasn’t detected screwworm through trapping flies or seeing the pest in any wildlife or feral animals, the primary ways officials might expect to keep tabs on the population and their movement.
“Zero cases in wildlife is unexplainable to me,” Texas Agriculture Commissioner Sid Miller told NOTUS. Miller said that he and horticulturists on his team have caught what they believed to be screwworm in traps, only to have the tests come back negative when sent to the USDA.
One current employee at the USDA’s Animal and Plant Health Inspection Service called the lack of cases in traps or wildlife “suspicious,” saying, “It doesn’t seem very likely to me that we haven’t trapped a single wild fly.”
The USDA says its playbook for fighting screwworm is working, which is why cases are so low. The New World screwworm is a fly that feeds on living tissue in animals and even humans. The infestation can be lethal to animals if left untreated and is considered a major threat to livestock in warm climates.
“The low case count is a direct result of the aggressive, science-driven five-prong strategy to eradicate NWS unveiled by Secretary [Brooke] Rollins a year ago,” a spokesperson for the USDA said in a statement. “This strategy, led by USDA and other federal partners, has increased tracking of the fly’s movement, inspections of impacted animals both wild and domestic, and substantially built up sterile fly capability through coordination with states and industry.”
The agency reopened the port of Douglas, Arizona, to livestock after more than a year of movement restrictions aimed at keeping the screwworm out of the U.S. The USDA conducts inspections of the livestock at the border and said it plans to reopen additional ports if successful in Douglas.
The agency also lifted movement restrictions on three counties this week ,two in Texas and one in New Mexico.
The USDA said on X that it has released 1 billion sterile flies, or male flies that mate with females but don’t produce offspring and effectively stop the lifecycle over time. Rollins and Homeland Security Secretary Markwayne Mullin wrote an op-ed Friday touting their efforts.
At the beginning of the summer, Rollins acknowledged that the agency needed 400 million more sterile flies a week to effectively combat the screwworm, a number that would likely take several months or a year to reach.
In the meantime, cases across Mexico and Central America have reached over 200,000, with 500 human cases in Mexico alone.
Some experts question whether the agency has the personnel and the resources to properly track and contain the screwworm. The USDA has undergone significant staffing reductions to many of its departments since the start of the second Trump administration.
“The critical mass is just not there to do the work that needs to be done,” the former USDA manager said.
And some former USDA officials question whether the current surveillance methods are working or whether screwworm is truly being contained effectively. “They’re using wildlife cameras to do surveillance, and USDA is claiming that it works well, but it’s also not catching anything,” a former USDA official who oversaw screwworm response told NOTUS.
Miller also raised doubts over the ability for cameras to identify open wounds and screwworm on animals, saying he has seen the video the cameras produce himself. “It’s not the best videos,” Miller said. He said the videos are fine for identifying animals, but are not the same as a medical examination.
The USDA said that the footage from these cameras is reviewed by wildlife biologists, with assistance from AI tools to identify animals with wounds.
Experts are also concerned that the current system does not incentivize ranchers to report cases. Current rules require cattle ranchers to keep their herds in quarantined areas, which can cause ranchers to lose money without any indemnity in return from the USDA.
Both former USDA officials said the USDA set up an indemnity program during the bird flu outbreaks beginning in 2024, something that has not been done for cattle ranchers with screwworm.
Both the current and former APHIS employees echoed this concern, saying ranchers would be uninterested in self-reporting due to concerns over movement restrictions or government surveillance.
The USDA said the agency provides indemnity for any animal that must be culled to prevent the spread of disease or parasites.
The U.S. eradication campaign that eventually beat back screwworm in the 20th century took the better part of a decade of sustained effort. USDA formally launched its sterile insect technique program across the southeastern U.S. in 1957. Florida was declared screwworm-free by 1958, and a large-scale push into Texas began in 1962, with ranchers reporting screwworm-free herds for the first time in modern memory by 1963. The U.S. wasn’t declared fully free of the pest until 1966 — nine years after the campaign began in earnest.
Even then, the win proved fragile, with a resurgence tied to reinfestation from Mexico in 1972. Even after the 1966 declaration, eradication pushed steadily south over the following decades, with Mexico declared screwworm-free in 1991 and Belize, Guatemala, Honduras and El Salvador following through the mid-1990s.
“I think it’s still too early to tell what the situation is with the flies,” the current APHIS employee said before expressing optimism that screwworm has been defeated once and can be defeated again; it’s “just a matter of how much time and money it will take.”
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