The evening before her visit to hell began, the young American traveller’s holiday at the Edenesque resort in the Dominican Republic had been idyllic. For a week, she swam in the ocean, hiked near an estuary, and sunbathed on the beach, once falling asleep under the light of the stars. Later, she’d felt slight irritation in one ear, and pulled out a fly. Then, on the flight home, the pain and bleeding began. Tiny larvae had begun to burrow their way through her flesh like screws, tearing their way towards her brain with their sharp hook-like mandibles.
Earlier this week, the United States reported its first new case of a human screwworm infestation in years, picked up by a patient on a visit to El Salvador. Thousands of cases in animals, and dozens in humans, have been reported from across Central America, as warming weather has enabled the resurgence of a parasite considered eradicated from the region in the 1990s.
For America’s increasingly inward-looking foreign-policy establishment, the life-threatening epidemic ought to provide a rapid education in the need for international cooperation. Ever since the 1950s, the United States used specially fitted aircraft to release millions of sterilised male screwworms that mated with the wild population. The programme destroyed screwworm populations, giving fertile wild males the chance to mate.
The air war against the screwworm wasn’t driven by altruism, or even to avoid relatively rare human infestations. The eradication of the screwworm, official estimates suggest, saves the United States’ livestock industry over $1 billion a year. The screwworm, however, has now breached the biological wall that was established at the Darién Gap in Panama over decades. The pest is relentlessly marching northwards—helped by President Donald Trump’s flailing regional foreign policy.
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A borderless war
For generations, nations in conflict have known that there is no option except to cooperate in the face of ecological threats. Even though the two countries were just emerging from the border conflict, India and Pakistan worked together against locust swarms that devastated crops in 2020. Israel, the Palestinian Authority, and Jordan ran joint programmes to combat the Mediterranean fruit fly. The ten countries that fished for North Sea herring agreed on a moratorium when schools became dangerously depleted in 1977, while the United States and the Soviet Union cooperated against smallpox.
Lessons learned in the long war against the screwworm repeatedly demonstrated the benefits of international cooperation. The technology to rear and sterilise large numbers of screwworm was available early in the 1950s. The first successful eradication project took place in Curaçao, in the Dutch Antilles, because its geographical isolation permitted rigorous observation of results. The success persuaded the United States to begin eradication efforts at home from 1957.
The effort was massive: the fly-breeding plant at Mission, Texas, needed more than 1,00,000 kilos of meat and over 35,000 litres of whole blood to produce the 200 million sterile males that were released each week. “In the earliest days of sterile-screwworm testing, the flies stank so badly that airlines refused to ship them,” journalist Sarah Zhang has recorded. “Workers learned to spray the boxes with cologne.”
Even this, however, wasn’t enough. In 1963, scientists discovered that screwworm flies could fly distances of over 250 kilometres. This meant that the United States south-west could not be free of screwworm unless Mexico was. In 1965, following a severe outbreak, Mexican and American livestock producers signed a declaration, calling for a joint programme for the eradication of the screwworm.
Their governments didn’t listen at first—but a massive outbreak in 1972 led the United States to propose a joint screwworm eradication commission. This led to the establishment of a biological wall at the Isthmus of Tehuantepec at the 93rd meridian, the narrowest point in Mexico. In 1974, a sterile fly rearing facility was set up at Tuxtla Gutierrez. Flight maps of the sterile fly releases show that methodical and massive efforts were made to eradicate the pest across the cattle-rearing regions of northern Mexico.
Less than a decade later, the screwworm had been eliminated from much of Mexico, and scientists began to consider pushing the wall further south, to a narrower region, easier to defend—the Darién Gap. This meant signing new cooperation agreements with Honduras, Nicaragua, Costa Rica, and Panama, followed by Guatemala and Belize. A new fly-rearing plant also had to be established in Panama.
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The return of the nightmare
The war against the screwworm was, in some ways, a victim of its own success. The next step in the programme ought to have expanded it further south, to eliminate the pest from South America’s livestock. However, with the wall along the Darién Gap working successfully, interest in the issue diminished. With funding for the programme in Panama coming overwhelmingly from the United States Animal and Plant Health Inspection Service, politicians balked at the idea of spending more on fighting the screwworm.
From 2023, the screwworm resumed its northward march. Explanations for the resurgence vary — large-scale smuggling of cattle grown in Central America’s rainforests through Mexico, the massive movement of immigrants through the Darién Gap, and the collapse of the inspection system during the coronavirus pandemic.
Today, even as the worm marches forward, the infrastructure to fight it has decayed. The fly factory in Panama has increased production from its usual 20 million flies a week to its maximum of 100 million. The eradication campaign in the 1980s, though, used 150 million flies a week just over the Isthmus of Tehuantepec. A new plant, under construction in Mexico, will add another 100 million flies. At its peak, the campaign in Mexico needed 550 million flies weekly.
This much is clear, though: Trump’s adversarial relationship with the United States’s southern neighbours hasn’t helped. In April 2025, Secretary of Agriculture Brooke Rollins wrote to her counterpart in Mexico highlighting concerns that Mexico was impeding eradication efforts. Local authorities, she alleged, were obstructing the aircraft hired to release sterile flies, and imposing import duties on equipment.
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The price of hostility
Likely, Mexico’s actions were linked to disputes over Trump’s bruising tariffs on imports, and his threats to use military force across the border. “Mexico still has not stopped the Cartels who are trying to turn all of North America into a Narco-Trafficking Playground,” Trump said in July. “Obviously, I cannot let that happen!” For her part, Mexican President Claudia Sheinbaum has claimed that the United States is weaponising the screwworm threat, pointing out that the country paused all imports of livestock across the southern border because of one single case in Veracruz.
To make things worse, Trump’s funding cuts have diminished the ability of the US Drug Administration to monitor animal disease control. The funding cuts specifically targeted programmes that address avian influenza and the screwworm. Emergency funding now organised will do little to help improve the technologies available to combat screwworm.
Forced to suspend cattle imports from Mexico, the United States’ emergency containment measures could hit consumers hard. The official Congressional Research Service warned this summer that the United States’ own cattle herd is at its lowest level since 1951, because of drought, making prices highly sensitive to any disruption in the supply chain.
The self-inflicted wounds to the United States’ interests will, hopefully, help the Trump administration understand why international cooperation matters.
Praveen Swami is contributing editor at ThePrint. His X handle is @praveenswami. Views are personal.
(Edited by Prashant)