An unpleasant parasite spread by flies was kicked out of the U.S. decades ago — but now it seems to be making its return. As a result, the U.S. Department of Agriculture is changing strategies to confront it.
New World screwworm is a livestock pest that can be a parasite of any warm-blooded animal, laying its eggs in even the tiniest of open wounds or cuts, including the bellybuttons of newborn animals, and in mucous membranes anywhere in the body.
Notably, they eat live flesh. Unlike other maggot-like infestations, "the screwworm eats live tissues, so it can cause significant tissue destruction and it can cause significant morbidity and even mortality," Isaac Bogoch, a general internist and professor of medicine at the University of Toronto who specializes in tropical infectious diseases, told Salon in a phone interview.
Bogoch noted that screwworm remains rare in returning travelers (although he treated one such case in Toronto this year involving a traveler returning from Costa Rica.) We have science to thank for eradicating it from the United States. And with outbreaks south of us working their way ever closer, we'll have science to thank if it stays that way.
That starts with our understanding of how this fly gets around, which can be pretty gruesome. After digging deep lesions with their powerful maggot jaws, anchoring themselves inside them with their external spikes, screwworm larvae feed on the living flesh, working their way to deeper tissues like the muscles. This continues — while the odor of the lesion may draw other pregnant females to the wound, and secondary bacterial infections can arise — until treatment or the death of the animal.
There isn't a medicine to treat screwworm. Instead, treatment involves removing the visible eggs and larvae from the wound with forceps and applying a larvae-targeting pesticide. Screwworm was once horribly endemic in the United States. If the preceding description wasn't enough, the screwworm's scientific name, Cochliomyia hominivorax, hints at how much we really, truly don't want it back — hominivorax translates as "man-eater," and refers to observations of a 1858 screwworm outbreak among prisoners on Devil’s Island in French Guiana.
Fortunately, screwworm was eradicated from the United States in 1966 thanks to a technique invented by USDA entomologist Edward F. Knipling of releasing massive numbers of sterile male flies. As the female screwworm fly gets only one chance to mate in her maximum 30-day lifespan, if you can get her to have sex with a sterile male, you've effectively ended her hopes of motherhood. Since she has the potential to produce 3,000 live-flesh-eating babies, with every interruption of reproductive sex you make a substantial dent in the future screwworm population.
"Human-mediated movement of infested animals remains a key driver. The trend could continue unless regional movement controls and surveillance are strengthened.""
Once the U.S. was rid of the parasite, the sterile male fly technique was used to push it south, eventually restricting it all the way down to South America, south of a permanent biological barrier established at the Darién Gap between Panama and Colombia. This biological barrier literally consists of APHIS' and partners' program, following the late Dr. Knipling's lead, of releasing sterile male screwworm flies in the millions and millions. Serious effort and the cooperation of the U.S. with countries throughout Central America kept it that way for decades.
Having been chased out of North and Central America by the hardworking sterile male flies, today the parasite is considered endemic in South America and in Cuba, Haiti and the Dominican Republic. But over the past two years, screwworm has been spreading in and north of Panama. There were 6,500 cases in that country in 2023, a surge from the more typical 25, with that outbreak in Costa Rica, including a human death last June, and cases in Nicaragua, Honduras, Guatemala, Belize, El Salvador and even as far north as southern Mexico.
According to APHIS, the USDA's Animal and Plant Health Inspection Service, that's because of "multiple factors including new areas of farming in previous barrier regions for fly control and increased cattle movements into the region," Dr. Anna Bagwell, APHIS' program manager for New World Screwworm, told Salon by email, adding that "increased human and animal movement — particularly illegal livestock transport — has contributed to the spread. C. hominivorax flies don’t travel far on their own, so human-mediated movement of infested animals remains a key driver. The trend could continue unless regional movement controls and surveillance are strengthened."
Screwworm monitoring funded by USAID (not USDA) has supposedly been on the chopping block, and in response to a question from Salon about whether this affects work through the Americas, Bagwell didn't address USAID funding, instead noting that USDA allocated $109.8 million two years ago, in December 2023, through Commodity Credit Corporation funding, to combat the outbreak in Central America and Mexico. A keyword search of a list of cancelled USAID awards put together by Health Policy Watch didn't turn up anything relating to screwworm. The sterile flies used now are bred in a cooperatively-funded sterile screwworm breeding facility in Panama run by COPEG, a joint U.S.-Panamanian commission to combat and prevent screwworm.
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But rather than restore and maintain the original biological barrier that kept screwworm from spreading north from Panama, APHIS decided in February to shift focus to Mexico, distributing most of its precious sterile flies there in hopes of preventing the parasite from crossing the border into the United States. Although also in February, APHIS reinstated imports of cattle that can carry it (but with enhanced surveillance and import requirements.)
"The reason Panama was chosen [in the first place] was because that's the bottleneck. That's the shortest distance across," Bogoch told Salon. "It's obviously beyond that New World screwworm sterile barrier and so essentially, you're finding an insect in areas where it should not be found, and this is one that can cause tremendous harm to livestock, to wildlife and to humans. So it's a huge public health issue. It's an animal health issue, it's a health security issue ... That's a five-alarm fire. It needs to be jumped on and taken care of promptly," Bogoch explained.
On Nov. 22nd, a single case of screwworm in Chiapas, a Mexican state at the border with Guatemala, triggered an American ban on imports of beef that transited through or came from Mexico. It didn't last long. Beef prices are high in the U.S. and it's a tight market. In December, USDA signed an agreement with SENASICA, its counterpart in Mexico, to set up measures to once again allow Mexican cattle to be exported to the United States, carefully.
"It's a huge public health issue. It's an animal health issue, it's a health security issue ... That's a five-alarm fire."
Also in December, APHIS received $165 million in emergency funding from the Commodity Credit Corporation to protect U.S. livestock and to increase efforts by the USDA to control screwworm's spread in Mexico and through Central America. That work was to involve animal health inspection checkpoints, re-establishing the biological barrier in Panama, and surveillance. Horse imports were restarted on Jan. 21st and imports started again on the 21st, while the cattle and bison import restrictions were loosened Jan. 31st and imports began again in early February.
But it's not clear to what extent the United States and Mexico are currently able to work together. On Saturday, Brooke Rollins, the U.S. Secretary of Agriculture, threatened to go back to restricting livestock imports from Mexico, referring to a lack of cooperation from that country. Rollins alleged in an April 26 letter to her counterpart, Mexican Secretary of Agriculture Julio Antonio Berdegué Sacristán, that Mexico is restricting USDA aircraft to six flights weekly rather, than allowing them to fly daily (to disperse sterile male flies), and is imposing "burdensome" import duties on aircraft parts, sterile fly shipments and dispersal equipment needed for an effective response to screwworm in Mexico, according to Rollins' post on X that links to the letter. "As the New World Screwworm outbreak is escalating, Mexico must eliminate restrictions on USDA aircraft and waive customs duties on eradication equipment. These barriers critically impair our joint response," Rollins wrote.
Also on X, Berdegué Sacristán soon responded that he had already sent a reply letter to Rollins. He further wrote in Spanish, as translated by Salon: "we collaborate, we cooperate, but we will never subordinate ourselves." Apparently the sudden U.S. desire for cooperation involving Mexican airspace is being viewed as an attempt to undermine Mexican sovereignty — a sentiment expressed by some commentators on the X post. Most other respondents argue that the Mexican government's insistent focus on sovereignty in this case amounts to posturing and populism over good sense, given that screwworm is a genuine public health crisis affecting both countries, and that the United States are making a genuine and urgent appeal for cooperation. One respondent goes as far as to call the Mexican secretary of agriculture a "worm talking about a worm." Nevertheless, it's clear that months of aggressive tariffs, insults and threats of drone strikes by the Trump administration are not creating a favorable environment for working together.
Historically, efforts to eradicate screwworm populations moved steadily south as soon as it was eliminated from the U.S. A history by the Food and Agricultural Organization of the United Nations tells much of the story. First, USDA researchers tried releasing sterile flies on the Florida island of Sanibel, then successfully used them to eliminate screwworm from Curaçao, eventually rearing 50 million sterile flies a week in a facility in Texas and creating a massive sterile fly-based barrier in the United States and northern Mexico, a massive invisible border to monitor and maintain.
Livestock farmers could see that clearing the parasites out of regions far further south would ensure they maintained their screwworm-free, or enzootic, status (an outbreak in Texas in 1976, while short-lived thanks to the sterile flies, did $283-375 million dollars' worth of damage.)
So Mexican and American farmers united to convince their governments to do more. A Mexico-U.S. Screwworm Eradication Commission was established in 1972. The Texas plant soon closed and a new, and globally unique, sterile fly rearing facility was built in Chiapas — the same state where the discovery of that single infested cow triggered the newly-eased ban on livestock imports in the final months of last year, and where two of the (so far) three human cases of screwworm infection in Mexico were identified this year.
By 1984, cooperation to release sterile flies reared in the facility in Texas had pushed the parasite south of the Isthmus of Tehuantepec, which, being narrow, created a bottleneck — much easier than monitoring the entire Mexico-U.S. border. And then further, ultimately eradicating screwworm from the Americas north of the Isthmus of Panama. (Again shutting down a sterile fly facility in order to open the cutting-edge, and again globally unique, COPEG facility in Panama.) Occasional outbreaks further north, including an outbreak among endangered Key Deer in Florida in 2016, were resolved quickly through emergency release of sterile flies. And indeed, recent documents, from late 2024, note a focus on re-establishing the Panama barrier.
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But as of this February, APHIS has decided that it would instead be wise to concentrate on the border with Mexico in keeping the parasite out of the United States: while work across Central America will continue, with sterile flies to be released at strategic points throughout the region, the goalposts have shifted, a situation Bagwell says is temporary. Asked by Salon whether the shift in focus is a decision made by the new U.S. administration, Bagwell instead simply replied that "the decision, announced in February 2025, was driven by scientific data and historical models." She also said that "the shift was a scientifically informed, emergency response to the northward movement of the outbreak. The goal is to contain northern movement and push New World Screwworm population southward to re-establish the original biological barrier at the Darién Province" and that despite the historical logic of focusing on restoring that narrow barrier as quickly as possible and the logistical challenge of the new strategy, given the current outbreak, it's the "most effective and economically prudent course of action."
As journalist Sarah Zhang notes in a 2020 Atlantic story about screwworm eradication, "protecting American livestock by dropping sterile flies over the narrow 50-mile Isthmus of Panama is cheaper than maintaining a barrier, even a virtual one, along the 2,000-mile U.S.-Mexico border."
So why give up on the full-on effort to restore the Panama barrier? As Zhang notes in her article, this was an American-led success in international cooperation — even though that cooperation stopped short of Cuba, at least until President Obama relaxed trade restrictions against Cuba in 2014, and scientists took tentative steps towards a cooperative eradication program there, where screwworm remains endemic, before relations went backwards again. The website for COPEG seems not to have been updated since 2024, and there is only a single post on X from the organization this year, although Bagwell says that the sterile fly breeding facility in Panama is fully operational and operating at maximum capacity.
Panama has become unexpectedly newsworthy lately, not for triumphs of international parasitic fly control but because President Donald Trump has spoken frequently about "taking back" the Panama Canal. Last week, the Panamanian government signed a memorandum of understanding with the United States that will allow first and free passage of U.S. warships through the canal and bring American troops back to the country for the first time since the last U.S. military bases were evacuated in 1999, a move that the opposition government has called "an invasion without firing a shot". The U.S. invaded Panama in 1989, killing hundreds of civilians and destroying infrastructure and homes.
If we are to remain free of parasitic blow flies that burrow deep into the flesh of animals when they can get them, and occasionally people when they cannot, without regard for nationality or immigration status, we're going to want earnest scientific cooperation and goodwill from the U.S. through Mexico and from Guatemala to Panama. Parasite partnerships over politics: Let's hope that persists — or returns.
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