A patient in Maryland was diagnosed in August with New World screwworm, a parasitic fly, after traveling to El Salvador. Doctors and veterinarians say the case poses very low risks for human health in the US, but it comes after an increase of cases in South and Central America and the Caribbean in recent years. It also highlights the importance of international cooperation on research and prevention.
For decades, the fight against the screwworm was a success story of scientific innovation and collaboration with other countries. There were devastating outbreaks of the parasite in the US in the first half of the 20th century until an ambitious program pushed it south, all the way to Panama.
But now, that progress has been threatened, with resurgences reported almost to the US border. And recent budget cuts to scientific research and foreign aid could further imperil the fight against the screwworm.
The New World screwworm “presents a significant potential for a public health emergency that could affect national security”, the US Department of Health and Human Services announced on 18 August.
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The emergency declaration paved the way to see whether existing anti-parasitic medications, like ivermectin, work to control the screwworm among animals.
There are no other established ways, like vaccines or medications, to address screwworm infestations, and the HHS announcement does not apply to anti-parasitic medications for people.
The screwworm case in Maryland was diagnosed before the insect could mature, which means there is little chance for onward transmission.
“This is not an instantaneous infestation,” said Heather Walden, an associate professor of parasitology at the University of Florida College of Veterinary Medicine. “This takes time – this is not like a bacteria or a virus that spreads quickly.”
Among people in the endemic areas of South and Central America and the Caribbean, “there’s only a few hundred cases in those countries all together, so it’s not something that rapidly transmits to humans”, Timothy Schell, acting director of the Center for Veterinary Medicine at the US Food and Drug Administration (FDA), said on an FDA podcast.
“This is really about protecting livestock,” Marty Makary, the FDA chief, added on the podcast.
“I had to learn a lot about it,” Makary said of the screwworm. “First of all, it looks disgusting.”
A rare case like this can serve “like a sentinel case”, indicating spread in the country where the patient acquired it, said Patrick Hickey, chair of the department of pediatrics at Uniformed Services University and a tropical medicine specialist.
“It probably isn’t a threat, certainly, to the larger public, and it probably isn’t a significant threat to the agricultural industry. But the bigger question is, what’s happening at the border right now with livestock importations?”
Female flies lay eggs in wounds on any warm-blooded animal. Once the eggs hatch, hundreds of screwworm larvae use their sharp mouths to burrow through living flesh, eventually killing their host if left untreated.
The New World screwworm, which affects humans and animals, was once endemic throughout much of the southern half of the United States, and it had enormous economic reverberations on the livestock industry.
But in the 1950s, scientists realized they could expose fly eggs to low doses of radiation and make them sterile – “a really elegant technique”, Hickey said.
The female fly typically only mates once, and encounters with sterile males meant the eggs were not fertilized and populations of the screwworm began declining.
The more successful the sterile-fly program was, the further south it went, eventually moving to the Darién Gap separating Panama from South America. The US has long worked with a screwworm breeding facility in Panama to control the flies.
Hickey said that experience showed the importance of international cooperation on research, monitoring and prevention.
Walden noted that the US “works very closely with other countries to try to mitigate the spread of New World screwworm to keep it out of the United States and other areas, and that’s an ongoing thing that we’ve done for decades”.
Around 2022, though, the screwworm began creeping northward again, probably aided by the cross-border trade of livestock. About 89,000 cases have been detected in animals over the past three years in Central America, Makary said.
Now, “it’s about a few hundred miles south of the US”, Schell said. “It could infest animals and come to the US and cause serious economic damage to our livestock production here, specifically areas like in Texas, where they have a very large cattle population – very susceptible.”
Officials also worry about wildlife bringing the parasite across the border. A 2016 infestation among deer in the Florida Keys was only brought to heel with the release of sterile flies.
Yet some global programs to fight the screwworm were cut by the Trump administration, and scientific research and international coordination have been shaken in recent months.
Facing reports of rising cases, the US Department of Agriculture (USDA) announced in June it would reopen a Texas breeding facility for sterile flies, followed by an $850m plan in August to fight the screwworm.
The US is also expanding its monitoring programs to trap flies, and the USDA is continuing inspections of cattle headed for slaughter.
“Government officials are monitoring this along the borders and in the cattle and any other livestock that might be crossing the borders,” Walden said. “It’s something that’s always on the radar, if you will – something that is always being looked for, just like many other pathogens that we try to keep out of the US.”
The USDA is closely inspecting animals intended for slaughter to make sure they don’t have infestations, Schell said.
And, Makary added, “there’s never been a reported case of somebody getting screwworm infection from eating beef”.
The emergency declaration from the HHS will allow regulators to evaluate existing animal medications and see whether any may be appropriate for emergency use authorizations, Schell said: “It’ll allow us to speed up the process to get the information we essentially need to try to get products on the market as soon as we can.”
“We currently don’t have anything approved for screwworm, but we do have some remedies that we think will work,” he added. One area of focus will be the amount of time that needs to pass before animals treated with anti-parasitic medications may enter the food supply.
But it’s not clear whether treatments like these would work as regular prevention, Hickey said: “Would that be a strategy, and particularly one to be done at scale, on an agricultural basis?”
The fight against screwworms highlights the connections with public health – between humans and animals, among doctors and veterinarians and entomologists, between local and federal agencies, and between countries, he said.
“Everybody has a slightly different piece of the story of how control and prevention is implemented,” Hickey said.
