The US confirms its first human case of New World screwworm. What is it?

The U.S. has confirmed its first human case of the New World screwworm, a flesh-eating parasite whose northward creep from South America has put the country's cattle industry on high alert in recent months.

The Centers for Disease Control and Prevention (CDC), in coordination with the Maryland Department of Health, confirmed the case on Aug. 4 in a patient who had returned from travel to El Salvador, Department of Health and Human Services spokesperson Andrew Nixon told NPR on Monday.

"This is the first human case of travel-associated New World screwworm myiasis (parasitic infestation of fly larvae) from an outbreak-affected country identified in the United States," Nixon said. "Currently, the risk to public health in the United States from this introduction is very low."

David McAllister, a spokesperson for the Maryland Department of Health, confirmed to NPR that the individual — a resident of Maryland — has recovered from the infection."The investigation confirmed there is no indication of transmission to any other individuals or animals," he wrote, calling the detection a "timely reminder for health care providers, livestock owners and others to maintain vigilance through routine monitoring."

The New World screwworm is a species of parasitic flies typically found in South America and the Caribbean, according to the CDC. Infestation occurs when fly larvae feed on the tissue or flesh of warm-blooded animals, primarily livestock and, rarely, humans.

"It's a fly, and it's the larvae that does the damage," says Max Scott, a professor in the Department of Entomology and Plant Pathology at North Carolina State University.

He says infestation in humans can be "quite painful," with high mortality rates if left untreated.

"Because once an infestation starts, that often attracts more flies that lay more eggs," he explains. "And depending where the wound is, the maggots can make their way into vulnerable tissue like the brain, or the wound can get quite big and then you get sepsis."

But, Scott says, screwworm is an insect, not a virus — so it's not contagious.

The pest poses a much bigger risk to livestock, and in the past year has been detected in cattle farms in Mexico. As the New World screwworm gets closer to the U.S. border, federal authorities have taken a series of steps to eliminate the threat — which they did successfully in the mid-20th century.

What exactly is New World screwworm?

An adult New World screwworm fly. They are commonly found in South America and the Caribbean.

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Denise Bonilla

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U.S. Department of Agriculture

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Screwworms are a type of blue-gray blowfly that look very similar to black flies found in the U.S.

The difference is that screwworms — specifically females — lay their eggs in live animals, usually in a wound or another entry point like a nasal cavity.

"The females can lay up to, like, 200 eggs at a time," Scott says. "And then when the eggs eat, they eat the animal alive."

After feeding, the larvae fall into the ground, burrow into the soil and emerge as adult screwworm flies, continuing the cycle.

The parasites are named after the screw-like way they burrow into tissue using their sharp mouth hooks. Their Latin name, Cochliomyia hominivorax, "literally means maneater," Scott says.

"It was named after a sort of unfortunate number of cases in the French penal colony of Devil's Island back in the 19th century," he explains.

Human cases are relatively rare these days, though counts are growing in some parts of South America.

The U.S. Embassy in Nicaragua said in July that it had confirmed 124 cases in the past year. In June, the U.S. Embassy in Costa Rica confirmed its seventh case since 2023 — and first human death "since at least the 1990s."

How was screwworm eradicated — and why is it spreading now?

Screwworm used to be in the U.S., mainly in Florida, Texas and, during the summer, sometimes as far north as the Dakotas, Scott says.

In the 1950s, scientists with the U.S. Department of Agriculture (USDA) pioneered a method of combating them known as the sterile insect technique, which Scott calls "one of the great success stories of the USDA of the 20th century."

Instead of using broad-spectrum insecticides, they decided to use the pest itself as a control agent. That involves mass-rearing insects inside factories, sterilizing them with radiation and then releasing them — either from the ground or, as is the case today, by "planes that fly very precise routes."

"If the females on the ground mate with a sterile male, at least with a screwworm, that's all they'll mate with … so that female won't produce any offspring," Scott says.

Through this technique, the U.S. managed to eradicate New World screwworm in 1966. Mexico followed suit in the 1970s, and Central America in the early 2000s. The U.S. also used this method to eliminate what the CDC calls a "small outbreak" in the Florida Keys in 2017.

"Over a 50-year period, screwworm was pushed back from the United States through Mexico, through Central America, to the Panama-Columbia border. That was about 20 years ago," Scott says. "It was stopped at the border and then was held for a long time until the barrier broke and screwworm came back."

Panama, Costa Rica, Nicaragua and Honduras have documented new cases in recent years, fueling concerns of a northward spread.

Scott says there are probably multiple explanations, including the movement of infested cattle and the possibility that the current strain of sterilized flies is less effective than in the past. The hope, he says, is that a bigger crop of sterilized insects will be able to contain the screwworm threat to southern Mexico, before it can reach the U.S.

The corrals were empty at the Union Ganadera Chihuahua cattle import facility in Santa Teresa, New Mexico, in June, after the U.S. reinstated its pause on cattle imports across the Southern border.

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Paul Ratje

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Bloomberg via Getty Images

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What is the U.S. doing about screwworm?

The U.S. briefly halted live cattle imports from Mexico in November, after a positive case was detected there.

It lifted the ban in February but reinstated it on a month-by-month basis in May, with Agriculture Secretary Brooke Rollins citing "the continued and rapid northward spread of New World Screwworm (NWS) in Mexico." She said it had been detected in farms as far north as Oaxaca and Veracruz, some 700 miles from the U.S. border.

In the months since, the federal government has faced mounting pressure from agricultural groups worried about the threat of screwworm and its potential impact on the supply chain. In an early August letter to Rollins, they used USDA estimates to calculate that a contemporary outbreak could cause a total economic loss of more than $10.6 billion.

The USDA appears to have heard those concerns. Earlier this month, it announced sweeping plans to combat the spread of screwworm, including building the U.S.' only sterile fly production facility at an air force base in Edinburg, Texas. It says it will produce up to 300 million sterile flies per week.

Scott says at its peak, the half-century eradication campaign was run from a facility in Mexico that could produce at least 500 million sterile flies per week. It was shut down for economic reasons in 2012. There is currently only one such facility in operation, in Panama, with a maximum capacity of some 100 million flies per week.

Announcing the new initiative in Texas, Rollins did not specify when the plant will be operational, but has previously said it will take two to three years to build, Reuters reports. The USDA is also supporting a separate facility in Mexico that is slated to open in 2026.

Other steps the USDA says it will take include ramping up the hiring of USDA-employed mounted patrol officers, called "Tick Riders," to focus on border surveillance; training dogs to detect screwworm infestations in livestock at the border and investing $100 million in technologies to combat screwworm.

Agricultural groups welcomed the announcement. Zippy Duvall, president of the American Farm Bureau Federation, said in a statement that the introduction of New World screwworm in the U.S. would only exacerbate an already-volatile cattle market.

"It took decades to eradicate this parasite from within and adjacent to our borders more than a generation ago, and this is a proactive first step," he added.
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