A flesh-eating parasite is creeping north from Mexico and Central America, spurring the U.S. to launch an aggressive eradication plan. Despite these efforts, a patient recently contracted the country’s first travel-associated case of New World screwworm infestation.
On Sunday, August 24, Department of Health and Human Services spokesperson Andrew Nixon told Reuters that health officials confirmed the case on August 4 in a Maryland resident who recently returned from a trip to El Salvador. David McAllister, a spokesperson for the Maryland Department of Health, told NPR the patient has recovered from the infection and confirmed that it has not spread to other individuals or animals.
“Currently, the risk to public health in the United States from this introduction is very low,” Nixon said.
What is the New World screwworm?
NWS infestations are rare in humans, despite what the maggot’s scientific name—Cochliomyia hominivorax, meaning “man-eater”—might suggest. Their common name is also disgustingly fitting. Adult flies lay eggs in wounds or mucous membranes of warm-blooded animals. When the screw-shaped larvae hatch, they burrow into the host’s flesh, feeding as they go. This causes deep, painful wounds that can kill livestock in droves, according to the Texas Farm Bureau.
This parasitic fly is endemic to Cuba, Haiti, the Dominican Republic, and countries in South America, according to the U.S. Department of Agriculture, but it has spread north before. In the 1930s, the parasite became a significant problem for U.S. farmers, prompting the USDA to launch a decades-long eradication effort that led to the development of its sterile fly release program. Mexico and Central America followed suit, and by 2000, NWS was contained below the Darien Gap—a stretch of jungle between Panama and Columbia.
Cases creeping north
In 2023, NWS detections exploded in Panama. Since then, outbreaks have reached Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and Mexico. The northward spread has created significant challenges for livestock farmers and led to many human infections. In June 2024, the U.S. Embassy in Costa Rica confirmed the country’s seventh case since 2023 and the first human death since at least the 1990s. In July, the U.S. Embassy in Nicaragua said officials had confirmed 124 human cases in the past year.
This has reignited the USDA’s sterile fly release efforts. The agency began ramping up its strategy in June, but on August 15, it announced the “largest initiative yet” in its plan to combat the reemergence of NWS. This includes researching and implementing new eradication techniques, constructing a new sterile fly breeding facility in Texas, deploying “tick riders” at the southern border who will check livestock for infestations before they cross into the U.S.
USDA officials—and U.S. farmers—hope this will keep the pests contained below the southern border. Zippy Duvall, president of the American Farm Bureau Federation, commended the agency’s announcement in a public statement. “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,” Duvall said. “Cattle markets are already volatile and the introduction of New World screwworm within the U.S. would only increase that volatility.”