The spread of a flesh-eating parasite that devastates cattle and can infect humans and other mammals has alarmed conservationists and cattle farmers. Fears it would reach the United States prompted officials in May to suspend Mexican cattle imports.

The New World screwworm fly (Cochliomyia hominivorax) has over the past three years crossed from South America into Panama and torn through Central America with the help of cattle smugglers, according to scientists and conservation workers. The parasite reached southern Mexico in May and threatens to undo a billion-dollar effort that eliminated the fly from North and Central America decades ago.

It threatens not only cattle but wildlife, say veterinarians and conservationists. The fact that the pest has broken through from South America suggests that other diseases will follow, they say.

“This is a public health emergency,” says Jeremy Radachowsky, Mesoamerica and Western Caribbean regional director for the nonprofit, U.S-based Wildlife Conservation Society (WCS). The parasite is a “major, major threat to the entire region, its economy, human health [and] wildlife,” he says.

The screwworm fly, an orange-eyed insect that is about twice the size of a house fly, lays eggs in the wounds of warm-blooded animals. Its larvae feed on live flesh and can severely harm or kill their host.

The pest was eliminated from the United States in the 1960s and Central America in the 1990s by a U.S.-led program that uses radiation to sterilize male flies. The flies are dispersed by airplane over areas where the screwworm is endemic and they mate with females, which mate only once in their 20-day life. For decades, the sterile flies and the dense jungle between Colombia and Panama known as the Darién Gap kept the screwworm at bay. But scientists, veterinarians and conservationists say a surge in foot-traffic in Darién in recent years helped the parasite breach that barrier. Migrants and people-smugglers, or coyotes, who traveled by the thousand through Darién, and domestic animals with them, likely brought the screwworm, says Kurt Duchez, WCS director of programs for Mesoamerica and the Western Caribbean.

Outbreaks of screwworm in Panama leapt from around 25 per year to more than 6,000 in 2023, according to the U.S. Department of Agriculture (USDA). Over the next two years, the parasite spread to Costa Rica, Nicaragua, El Salvador, Honduras, Belize and Guatemala and was detected in late 2024 in Mexico. Mexico reported infestations of the fly in cattle as far north as the states of Oaxaca and Chiapas in May, including a handful of human infections.

Cattle-smuggling a factor, too

The disease “caught fire” in Nicaragua, spreading along cattle-smuggling routes to Mexico, experts say. Cattle smuggling—and illegal ranching in protected areas such as the Maya Biosphere Reserve in northern Guatemala—has boomed over the past 15 years along with trafficking of drugs, contraband and migrants trying to reach the United States.

Alejandro Zaldívar-Gómez, a PhD student at the National Autonomous University of Mexico (UNAM) who applied mathematical modelling to the proliferation of the screwworm, says the pest’s spread through Central America indicates it is traveling in trucks. On its own, the fly can move only 1.6 to 1.9 kilometers (1 to 1.2 miles) per day, says Zaldívar.

Outbreaks in quick succession dozens of miles apart mean the fly or its larvae must be traveling on cattle being transported, most likely by smugglers, he says. (Authorities limit movements of legally registered cattle and check animals for disease before travel, so they are a less likely vector, experts say.)

Zaldívar and Radachowsky note the geographic pattern of outbreaks correlates with a map of cattle-smuggling routes published by InSight Crime, a Washington, D.C.-based research group that studies organized crime in Latin America. The screwworm outbreak reflects “a whole socioeconomic phenomenon” of smuggling and migration, Zaldívar says.

Cattle smuggled through Central America to Mexico—some 800,000 a year—are in most cases fattened in Mexico and slaughtered there principally for local consumption, experts say. Cattle that meet U.S. standards are bred in Mexico for sale in the U.S. Concern that these cattle might carry screwworm prompted the United States to close its border to Mexican cattle and other livestock from November to February and again in May. The suspension will be reviewed monthly, according to the USDA.

Alvaro Iván Bustillos Fuentes, president of the Regional Cattle Ranchers’ Union of Chihuahua in Mexico, says the loss of trade across the United States border was costing cattle ranchers millions of dollars a day. Speaking on the Mexican television show Aristegui Noticias on May 14, Bustillos said the New World screwworm threatened not only Mexico’s cattle, but also its “deer, sheep, horses, even public health.”

Sterile flies being released in Mexico

A longtime U.S.-Panamanian initiative that produces sterile flies in Panama and deploys them to prevent screwworm spread says it is now dispersing flies over the Isthmus of Tehuantepec in southern Mexico. Experts say the effort, by the Panama–United States Commission for the Eradication and Prevention of Screwworm (Copeg), will not work without a crackdown on Central American smuggling. “You need to bring in law enforcement and security agencies,” says Radachowsky. “This needs to be treated as a national security issue.”

Governments must also educate the public about preventing and treating infestations, experts say. Luis Fernando Guerra, regional health coordinator for WCS in Mesoamerica and the Western Caribbean, says animals infected with screwworm can be treated with insecticide and deworming medication and their wounds can be healed with silver sulphate. This is of course impractical when it comes to wild animals. The extent of screwworm infestations in wild animals is not known, but infection from the parasite has been reported in Costa Rican tapir and in a Maya Biosphere Reserve camera-trap image of a puma, Guerra says.

Only an effort coordinated at the highest level of government, across agencies and across the region, will stop screwworm, experts say. Says Zaldívar, “It took us nearly 30, 40 years to eradicate this. To lose everything in just two or three years would be distressing.”