New World screwworm close to US. More on the flesh-eating parasite

A flesh-eating parasite inching its way toward Texas is raising alarms among health and agriculture officials, decades after it was eradicated in the United States.

The latest U.S. Department of Agriculture data showed New World screwworm was detected about 25 miles from the southern border, in the Mexican state of Coahuila, south of Texas.

Officials in late May identified a goat infected with the flesh-eating infection, which is spread via screwworm flies. Hotter temperatures and changing weather are allowing it to spread back northward across the Americas in recent years after groundbreaking innovations led to its believed eradication 60 years ago.

The latest case is the closest identified detection to the United States to date, and officials are worried about the parasite’s effects on the American cattle industry.

A Republican Texas state lawmaker on June 1 said there was a reported detection as close as a mile from the southern border, though USDA said this was untrue and would provide updates on confirmed nearby cases.

Regardless, USDA data show detections have only moved closer in recent months.

"It does significantly raise the urgency and perhaps may lead to an increase in resources in order to accelerate the deployment of plans" to prevent screwworm from crossing the border, Dr. Michael Payne, a cattle disease expert at the University of California, Davis, School of Veterinary Medicine, told USA TODAY.

The USDA didn't respond to a request for comment.

The USDA has said New World screwworm is not currently in the United States. Officials have previously said there’s no current risk to people, livestock or pets in the country.

Texas Rep. Don McLaughlin, the GOP lawmaker who recently announced that there was a detection a mile from the border, said the threat of screwworm is no longer a distant possibility.

"It is at our doorstep," McLaughlin said in a June 1 statement. "Texas cannot afford to wait until the New World screwworm crosses the border and begins devastating our livestock and wildlife populations."

In a statement to USA TODAY, Texas Agriculture Commissioner Sid Miller said it was "just a matter of time" before the parasite impacts Texas directly. He urged deploying all available means to combat screwworm, adding, "this threat won't wait."

What are New World screwworm flies?

Screwworm flies, around the size of a common housefly, feed on warm-blooded animals, including humans, and create painful, foul-smelling wounds, the Centers for Disease Control and Prevention said.

They’re able to spread when female flies lay eggs inside an open wound, which can be as small as a tick bite, or into an animal’s nose, eyes, ears or mouth. Those eggs hatch maggots that burrow and eat live tissue. It can be deadly to the animal as wounds fester and become larger and deeper, with more maggots hatching and feeding on the animal.

The larvae then drop and burrow into the soil after a few days, later emerging as mature flies. Adult screwworm flies are about the size of a common housefly and have orange eyes, a metallic blue or green body, and three dark stripes along their backs, the USDA said.

How did the United States eradicate them?

The United States was declared free of indigenous screwworms around 1966, according to the USDA. By the 2000s, North America eradicated screwworm.

A small outbreak last occurred in 2017 in the Florida Keys. There have been isolated cases of people in the United States infected by the parasite through international travel, where it’s endemic.

Eradication involves sterilizing male flies using radiation and dispersing them where screwworm flies are indigenous, according to the American Society for Microbiology. Since female flies normally mate only once, they then lay unfertilized eggs. Gradually, the New World screwworm population declines. The USDA said the process is safe, environmentally friendly and a is an alternative to chemical pesticides. It also doesn’t pose a risk to other animals.

Payne, the UC Davis researcher, previously said sterile screwworm flies are the main tool used in the eradication of the pest. New sterile fly production and dispersal facilities are being brought online rapidly in both Mexico and the United States, he said in emailed responses.

Why are they returning?

In subsequent decades, the parasite has steadily marched back toward the United States from where it is endemic in South America. Central America and Mexico, the latter a major cattle supplier to the United States, have experienced screwworm outbreaks.

American officials have closed southern ports of entry to livestock trading to prevent New World screwworm's spread.

A 2025 Nature study said climate change is contributing to New World screwworm’s reemergence with warming temperatures and extreme weather patterns. Conditions are becoming more favorable for its spread in the southern United States, particularly in Texas. Losses from cattle afflicted by the parasite could run into the billions of dollars, with larger effects across the American economy, according to USDA estimates.

This story was updated with new information.

Contributing: Natalie Neysa Alund, USA TODAY

Eduardo Cuevas is based in New York City. Reach him by email at [email protected] or on Signal at emcuevas.01.