What We Know About the Flesh-Eating Screwworm Outbreak

The U.S. Department of Agriculture has confirmed 74 animal cases of New World Screwworm (NWS) infestation since the parasitic fly re-entered the country in June, according to Ars Technica's review of USDA data. The infestation, once eliminated from the U.S. in the 1960s, now spans three states after Arizona confirmed its first case this week.

What is a screwworm and why does it attack living animals?

The New World Screwworm is not a worm but the larval stage of a fly that lays hundreds of eggs in open wounds and moist orifices of warm-blooded animals, Ars Technica reported. The hatched maggots, which resemble screws, bore and twist into living tissue as they feed, producing wounds that can become severe and in some cases fatal before the larvae drop to the ground to mature into adult flies. The species was wiped out domestically decades ago through aerial releases of sterile male flies, a technique that disrupts breeding by ensuring fertilized eggs never develop.

How many U.S. animals have been infected, and where?

By the numbers:

• 74 total animal cases confirmed in the U.S. since the flies reemerged in June, per USDA data cited by Ars Technica.
• 10 of those cases involve domestic pets — mainly dogs, plus one domestic cat in New Mexico — with four new pet cases recorded in a single week in late October, including the Arizona index case.

Most of the 74 cases remain concentrated in livestock, the primary economic concern for the agency, but the pet total has climbed as the fly's range has widened. One case has also been confirmed in a wild elk.

Which states have confirmed cases so far?

Texas and New Mexico were the first two states to record infestations after the fly's return. Arizona became the third this week, with its index case identified in a dog in Cochise County, a border county that touches both New Mexico and Mexico, Ars Technica reported, citing the Arizona Department of Agriculture. A local veterinarian examined the dog on October 26 and found a wound "heavily infested with maggots consistent with NWS characteristics," according to the state agency's announcement.

"We remain prepared and vigilant, and we urge all livestock owners, pet owners, and other animal industry partners to continue to monitor animals closely and report any unusual wounds consistent with NWS immediately," the Arizona Department of Agriculture said.

The USDA confirmed the Arizona infection and said it is now working with the state on a joint response to the expanded infestation, Ars Technica reported.

How did screwworms get back into the United States after being eradicated?

For decades, the U.S. and Mexico held screwworms at bay using a biological barrier along the Darién Gap, the stretch of jungle at the border of Panama and Colombia where sterile flies were continuously released to prevent northward spread. That barrier was breached in 2022, allowing the flies to migrate back through Central America and eventually reach the U.S. border this year, according to Ars Technica's account of the outbreak's origins.

Is the outbreak affecting humans too?

Screwworms are best known as a livestock threat, but they can also harm wildlife, pets and, rarely, people. Mexico's Ministry of Health has confirmed 806 human screwworm infestations since the flies reemerged, with nine deaths attributed to the infections, Ars Technica reported citing the ministry's data. No human cases have been reported in the U.S. as of the latest USDA figures.

What should pet owners and livestock producers do now?

State agriculture officials in Arizona have urged livestock owners, pet owners and industry partners to monitor animals closely for unusual wounds and report any suspected cases immediately rather than waiting for symptoms to worsen. The USDA's joint response with Arizona focuses on identifying additional cases quickly and containing further spread, following the same surveillance approach already in place in Texas and New Mexico.

The situation remains under active monitoring by federal and state agriculture agencies, with case counts likely to be updated as more animals are examined near the border region.