Reuters Investigation: US Left Exposed to Screwworm as Cases Reach 120

A Reuters investigation published on Monday has exposed how the United States was left exposed to the flesh-eating New World screwworm, with 120 confirmed cases now recorded after a shortage of sterile flies let the parasite break through a containment barrier in Panama and spread north through Mexico into American herds.

Key Points

• The United States has now recorded 120 confirmed New World screwworm cases, up from 113 reported last week.
• The parasite broke through a sterile-fly barrier in Panama in early 2023 and spread through Mexico, which has about 51,000 confirmed cases.
• Mexico and Panama together produce only about 150 million sterile flies a week, still short of what is needed three years into the outbreak.
• The United States has no sterile-fly plant of its own, and one under construction in Texas will not start production until spring 2027.
• The Trump administration awarded a $610 million contract in March to M.A. Mortenson for the Texas plant, after the USDA began discussing one in November 2024.

What Happened

Reuters reported on Monday that the screwworm, which the United States eradicated from its own territory in the 1960s, is now established on American soil. The US case count has reached 120, the investigation found, after the parasite broke through a barrier of sterile flies in Panama in early 2023 and moved steadily north through Mexico.

Mexico has recorded about 51,000 confirmed cases, and the scale of the outbreak there has overwhelmed the sterile-fly supply. Mexico and Panama together now produce only about 150 million sterile flies a week, the investigation found, still short of what the outbreak demands. The rise from 113 cases to 120 confirms the outbreak is still growing on American soil, even as the border with Mexico has begun to reopen to cattle imports.

Background and Context

The New World screwworm is a parasitic fly whose larvae feed on the living flesh of warm-blooded animals. Females lay eggs in open wounds, and the maggots burrow into the tissue, causing wounds that can kill livestock and wildlife if untreated. The US wiped the pest out in the 1960s with the sterile insect technique, releasing sterilised males so wild females lay eggs that never hatch.

The technique needs a vast, steady supply of sterile flies, and for decades a barrier of releases in Panama, run by a joint Panama-US commission, kept the pest confined to South America. The barrier broke in early 2023, and the fly moved through Central America into Mexico, where cases climbed into the tens of thousands.

The US closed its border to Mexican cattle imports for more than 15 months from May 2025, disrupting a multibillion-dollar trade. The first US case was confirmed on June 3, 2026, and dozens more followed in US cattle as the border began to reopen. Sterile-fly drops shifted to the Texas-Mexico border in February 2026. As our earlier report on the outbreak reaching 113 US cases detailed, those cases spanned 32 counties in Texas, New Mexico and Arizona, mostly in cattle.

Screwworm: Why the Sterile-Fly Supply Fell Short

At the heart of the Reuters investigation is a simple numbers problem. The sterile insect technique only works when sterile males vastly outnumber wild males, so wild females mate with sterile partners. When the supply falls short, enough wild matings succeed to keep the population growing. That is what happened in Panama in early 2023, and the supply has never caught up.

The United States has no sterile-fly plant of its own. The Agriculture Department began discussing one in November 2024, and the Trump administration awarded a $610 million contract to M.A. Mortenson this March to build it in Texas. But production is not expected until spring 2027, leaving the US dependent on Panama and Mexico — which helps explain why the border closure, the shifted drops and months of surveillance have slowed the pest without stopping it.

Reactions and Quotes

A USDA spokesperson told Reuters the agency has "acted urgently and pursued every practical avenue to get more sterile flies into the field as quickly as possible". It is the department's public defence of a response the investigation portrays as constrained from the start by the missing domestic production capacity.

The investigation's central finding is not that officials ignored the outbreak, but that the tools available were too small for the scale of the problem. The programme that eradicated the pest was built for a contained enemy; the outbreak that escaped Panama in 2023 arrived with a supply chain never sized for a continental spread.

Why It Matters

The outbreak matters first to American ranchers. Cattle are the main target, and every case means treatment, quarantine and lost value, on top of the 15-month border closure's disruption. Mexico is one of the largest suppliers of feeder cattle to the United States, and the closure and slow reopening have rippled through beef prices on both sides of the border.

It matters beyond the ranch gate too. The pest attacks any warm-blooded animal with an open wound, including pets, wildlife and, rarely, people, and its spread into three states shows it is not a distant foreign problem. It is also a test of preparedness: the technique is proven, but the capacity to use it at scale shrank until one barrier breach could grow into a continental outbreak.

What Happens Next

The immediate focus is the Texas plant. Construction is under way under the $610 million Mortenson contract, with production expected to begin in spring 2027. Until then, the United States will keep relying on sterile flies produced in Panama and Mexico, and on the drops now concentrated along the Texas-Mexico border. Surveillance will continue across the southern states, and the case count will be watched closely as the border reopens further to Mexican cattle. The investigation is likely to sharpen questions in Washington about why the plant was not contracted sooner: the Agriculture Department began discussing a facility in November 2024, yet the contract was not awarded until March, and production is still more than a year away.

Frequently Asked Questions

What is the New World screwworm?

A parasitic fly whose larvae feed on living flesh. Females lay eggs in open wounds; the maggots burrow in and can kill livestock, pets and wildlife if untreated. The US eradicated it in the 1960s.

How many screwworm cases has the US recorded?

120 confirmed cases, per the Reuters investigation of November 9, 2026, up from 113 the previous week. Mexico has about 51,000 confirmed cases.

Why did the outbreak reach the United States?

It broke through a sterile-fly barrier in Panama in early 2023 and spread through Mexico. Mexico and Panama make only about 150 million sterile flies a week, and the US has no plant of its own.

When will the US have its own sterile-fly plant?

A Texas plant is under construction under a $610 million contract awarded to M.A. Mortenson in March 2026, with production expected from spring 2027.

What is being done to fight the outbreak now?

Sterile-fly drops moved to the Texas-Mexico border in February 2026, surveillance continues in the southern states, and the cattle border with Mexico, closed for over 15 months, has begun to reopen.