Flesh-eating screwworm confirmed in Texas calf as parasite crosses border from Mexico

CHICAGO/MEXICO CITY – New World screwworm, a parasitic fly that eats warm-blooded animals alive, has been found in a calf in Texas, the US Department of Agriculture (USDA) said on the evening of June 3, exposing the nation’s cattle herd to a serious new threat.

A case was confirmed in a calf in La Pryor, Texas, a town 48km north-east of the US-Mexico border – a blow to US cattle ranchers who have been bracing themselves for a domestic screwworm outbreak as the pest has advanced north through Mexico over the past year.

USDA secretary Brooke Rollins said the case, the first in Texas since 1966, was the only confirmed infestation in the country.

The pest could further shrink the US cattle herd, which is at its lowest level in 75 years. Tight supplies have already reduced beef production and driven prices to record highs for consumers.

The detection also threatens Texas’ livestock industry, which could face up to US$1.8 billion (S$2.3 billion) in estimated economic losses if the pest spreads, and represents a setback for US efforts that cost millions of dollars to keep the pest out, experts said.

Texas agriculture commissioner Sid Miller criticised USDA for allowing screwworm to reach the US.

“Instead of using every available tool, USDA moved too slowly and relied solely on a partial solution that takes years to fully implement,” he said.

Rollins said that the release of sterile flies was the most effective tool and that USDA had invested heavily in production. Sterilised male screwworm flies mate with wild female screwworms to produce infertile egg.

Reuters reported on the morning of June 3 that samples of a suspected screwworm infestation at a ranch in La Pryor had been sent for testing to the federal government’s lab in Ames, Iowa.

Chicago Mercantile Exchange feeder cattle futures extended losses, ending 1.7 per cent lower, with traders worried that confirmation of a US case may reduce demand for beef.

Screwworms are parasitic flies whose females lay eggs in open wounds and mucous membranes on any warm-blooded animal.

Once the eggs hatch, hundreds of screwworm larvae use their sharp mouths to burrow through living flesh, eventually killing their host if left untreated. They mostly spread through the movement of infested animals.

The fly can infest people and pets as well, but the risk to humans is low and human cases of screwworm are rare, experts said. The fly poses no food safety issues.

The USDA said it had halted movement of any animals in a 20km area around the site of the detection in a bid to contain the parasite, released sterile screwworm flies in the area, implemented additional wildlife surveillance, and sent a response team to the area.

Rollins also said that a plane was headed to South Texas with stockpiles of treatments for screwworm.

“Protecting our livestock industry is a national security issue of the utmost importance,” said Dudley Hoskins, a USDA under-secretary.

Impact on cattle supply, beef prices

An extensive outbreak could deal a huge economic blow to ranchers in Texas, the biggest cattle-producing state, through livestock deaths, labour costs and medication expenses.

Washington has kept its border with Mexico closed to livestock imports for more than a year in an effort to prevent the parasite from reaching US border states, spending millions of dollars to slow its advance through Mexico, investing in sterile fly production facilities, expanding trapping efforts and increasing livestock surveillance.

Mexico has confirmed 27,449 cases of screwworm since November 2024, with 2,094 cases classified as currently active.

It has also increased livestock inspections and said that a sterile fly production plant in the country’s south, which the US helped fund, would be operating by the end of June.

The US previously imported more than a million cattle from Mexico a year to fatten in feedlots and process into beef.

Infestations can be cured, but treatment involves removing hundreds of larvae and thoroughly disinfecting wounds, a time-consuming, pricey and labour-intensive process.

Screwworms have been travelling north through Mexico from Central America, and are endemic in many countries in the Caribbean and South America.

In August 2025, the US said a person who returned from El Salvador became the country’s first human case of travel-associated screwworm linked to the current outbreak.

The last outbreak in the US border states in the 1960s decimated the local wildlife population and caused millions of dollars in damage to ranchers.

Screwworms were eradicated from the US in the 1960s when researchers began releasing massive numbers of sterilised male screwworm flies.

A production facility for sterile flies is not expected to open in Texas until late 2027.

On June 3, most active August feeder cattle fell 5.8 US cents to finish at 342.625 US cents per pound. August live cattle futures fell 1.8 US cents to 237.85 US cents per pound. Shares of major meatpackers Tyson Foods and JBS also fell. REUTERS