A case of New World screwworm, or screwworm fly, has been detected in a herd in south Texas, the USDA (U.S. Department of Agriculture) said.
This is the first time in decades that this parasitic fly – whose larvae feed exclusively on the tissue of warm-blooded animals – has been detected in livestock in the US.
New world screwworms are the carnivorous larval form of Cochliomyia hominivoraxa species of blow fly.
Although it does not pose a food safety risk, an infestation can affect food production and could cost the economy billions. It would also increase the price of beef at a time when Americans are already paying record prices.
The USDA said its National Veterinary Services Laboratories in Ames, Iowa, tested a sample from a three-week-old calf in La Pryor, Texas, and confirmed on Wednesday that it was the New World screwworm.
The agency has mobilized teams in Texas to contain and eradicate the parasite, including a 20-kilometer infestation zone and implementing quarantines, movement controls and surveillance in the area.
The agency is also speeding up , which are used to overwhelm fertile flies in the area and limit spread.
“If we all work together and follow animal care protocols and movement restriction guidelines, there is no reason to believe that this incursion will result in the establishment of the pest in our country,” US Secretary of Agriculture Brooke Rollins said during a press conference.
The New World screwworm had been declared eradicated in the US decades ago, thanks to efforts such as the creation of sterile flies, awareness campaigns and initiatives to control the pest in other countries.
However, there has been a sudden increase in cases in South America, which has been closely monitored by public health experts and the Department of Agriculture.
Deadly parasite
Fly is not a contagious disease that spreads from animal to animal. Instead, adult females lay their eggs in fresh wounds of warm-blooded animals.
The larvae feed on the host and can damage vital organs or cause serious bacterial infections. Cases can be severe and even lead to the death of the host animal.
The parasite also poses a threat to wildlife and pets. Veterinarians in Texas, Arizona and New Mexico have been told to be on the lookout for new infections.
On Wednesday, the U.S. Secretary of Agriculture warned pet owners to be alert for signs of discomfort, open wounds or larvae or eggs near holes in the body.
These are rare but can be fatal. The last person in the U.S. to contract the blowfly infection was a case in Maryland in August of a person who had traveled outside the country. She recovered completely.
Low threat to humans
Brooke Rollins said Tuesday that the current threat to human health is extremely low and that the blowfly does not pose a risk to food security, but that “there is no doubt that this is a very, very serious threat to food.”
People most at risk of contracting New World screwworm are those who work with livestock or other warm-blooded animals in areas where flies are present and those who spend a lot of time outdoors, especially if they sleep outdoors.
People with health problems that cause bleeding or open wounds may also be vulnerable to infection.
Any open wound, even a small break in the skin like a scratch or insect bite, can attract these flies, according to the U.S. Centers for Disease Control and Prevention.
In August, the U.S. Department of Health and Human Services began allowing emergency use of medications to treat or prevent infestations in animals. A shipment of that treatment is on its way to South Texas, Rollins said Wednesday.
The USDA says it has created new monitoring, testing and quarantine protocols in response to the outbreak spreading across Central and South America.
In May 2025, the agency suspended the import of live animals through U.S. ports of entry along the southern border.
The department has stationed sniffer dogs on the border with Mexico capable of detecting the New World screwworm and sent teams to the neighboring country and Panama to help increase the production of sterile flies.
The US has also committed $750 million to build a facility in Texas that will produce hundreds of millions of sterile flies per week. This facility is expected to open next year.
Infestation can be devastating
A blow fly infestation in the US could be financially devastating, experts say.
The nation’s worst outbreak occurred in 1972, when the Department of Agriculture estimated 90,000 cases, and another outbreak of that magnitude could cost the U.S. Southwest more than $3 billion, according to the Federal Reserve Bank of Dallas.
Max Scott, an entomologist and plant pathologist who genetically modified the blow fly so that it cannot reproduce, said outbreaks are typically controlled by breeding flies in a factory and sterilizing the insects through exposure to gamma rays while they are still in the pupal stage.
Scientists then release enough sterile flies to overwhelm the local population of fertile flies. When the fertile female mates with a sterile male, they do not produce offspring.
“The USDA, over 50 years, has managed to eradicate this fly all the way to the Panama-Colombia border,” said Scott, a professor at North Carolina State University.
“It was a huge effort by a lot of people, but unfortunately it’s a little-known feat, even though it was a success story. They managed to keep it there, far from here, for 20 years, but the barrier failed a few years ago, and the flies quickly spread across Central America until we got to where we are today
