Flesh-eating, hook-mouthed maggots have returned to the United States. As of June 21, 15 cases of the New World butcherworm have been confirmed in Texas and New Mexico in cows, goats, sheep and even a dog.
The endless screw (Cochliomyia hominivorax) threatens the health of animals and humans as well as the beef industry. Females of this subtropical fly, native to South America, deposit their eggs in the open wounds of mammals. The larvae burrow into living flesh and feed on it as they grow. The digging process can be painful to the host and, if left untreated, can cause secondary infections or even death. Flies like mammals and are not selective. They parasitize wildlife, pets, livestock – such as cattle, sheep and goats – and sometimes humans.
After eradicating the cork rootworm in the United States in 1982, this pest was gradually creeping north. THE first new US case was detected June 3 in a three-week-old calf in Zavala County, Texas, according to the United States Department of Agriculture.
Scientific news I spoke with experts about how concerned we should be about this new invasion and what steps are being taken to combat it.
What are the risks of re-emergence of the New World butcher caterpillar?
In humans, the risk is low, experts say. There is currently no human cases reported of locally acquired dung worms in the United States, although in 2025 one infection was reported in a person returning to the country after visiting El Salvador. However, since Panama and Costa Rica first identified an outbreak in 2023, 2,100 human cases have been reported in Mexico and Central America.
Still, human cases are extremely rare, says Edwin Burgess, a veterinary entomologist at the University of Florida in Gainesville.
“You can look online and find horror stories about the number of human cases, but you have to think about the total population over that time period,” Burgess says, “and that number is tiny in terms of risk.”
Even so, a dung worm infestation in a wound is incredibly painful and foul-smelling, meaning it’s difficult for a human case to go unnoticed. In rare cases of human infection, the treatment of removing larvae from the infected wound is simple.
To prevent infection in an infested area, experts recommend wearing loose clothing that limits exposed skin, using insect repellent, and keeping open wounds clean and covered.
People living near an outbreak site in Texas and New Mexico should pay special attention to their animals and wildlife. Although the number of cases in the United States remains relatively low, even a single case of butcher worm is concerning because it is a very damaging pest, says Phillip Kaufman, a veterinary entomologist at Texas A&M University in College Station. A 2024 USDA Economic Report estimated that, based on losses to the Texas economy during a 1976 outbreak, the costs of a similar screwworm outbreak would exceed $1.8 billion per year in Texas alone.
Despite these concerns, experts emphasize that the food supply is safe. Dungworms do not infest meat, fruit, vegetables, or other food products, and when animals are found and their wounds treated and dressed, they can make a full recovery.
“You won’t have dung worms in your meat,” Kaufman says.
How have we eradicated the butcher caterpillar before?
During the epidemic in the 1950s and 1960s that spread through Texas and the southeastern United States, federal agencies resorted to so-called sterile insect technique, or SIT.
Female dung worms only mate once. The technique involves breeding males using radiation to sterilize them, then deploying them with trucks and planes to infested regions. If sterile males outcompete fertile males and mate with females, the females produce nonviable eggs and their population declines over time.
The sterile insect technique has been highly successful for decades and has been continually used to suppress breakthrough epidemics throughout South and Central America.
Why are New World dungworms spreading north again?
There is currently no consensus on the causes of the northward spread of butcher worms. Some studies suggest that global warming could be a key factor in the re-emergence and movement of the fly, as it prefers warmer climates. One study estimated that global warming over the next two decades could expand the reach of butcher worm in North America and intensify the severity of epidemics.
Closures of SIT breeding facilities may also have played a role. After successfully mitigating worm infestations in the 1960s and 1970s, many facilities farm due to the costs and complexities of keeping them open.
Until recently, the only facility producing sterile flies was Panama-United States Commission for the Eradication and Prevention of Bacterial Luctose Infestation of Livestock (COPEG) installation in Panama. Since the detection of screwworms across the Guatemala-Mexico border in 2024, COPEG has increased from 20 to 40 million flies per week to approximately 115 million flies per week. Even with COPEG’s intensified breeding efforts, the facility produces only about 20 percent of what might be needed to eradicate the flies from North and Central America, Kaufman says. In the United States, the latest eradication effort required the export of 500 million flies per week.
Additionally, Kaufman suggests that the movement of livestock across Central America without proper inspection and treatment could have hindered the ability to suppress the northward movement of screwworm. Illegal cattle crossings across the Guatemala-Mexico border could also be causing the spread of butcher worms, according to the Wildlife Conservation Society.
What are we doing this time to fight against the New World butcher worm?
Federal agencies are deploying new tools to mitigate the butcher worm outbreak. The USDA now uses epidemiological and weather monitoring data to predict where flies are and where they are likely to go, then uses that data to determine where to release sterile males.
Multiple fly production facilities are under construction or renovation, such as an existing facility in Metapa, Mexico, and a new dispersal facility at Moore Air Force Base in Edinburgh, Texas. For now, however, efforts are aimed more at slowing the spread than at eradicating the pest, Kaufmann says.
“We have help on the way,” he said. “It will take time before we have a sufficient number [of sterile flies] that we can push this south.
The USDA recently unveiled a new sterile fly rearing technology, New fly strain, aiming to double the number of male flies produced by eliminating female flies from each brood. Using this strain, all female larvae will die early, allowing a given clutch to be 100% male.
“We can basically put twice as many eggs in a breeding container, knowing that half will die and half will be male,” Kaufman says. “So instead of producing 50 million males and 50 million females per week, we are now producing 100 million males.” The USDA is seeking approval from the U.S. Environmental Protection Agency for use of the Novo Fly, which experts hope will significantly increase sterile fly production in the United States, Mexico and Panama.
The United States Food and Drug Administration has also issued several emergency use authorizations for the treatment and prevention of screwworm infections in a wide range of animals, since cattle, sheep and goatshas cats, dogs, birds and even wild mammals. Many of these medications have been used for decades to treat various types of parasitic infections in animals and show promise in their effectiveness in treating dung worm infestations.
“We need chemicals with unique modes of action [in how] they affect the physiology of the fly,” says Burgess. [is to] balancing available therapeutic options while maintaining the safety of animals and the food supply.
Injectable insecticides like Dectomaxwhich received emergency use authorization on May 19, are systemic. They work by diffusing through the animal’s tissues, targeting the nervous system of the larvae which ingest the flesh.
The road ahead in the fight against the New World butcherworm is still quite long and costly. It will take another year for the Edinburg, Texas, facility to be operational, and butcher worms could spread further in the meantime. USDA spent $21 million for the Mexico facility and will spend an additional $8.5 million on the Texas facility. On June 16, the USDA also announcement that they would grant $105 million to fund innovative efforts to combat the parasite, including new ways to sterilize flies and new traps to help strengthen early warning systems. Kaufman and other experts agree that the cost is worth it.
“Every dollar we spend to eliminate this fly is worth it, and we will continue on this path and we will be successful,” he said.
