Flesh-Eating New World Screwworms Reemerge in the United States
As of June 21, 15 cases of New World screwworm have been confirmed in cows, goats, sheep, and a dog across Texas and New Mexico, according to the U.S. Department of Agriculture. The parasitic fly feeds on living flesh in open wounds, though experts state the risk to humans remains low.
The New World screwworm (Cochliomyia hominivorax) is a subtropical blowfly native to South America. Females deposit eggs into open mammal wounds, where larvae burrow into and feed on living tissue. If left untreated, these infestations can lead to secondary infections or death.
The USDA detected the first new U.S. case on June 3 in a three-week-old calf in Zavala County, Texas. This marks a significant return for a pest that was eradicated in the United States in 1982.
Are humans at risk from screwworm infestations?
Experts say the risk to humans is low. There are currently no reported cases of locally acquired screwworms in the U.S., though one person returning from El Salvador reported an infection in 2025.
Since 2023, 2,100 human cases have been reported in Mexico and Central America following outbreaks in Panama and Costa Rica. Edwin Burgess, a veterinary entomologist at the University of Florida, describes the risk to the general population as “itty-bitty.”
Burgess notes that infestations are foul-smelling and incredibly painful, making them difficult to miss. For those in infested regions, experts recommend wearing loose-fitting clothing, using insect repellent, and keeping open wounds covered.
Why is the New World screwworm returning?
There is no consensus on the cause of the northward spread. Some studies suggest global warming may be a factor, as the flies favor warmer climates. One study estimates climate warming could intensify outbreaks and widen the fly’s reach over the next two decades.
The closure of sterile insect technique (SIT) breeding facilities may have also contributed. Until recently, the COPEG facility in Panama was the only producer of sterile flies. While COPEG increased production from 40 million to 115 million flies per week, Phillip Kaufman of Texas A&M University says this is only 20 percent of what may be needed for eradication.
The Wildlife Conservation Society suggests illegal cattle crossings across the Guatemala–Mexico border may drive the spread. Kaufman added that moving cattle across Central America without proper inspection could have hindered suppression efforts.
How is the USDA fighting the outbreak?
Federal agencies are utilizing epidemiological tracking and meteorological data to predict fly movement and target the release of sterile males. A new dispersal facility is being established at Moore Air Base in Edinburgh, Texas, while an existing facility in Metapa, Mexico, is being renovated.
The USDA is seeking EPA approval for the “Novo Fly” strain. This technology eliminates female larvae early in the brood, allowing facilities to produce 100 percent male flies and potentially doubling production capacity.
On June 16, the USDA announced $105 million in funding for innovative fight efforts, including new sterilization methods and early warning traps. This follows $21 million spent on the Mexico facility and $8.5 million allocated for the Texas site.
The FDA has also issued emergency use authorizations for various animal treatments. This includes the systemic insecticide Dectomax, authorized on May 19, which targets the nervous system of larvae within animal tissues.
Phillip Kaufman warns that efforts may initially slow the spread rather than achieve immediate eradication. He notes that the Edinburgh facility will take another year to go live, meaning the pest could spread further in the interim.
Frequently Asked Questions
Is the food supply safe from screwworms?
Yes. According to Phillip Kaufman, screwworms do not infest meat, vegetables, or other food products.
What is the sterile insect technique (SIT)?
SIT involves using radiation to sterilize male flies. When these males mate with females, the females produce unviable eggs, causing the population to decline.
What happens if an animal is infested?
The larvae feed on living flesh, which is painful for the host. However, Kaufman states that if animals are found and their wounds are treated and dressed, they can make a full recovery.
How should pet owners and livestock farmers monitor for these parasites in their regions?