New World screwworm returns to the U.S.—and tests resilience

After the pest was eradicated in the United States in 1982, the New World screwworm has crept back north. As of June 21, 15 cases have been confirmed in Texas and New Mexico—in cows, goats, sheep, and even a dog—sparking an expanded campaign of sterile insect

The first report came with a kind of dread farmers recognize immediately: flesh-eating, hooked-mouth maggots—New World screwworm larvae—had been found again in the United States.

As of June 21, 15 cases have been confirmed in Texas and New Mexico in cows, goats, sheep, and even a dog. The species—Cochliomyia hominivorax, a subtropical blowfly native to South America—spreads by laying eggs into open wounds of mammals. The larvae then burrow into living flesh, feeding as they grow. That burrowing can be agonizing for the host. and when it’s left untreated. secondary infections and even death can follow. Screwworm flies don’t discriminate: they parasitize wildlife, pets, and livestock, including cattle, sheep, and goats—and occasionally humans.

This time, the reappearance comes after the pest was eradicated in the United States in 1982. The first new U.S. case was detected June 3 in a three-week-old calf in Zavala County, Texas. From there, the number of confirmed cases climbed, and the response moved quickly from alarm to action.

While the infestation is unsettling—especially for people whose livelihoods depend on healthy animals—experts stress that the stakes are different depending on who you are.

In humans, the risk is described as low. There are currently no reported human cases of locally acquired screwworm in the United States. though in 2025 an infection was reported in a person returning to the country after visiting El Salvador. Since Panama and Costa Rica first identified an outbreak in 2023. there have been 2. 100 reported human cases in Mexico and Central America.

Edwin Burgess, a veterinary entomologist at the University of Florida in Gainesville, puts the fear in its numerical place. “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 itty-bitty in terms of a risk.”.

Still, human cases aren’t something to casually ignore. An infestation in a wound is described as incredibly painful and foul-smelling—conditions that make it difficult for a human case to go unnoticed. And in the rare event of infection, larvae removal treatment of the infected wound is described as straightforward.

To prevent infection in an infested region, experts recommend wearing loose-fitting clothing that limits exposed skin, using insect repellent, and keeping any open wounds clean and covered.

For families living near outbreak sites in Texas and New Mexico, the message is blunt: watch animals and wildlife closely. Even when the number of U.S. cases remains relatively low. Phillip Kaufman. a veterinary entomologist at Texas A&M University in College Station. says that even a single screwworm case is concerning because the pest is so damaging.

The economic hit can also be severe. A 2024 USDA economic report estimated that, based on the losses the Texas economy faced during a 1976 outbreak, the costs of a similar outbreak of screwworm would exceed $1.8 billion annually in Texas alone.

For consumers, reassurance comes in a single sentence: the food supply is described as safe. Screwworms do not infest meat, fruits, vegetables, or other food products. When animals are found and their wounds are treated and dressed, they can make a full recovery. “You are not going to have screwworms in your meat,” Kaufman says.

What makes this outbreak feel especially sharp is how hard the U.S. had once fought—and the method that worked.

During the 1950s–60s outbreak that spread throughout Texas and the southeastern United States. federal agencies used what is known as the sterile insect technique. or SIT. The biology matters: female screwworms mate only once. In SIT, males are bred, sterilized using radiation, and then deployed with trucks and aircraft into infested regions. If sterile males outcompete fertile ones and mate with females. those females produce unviable eggs. and the population declines over time.

The sterile insect technique has been widely successful for decades and has been continuously used to suppress breakthrough outbreaks across the American South and Central America.

So why is the screwworm spreading north again?

There is currently no consensus on what drove the northward reemergence. Some studies suggest global warming could be a key factor, because the flies favor warmer climates. One study estimated that climate warming in the next two decades could widen the reach of screwworms in North America and intensify the severity of outbreaks.

Other explanations focus on the supply chain behind eradication. Shutdowns of SIT breeding facilities might have played a role. After successful mitigation of outbreaks in the 1960s and 1970s, many facilities closed due to the costs and complexities of keeping them open.

Until recently. the only facility producing sterile flies was the Panama–United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG) facility in Panama. Since the detection of screwworms past the Guatemala–Mexico border in 2024. COPEG has increased production from typically 20 million to 40 million flies per week to about 115 million flies per week. Even with that ramp-up. Kaufman says the facility is producing only about 20 percent of what may be necessary to eradicate the flies from North and Central America.

That shortfall matters because eradication in the United States alone once required 500 million flies to be exported per week.

Kaufman also suggests that the movement of cattle across Central America without proper inspection and treatment may have hindered efforts to suppress the screwworm’s northward movement. Illegal cattle crossings across the Guatemala–Mexico border may also be driving the spread, according to the Wildlife Conservation Society.

To fight back this time, federal agencies are deploying new tools built for speed and prediction rather than guesswork.

The USDA is using epidemiological tracking and meteorological data to predict where the flies are and where they’re likely to go, then using that data to determine where to release sterile males.

Several fly production facilities are being built or renovated, including an existing facility in Metapa, Mexico, and a new dispersal facility at the Moore Air Base in Edinburgh, Texas. For now, experts say, the aim is most likely to slow the spread rather than eradicate the pest immediately.

Kaufmann says: “We have help on the way,” adding, “It’s just going to take us time before we have a sufficient number [of sterile flies] that we can push this back south.”

Part of that help is a new breeding approach the USDA says it has unveiled: the Novo Fly strain. The goal is to double the number of male flies produced by eliminating female flies from each brood. With this strain, any female larvae will die early on, allowing a brood to be 100 percent male.

Kaufman explains the math this way: “We can essentially put twice as many eggs into a rearing container, knowing that half will die, and the other half will be male. So instead of producing 50 million males and 50 million females a week, we’re now producing 100 million males.”

The USDA is seeking approval from the U.S. Environmental Protection Agency for the use of the Novo Fly. Experts hope it will greatly increase sterile fly production across the United States, Mexico, and Panama.

Alongside that, workers are already managing visible parts of the campaign—like a crate of sterile pupae dyed green to distinguish them from fertile flies—installed at Chapparosa Ranch in La Pryor, Texas.

And because prevention and treatment can’t wait for sterile flies alone, the U.S. Food and Drug Administration has issued emergency use authorizations for treatment and prevention of screwworm infections across a wide range of animals: cattle. sheep. and goats; cats and dogs; birds; and even wild mammals.

Many of these drugs have been used for decades to treat various types of parasitic infections in animals. and they show promise for efficacy against screwworm infestations. Burgess says the hunt is for chemistry that works in the right way: “We need chemistries that have unique modes of action [in how] they affect the fly’s physiology. The goal [is to] balance available options for therapeutics while also maintaining safety for the animals and for the food supply.”.

Injectable insecticides such as Dectomax—approved for emergency use authorization on May 19—are systemic. They work by diffusing through the tissues of the animal, targeting the nervous system of larvae that ingest the flesh.

There’s no illusion that this will be quick. Kaufman and other experts agree the road ahead is long and expensive. It will take another year for the Edinburgh facility in Texas to go live, and screwworms may spread farther in the meantime.

The financial commitment is already large: the USDA spent $21 million for the Mexico facility and will spend another $8.5 million on the Texas facility. On June 16. the USDA announced it would grant $105 million to fund innovative efforts to fight the parasite. including new ways to sterilize the flies and novel traps to help bolster early warning systems.

Kaufman frames the spending as a necessity. “It is worth every dollar we’re spending to eliminate this fly, and we’re gonna keep at it, and we’re gonna be successful,” he says.

That resolve is what the outbreak now demands: fast detection, rapid treatment, and the slow build-up of sterile fly production needed to push the pest back south. For people watching their animals in Texas and New Mexico, the countdown has already started.

New World screwworm Cochliomyia hominivorax USDA sterile insect technique SIT Novo Fly strain Dectomax emergency use authorizations Texas A&M University of Florida livestock health invasive species