What to know about the New World screwworm
By Jen Christensen, CNN
(CNN) — Earlier this month, Texas Gov. Greg Abbott told residents to brace themselves for an “extraordinarily challenging summer.” A flesh-eating parasite that did hundreds of millions of dollars of damage across the state in the 1960s and ’70s, one that had been considered eradicated from the US for decades, has come back and poses a serious threat to cattle, wildlife and pets.
The first case of New World screwworm in this latest outbreak was confirmed June 3 in a 3-week-old calf in South Texas that has since recovered. But despite enhanced surveillance and restrictions on movement, cases continue to turn up hundreds of miles beyond the southern border.
If the parasite’s advance can’t be halted by the state and federal governments’ stepped-up surveillance, containment and treatment efforts, experts say, the outbreak could cost the southwestern United States alone billions of dollars.
The larvae of the parasitic fly feed on the tissue of any warm-blooded animal, even humans, but officials say the risk to people is low. While it’s not a direct food safety issue, the infestation could raise the cost of beef at a time when Americans are already paying record high prices.
Screwworm by the numbers
The latest outbreak started in Central America in 2023 and has been extensive, according to the US Centers for Disease Control and Prevention. There have been more than 185,000 animal cases and 2,100-plus human cases reported in Mexico and Central America.
In the US, there have been 15 cases identified as of Sunday, according to the US Department of Agriculture. Cattle have been affected most so far. Other animals that have been infested include sheep, goats and a dog. Texas, the largest beef producer in the country that produces the most beef in the world, has had the majority of cases. The dog lives in New Mexico.
No human cases have been reported as part of this outbreak. The last case in the US was a travel-related one in Maryland in 2025.
The CDC has designated the outbreak a Level 3 public health emergency, its lowest level, which means the CDC is actively monitoring the situation and using its experts to manage the event. The agency has also been urging doctors to be on the lookout and report any cases.
What is a New World screwworm?
The name of the pest may confuse people, said livestock entomology and parasitology expert Dr. Jonathan Cammack. It’s a fly, not a worm. It’s not even a contagious disease that spreads from animal to animal; it’s an infestation.
It’s called a screwworm because, unlike many fly larvae that feed on dead or decaying flesh, New World screwworm larvae screw into the flesh of warm-blooded animals to feed.
Emory Cushing, an entomologist with the USDA’s Bureau of Entomology, and Walter Patton, a scientist at the University of Liverpool in England, first identified New World screwworm as a distinct species of fly in 1933.
Scientists used to call it only a screwworm. But in the 1980s, when the species turned up in North Africa, a region that had its own distinct screwworm, scientists needed a way to distinguish between the two, said Cammack, an assistant professor and the state extension specialist at Oklahoma State University in Stillwater.
Scientists named the fly commonly found in Central, South and now North America the New World screwworm and labeled the kind found more typically in Africa and Southeast Asia the Old World screwworm.
How screwworms hurt animals
The New World screwworm makes warm-blooded animals sick when the adult female deposits eggs into an open wound or a mucous membrane, like in the eye or nose.
The eggs hatch within about 12 to 24 hours. Larvae emerge and immediately start to feed, tearing into the host animal’s tissue, causing sores to expand and deepen and sometimes creating secondary bacterial superinfections, according to the CDC.
The smell that comes from the deterioration in the animal’s flesh can also attract additional screwworm to lay more eggs, and it may attract other species of fly that can co-infest the lesions.
Larvae can rupture arteries, causing severe bleeding and leaving the animals severely anemic.
If left untreated, screwworm infestations can eventually kill the animal host within seven to 14 days.
How screwworm infections are treated
If it’s caught early, animals can be cured of a New World screwworm infestation.
Veterinarians typically clean out visible eggs and larvae and then use an insecticide that kills larvae and protects against reinfestation.
In August, the US Department of Health and Human Services issued a declaration allowing the US Food and Drug Administration to provide emergency use authorizations for animal drugs to treat and prevent infestations of screwworm. There are a handful of medicines to treat and prevent infestations in pets and in livestock, zoo and wild animals.
Animals that fully recover from an infection can eventually enter the food supply after a rigorous USDA safety inspection.
Preventing infestation
Just because one calf is sick doesn’t mean the entire herd will get it, Cammack said.
“It takes a lot of very specific steps to happen, so it’s not like this is just going to pervasively spread through any of our herds of animals,” Cammack said.
To control infestations, the government has encouraged producers to keep a close eye on their animals and watch for wounds that could make the animal a target.
But that may not be as easy as it sounds. Wounds as small as tick bites can attract the flies. Although it may be easy for a person with one pet cow in the backyard to keep a close eye on their animal, bigger operations like those in Texas face a tougher task. “We’ve got producers on the other end of the spectrum that could be interacting with their animals two or three times a year,” Cammack said.
How the US stopped screwworm in the past
Before it was eradicated in the 1960s, screwworm caused huge livestock and wildlife losses, mostly in the warmer states, with infestations concentrated in Texas, Mississippi, Alabama, South Carolina, North Carolina, Georgia and Florida. Cattle producers in affected states lost up to $50 million to $100 million per year before full eradication, according to a 2025 analysis from the USDA.
In the 1950s, scientist Edward Knipling discovered that he could sterilize flies with radiation. Female New World screwworms mate only once, and Knipling theorized that if he created enough sterile flies for the females to mate with, the fly population would die off.
In 1958, the US created its first fly-rearing facility in Florida and dropped sterile flies in affected areas. By the next year, outbreaks in Florida, Alabama and Louisiana were controlled. Building new sterile fly production facilities in the Southwest helped eradicate the flies in that region too, and by 1966, the US was declared officially screwworm-free.
Some outbreaks still cropped up in the US after that – the largest in 1972, when Texas alone had 90,000 cases – but experts say the effort was beyond successful.
“Using the sterile insect technique was one of the USDA’s major achievements of the 20th century,” said Max Scott, an entomology and plant pathologist who is a professor at North Carolina State University. “The USDA, over a 50-year period, succeeded in eradicating this fly all the way down to the Panama-Colombia border.”
Why screwworm came back
The screwworm barrier in Panama eventually failed. The latest outbreak started in 2023, with several cases popping up in Panama and Costa Rica, and then slowly started spreading north toward the United States.
The spread happened for a few reasons, Scott said.
In hindsight, he said, the strain of infertile male flies used recently was “probably not as good,” so the female flies were no longer choosing to mate with the less-fit sterile males.
A recent surge of migrants across the Panama-Colombia border also included animals that may have been infected, he said. The fly doesn’t travel big distances and moves more frequently on infested livestock. Livestock was also smuggled across borders without proper health checks, according to a 2022 report from the think tank InSight Crime.
USDA Secretary Brooke Rollins has blamed the Mexican government for not cracking down on “cartel trafficking and immigration” while some Democrats point the finger at the USDA for not doing enough.
In a letter to the USDA on June 9, several US senators expressed concerns that “significant staffing reductions” due to retirements and resignations, as well as Trump administration funding and staffing cuts, affected the national screwworm response. Rollins has repeatedly denied that staffing cuts had an effect and says President Donald Trump has given her all the resources she’s needed.
Even some Texas Republicans have been critical of the Trump administration’s response. Texas Agriculture Commissioner Sid Miller critized the federal government’s efforts as “slow, bureaucratic, and incomplete.” He wants the federal government to use a targeted chemical baiting method as a bridge before more sterile fly facilities can be built.
Although the screwworm infestation has created “unfortunate circumstances,” Rollins remains “bullish and buoyed and inspired and just invigorated by these incredible producers that we’re up there fighting for every single day, and we’re going to beat this,” she said June 11th. “We beat it before, and we’re going to beat it again.”
What the US is doing now
The US is taking several steps to stop the screwworm.
Sterile fly facilities
The USDA has invested millions to modernize a facility in Mexico that produces sterile flies. Rollins says that facility should open this month and supplement the flies already produced in Panama. The USDA also broke ground on a $750 million fly-producing facility in Texas, but that’s not supposed to open until next year.
As of now, the USDA is bringing 100 million flies from its Panama facility to affected areas in the US every week.
The agency is also seeking approval for the NovoFly, a genetically altered all-male fly that would create stronger flies faster, but the US Environmental Protection Agency has not said when it would decide whether to approve the fly.
Changes at the border
The Biden administration closed southern borders to Mexican cattle in November 2024, after the first screwworm case was confirmed in Mexico, but after pressure from the beef industry, Trump briefly reopened the border to cattle imports in February 2025. The USDA shut southern ports to Mexican cattle again in May 2025. As of June, exports of horses, pigs, ferrets and birds were halted from all US states to Mexico by the USDA.
The USDA has staged sniffer dogs to look for screwworm at the southern border and increased Tick Rider border patrols — horseback riders looking for the pest.
“We now know what the enemy looks like. We now understand what we have to do,” Rollins said last week.
Shifting experts to Texas
The USDA has temporarily relocated its Iowa lab team that works with screwworm to Kerrville, Texas, to bring testing closer to the affected area. The agency also launched a $100 million innovation initiative to incentivize scientists to come up with additional ways to eliminate screwworm. The USDA has announced funding for some of these new approaches, including using drone technology, developing novel traps and lures, and developing genetic strains to produce flies faster.
States take action
Several states have also taken action to stop screwworm. New Mexico stepped up its surveillance, and some states including South Carolina and New York have created interstate movement health restrictions for warm-blooded animals entering from impacted areas. California is working with the University of California, Riverside to conduct prevention trapping and has sought emergency use authorization for additional treatments.
And in Texas, the state with the most cases, Abbott expanded a disaster declaration to authorize the use of all state government resources to respond to screwworm cases, several counties have done the same. The state has set up additional surveillance and created infested zones while implementing quarantines, movement controls and surveillance in affected areas. Texas has also developed a free course to expand the number of New World screwworm inspectors across the state.
The state government is working closely with private landowners to do additional fly trapping in areas frequented by Texas’ abundant wildlife. With about 5.4 million whitetail deer alone, deer hunting adds about $9.6 billion to the state’s economy, said Alan Caine, wildlife division director with the Texas Department of Parks and Wildlife,
Abbott encouraged ranchers and others in the state to pay close attention to their animals and to report any cases quickly.
“Texas is resilient. Our producers, veterinarians and state officials are among the very best in the United States. It is critical to stay vigilant and stay informed. We prevented and eradicated this before; we can do it again.”
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