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An infestation of the flesh-eating flies has been confirmed in South Texas, setting off alarm bells for the state’s cattle industry.
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A sample from a 3-week-old calf from La Pryor in Zavala County tested positive for the country’s first case of New World screwworm, the U.S Department of Agriculture Secretary Brooke Rollins said during a June 3 news conference about the case.
There have been no other detections so far.
The USDA said in a social media post earlier that it had already activated personnel on the ground and was working with local partners. The federal agency also said that issues with screwworms shouldn’t immediately cause food supply chain issues, as screwworms do not infect meat, fruits or vegetables.
However, an outbreak of screwworm threatens to do $1.8 billion in damage to Texas’ economy alone, according to a U.S. Department of Agriculture estimate. This potential fly pandemic could also increase already record-high beef prices nationwide, given the state’s importance to the cattle industry.
Here is a breakdown of why the cattle industry and government leaders are concerned about a parasitic fly:
What is a screwworm?
The New World screwworm is a type of blowfly that lays its eggs in the flesh of living animals rather than dead ones. Cuts as small as a tick bite may attract a female fly to lay her eggs. One female can lay 200 – 300 eggs at a time and may lay up to 3,000 eggs during her 10- to 30-day lifespan.
The eggs hatch into maggots that burrow into the wound to feed on the living flesh. The larvae in their final stage develop little hooks and dig deeper into the flesh, hence the name screwworm. After feeding for about seven days, larvae drop to the ground, burrow into the soil, and emerge as adult screwworm flies, repeating the cycle of infestations.
Multiple infestations are common with multiple stages of larva in one wound, sometimes leading to a “feeding frenzy” resulting in rapid tissue damage.
New World screwworm is typically found in South America and parts of the Caribbean. However, the fly has steadily moved northward from South America, through every country in Central America, and into Mexico, since 2023. Federal and Texas officials started monitoring cases the following year.
Can the screwworm infect humans and pets?
Most screwworm infestations occur in mammals, like cattle, pets, and wildlife, but they can also occur in people.
Dogs and cats are particularly vulnerable to screwworm because minor injuries can attract flies. The Texas Farm Bureau encourages pet owners to check animals daily for wounds — including areas of previous tick bites — and monitor for foul odors, tissue damage or signs of fly larvae in wounds.
It’s also advisable to assess the pet’s environment for items that may cause injury, such as metal collars, wire fencing, and sharp vegetation that could cause small wounds.
If left untreated, these infestations can quickly become fatal for pets.
Although human cases are uncommon, they have usually occurred after someone has visited a place where the fly thrives, such as South America, Cuba, Haiti, or the Dominican Republic.
Even small injuries, like scratches, insect bites, or surgical wounds, can provide an entry point for screwworms on a human. Infestations can also occur in the nose, mouth, or eyes.
Signs of infestation can include skin lesions that do not heal or worsen over time, painful bleeding, or sores. In some cases, individuals may feel movement in the wound or notice larvae present.
Preventative measures for humans include keeping wounds clean and covered as well as wearing loose-fitting, long-sleeved clothing and socks.
Why are ranchers and agriculture leaders so concerned?
Screwworm in the flesh of an infested animal will cause a lot of damage to the wound, as flies repeat a cycle of flying into the wound, laying eggs, and making the wound bigger, leading to illness and death.
Even if the animal survives, there’s usually extensive damage to the hide and to the health of the animal.
Cattle are particularly susceptible to screwworm due to their inability to protect an open wound, and their large frame can allow eggs to develop in multiple locations from a single cut.
“Texas livestock producers and all Texans must now be vigilant in their efforts to manage and combat the invasive pest,” Farm Bureau President Russell Boening said in a statement after screwworm had been confirmed in Texas.
Though screwworm was eradicated in the 1960s, the U.S. experienced a resurgence of the pest from 1972 to 1976. A screwworm outbreak in the U.S. infested tens of thousands of cattle across six states, cost tens of millions of dollars to contain, and was only defeated after a massive eradication effort.
U.S. producers have not treated livestock for screwworm in more than 40 years, putting the future of the industry on shaky ground.
Texas leads the U.S. in cattle production, bringing in $15 billion a year to the state.
How did they get here?
The spread of New World screwworms across Central America has been very rapid, seemingly traveling hundreds of miles within weeks.
Animal experts believe the movement is too quick to be done by the fly itself, meaning people were clearly shipping infested livestock around Central America.
Screwworms usually don’t stray too far from their natural tropical and subtropical climates on their own. The parasitic flies do not tolerate prolonged periods of very dry, hot, or very cold weather. Rapid spread in a country is mainly due to humans moving infested animals over large distances.
After Mexican officials confirmed a case of screwworm in November 2024, the USDA, under former President Joe Biden, closed southern ports of entry to live cattle imports to prevent the spread of screwworm into the U.S. However, the move also strained the supply of cattle in the state, hitting some in the cattle industry hard.
The USDA reversed course in February 2025, after President Donald Trump took office, announcing the opening of the ports, only to close them again in May 2025.
In an effort to prevent its spread, the USDA shut down the southern border to live animal imports in May 2025, preventing cattle from Mexico from entering the U.S. and limiting the supply of cattle in Texas.
It’s currently unknown how the screwworm infestation reached South Texas.
How do you stop these flies?
The federal government has also invested $100 million into research, traps, and mounted patrol officers known as tick riders at the border.
U.S. officials are working with officials in Mexico and Panama to eradicate the screwworm again, using the sterile fly method. This practice consists of producing male sterile flies to have them reproduce unviable eggs with female flies who can only mate once in their lifetime.
The sterilization method was successful for more than 20 years, with a key facility in Panama.
American microbiologists said in a 2025 report that the South American region served as a natural barrier to screwworm due to the difficult terrain of the Darien Gap, a remote land bridge composed of dense, mountainous rainforest between Colombia and Panama.
In 2023, the flies suddenly overpowered the insect barrier and the difficult terrain of the Darrien Gap. The exact cause of this breach is unclear, but the scientists believe it is most likely due to multiple factors, including interruptions in sterile fly production caused by the COVID-19 pandemic and illegal cattle imports stemming from surveillance gaps in geographically difficult regions.
At the time of their spread from Central America into Mexico, there was only one sterile fly production facility, located in Panama. Since then, U.S. officials have helped launch another in Metapa, Mexico, which is expected to open later this month, and are building another in Edinburg, Texas, which Rollins said is slated to open in fall 2027.
They’ve also launched two fly dispersal facilities, which help distribute sterile flies in needed areas, in Tampico, Mexico, and Edinburg. The USDA has already been dispersing sterile flies in South Texas, within 50 miles north of the border.
In response to the first confirmed case, the USDA implemented a quarantine zone, controlling the movement and surveillance of animals in the area.
Officials are also increasing trapping and surveillance for screwworm along the border, just outside the 50-mile dispersal area, and are ramping up surveillance and monitoring of wildlife.
Earlier this year, the U.S. Food and Drug Administration issued multiple emergency orders authorizing topical sprays, powders, injectable drugs, and ointments to treat New World screwworm in cattle, dogs, cats, goats, horses, pet birds, and wild and exotic animals.
Texas Agriculture Commissioner Sid Miller criticized the USDA’s response to the threat of screwworm as slow, bureaucratic and incomplete.
He urged Trump to direct the agency to deploy the Screwworm Adult Suppression System, a method that uses attractants and insecticides in pellets. However, the USDA called the method outdated and ineffective because it indiscriminately attracted all kinds of flies, not just screwworm.
The technology used is also considered to be a probable carcinogen that is toxic to wildlife and humans, said Scott Hutchins, under secretary for research, education and economics at the USDA.
“This particular technology is not effective and has not been,” Hutchins said.
What to look out for?
Animal experts say continual and regular monitoring and evaluation of all livestock is important during a screwworm outbreak.
Cattle producers and pet owners should be alert for a foul-smelling wound with visible maggots, animals biting or licking at wounds, lesions, and unusual restlessness or lethargy.
The best way to identify screwworm larvae is by the spikes that protrude from the body and wrap around them.
The USDA said residents near the affected areas should check their pets for signs of screwworm infection, which include infected wounds and screwworm eggs or larvae.
Treatment for screwworm often involves the manual removal of larvae, cleaning and dressing the wound and prescribed pesticides by a veterinarian.
There are 12 FDA-approved products available for the treatment of various species. Agency officials warn they have not approved a medicated feed option, though they said they are working on making one available as soon as possible.
Those who potentially see a screwworm infestation are recommended to report it to a local veterinarian or wildlife biologist.
“If we all work together and follow the animal treatment protocols and movement restriction guidance, there is no reason to believe that this incursion will result in an establishment of the pest in our country,” Rollins said.