Flesh-eating screwworm returns: Texas calf triggers U.S. watch

Federal inspectors found New World screwworm larvae on a 3-week-old calf in Zavala County, Texas, near the U.S.-Mexico border. Officials say they’re prepared to contain the parasitic fly as it has been moving north from Central America to Mexico since 2023, ra

By Thursday. the danger had a name and a location: a 3-week-old calf in Zavala County. Texas—more than 60 miles from the northern Mexico border—was found with New World screwworm larvae in the calf’s umbilical area. The discovery came from federal agricultural inspectors, and by Friday morning, there were no additional cases of infected animals reported.

The case lands near the region where ranchers have spent months bracing for the same thing. New World screwworm is caused by larvae from a parasitic fly known scientifically as Cochliomyia hominivorax. The larvae don’t wait for an animal to die. They burrow into living tissue—attacking flesh—turning wounds into something that is both painful and foul-smelling. according to the Centers for Disease Control and Prevention.

Officials say the timing matters. The USDA confirmed the detection on Thursday and anticipated the screwworm’s arrival in the United States, saying it is prepared to contain it.

“The fly does not survive at temperatures at or below freezing. ” Sally DeNotta. director of the University of Florida’s Equine Performance Laboratory. said. But she added that infected animals could still carry the parasite northward and spread infection during the summer months. She also warned that the temperate climate of Southern California could support year-round New World screwworm populations.

That mix—less survival in cold weather, but the potential for movement and spread—has been the focus of an increasingly anxious countdown along the border.

On Thursday, the USDA confirmed the case in Texas. As of Friday morning, there were no additional cases reported.

The CDC has described New World screwworm as endemic in South America and parts of the Caribbean. but movement is the reason ranchers are watching so closely. The parasitic fly has been moving north from Central America to Mexico since 2023. and the USDA says it has actively monitored the fly’s movement.

In a sign of how close it has gotten. Mexico’s Secretary of Agriculture and Rural Development said last month that the USDA was aware of more than 200 active screwworm infestation cases in the border states of Nuevo León and Tamaulipas. Mexico currently has more than 2,000 active cases throughout the country.

Dudley Hoskins. undersecretary for Marketing and Regulatory Programs for the USDA. said in a statement that the country “was able to buy time for this moment. ” adding: “thanks to the hard work across the entire Trump administration and our industry. state. and local partners.” His comments framed the Texas finding as part of a longer push to get ahead of something the government believed was coming.

DeNotta, meanwhile, focused on the economic stakes for a ranching industry that can’t afford even short disruptions. She said the potential economic impact of New World screwworm on the cattle industry—through import restrictions, reduced productivity, and animal loss—is substantial.

Last year, 175 key agricultural organizations signed a letter urging additional federal funding for screwworm-control measures. The letter emphasized USDA estimates that a New World screwworm outbreak in the U.S. could cost producers $4.3 billion annually and cause economic losses of more than $10.6 billion across the southern United States.

In ranching communities, the mood has been less about surprise than preparation. Tom Talbot, a veterinarian and member of the California Cattlemen’s Assn., said the arrival of screwworm has been a topic of nonstop conversation “from the federal to the local level” on how to combat it.

“My expectation is that there will be a minimal number of cases of [New World screwworm] in California,” Talbot said. He pointed to several stations on the border in Southern California that are collecting data, monitoring for any incidents of the parasitic fly, and trapping them.

Talbot said he’s confident that proactive measures by the federal government will mitigate screwworm’s reach and prevent impact to the beef supply locally or nationally.

The core of the threat is biological and brutally simple: female screwworm flies are attracted to the smell of wounds. even injuries as small as a tick bite. and to body openings like the nose. eyes. ears. and mouth where they can lay eggs. According to the CDC. a female screwworm fly can lay 200 to 300 eggs at a time and may lay up to 3. 000 eggs during her 10 to 30-day lifespan.

When eggs hatch into maggots, the maggots eat live tissue. The CDC describes it as a worsening wound with a foul smell and often intense pain.

The United States has faced this before. The last major outbreak in the southwestern region occurred in 1965, according to the National Agricultural Library. That outbreak prompted Mexican and U.S. livestock producers to sign a declaration to establish a joint program for the eradication of screwworm from states on both sides of the Mexico-U.S. border.

By 1966, the United States eradicated screwworms—but livestock remained vulnerable to reinfestation from screwworms migrating from Mexico. Eradication relied on the sterile insect technique, using gamma radiation to irradiate screwworm pupae and create sterile male flies.

Today. the USDA’s Animal and Plant Health Inspection Service maintains a screwworm pupae sterilization facility in North America and is building a new center in southern Texas. The USDA says that when sterile male flies are produced and released in large numbers. they mate with wild female flies. which then lay unfertilized eggs. USDA officials say that because female screwworm flies normally mate only once, the population progressively reduces and is ultimately eradicated.

But funding and monitoring are part of the story too. Last year, the Trump administration cut thousands of grants and programs from the U.S. Agency for International Development. including U.S.-funded animal disease monitoring projects operated by the United Nations Food and Agriculture Organization. Argi-Pulse Communications reported. Among the slashed programs were some dedicated to monitoring and containing New World screwworm in Central America.

So while screwworm infestations aren’t a regular occurrence in the U.S., cases have occurred before—typically in travelers returning from areas where the flies are present, the CDC says.

There is also a treatment path, though it requires speed. DeNotta said infected wounds are cleaned and debrided to remove larvae, then the animal is treated with an approved insecticide.

Last month, the U.S. Food and Drug Administration issued an emergency use authorization for several insecticides known to be effective against screwworm. DeNotta said there are approved systemic and topical options for a variety of species, including cattle, horses, small ruminants, cats and dogs.

“Multiple days of treatment are often required. and antibiotics and analgesics may also be administered to treat secondary infection and control pain. ” she said. If left untreated. DeNotta warned. tissue destruction can be extensive and severe. often resulting in debilitation and eventual death of the host.

Animals that survive may still pay a price, DeNotta said, with weight loss, poor growth, and reduced productivity driven by pain and discomfort.

Humans can be infected, though DeNotta said it is rare. The CDC says people are at risk if they travel to areas where the flies are present. such as South America and the Caribbean. CDC officials said risk increases if a person spends a lot of time outdoors during the day. especially if sleeping or unable to keep flies at bay. or if they have open wounds.

They also note that a small break in the skin—including from a scratch. insect bite. or recent surgery—may attract screwworm flies. People with medical conditions that cause bleeding or open sores. such as from skin or sinus cancer. or from treatments that can create breaks in the skin. face greater risk. The CDC also says risk rises when someone lives. works. or spends extended time with or near livestock or other warm-blooded animals in areas where screwworm flies are present.

For symptoms, the CDC lists feeling maggots move or seeing maggots within a skin wound, sore, or body opening; painful skin wounds or sores that worsen within a few days; and foul-smelling odor from the site of infestation. It also includes bleeding from open sores.

The CDC adds that bacteria can infect wounds where screwworm maggots are present, and that can lead to symptoms like fever or chills. Treatment for people follows the same combination DeNotta described for animals: wound debridement and insecticides.

The sequence that has led to this moment—movement north from Mexico after years of endemic spread elsewhere. active USDA monitoring since 2023. hundreds of active cases in border states last month. and now a confirmed infection on a calf in Texas—has put the countdown back in the open. And with no additional infected animals reported as of Friday morning. the question now facing ranchers and regulators is how quickly containment can turn an arriving threat into a contained one.

New World screwworm Cochliomyia hominivorax USDA Texas cattle Zavala County U.S.-Mexico border sterile insect technique CDC cattle industry impact animal disease monitoring