New World screwworm flies during the inauguration of a sterile screwworm fly production facility in Metapa, Chiapas state, Mexico, on June 27, 2026. —Mauricio Palos/Bloomberg—Getty ImagesIn early June, the U.S. Department of Agriculture (USDA) first reported that the New World screwworm—a fly that lays flesh-eating larvae—was found in a baby calf in Texas. There are now close to two dozen cases in domestic animals in the U.S.—all in Texas, including cattle, a goat, and a dog that was identified in Texas but lives in New Mexico.Here’s what to know about screwworm and the ongoing clash about how to control it.What's a screwworm, anyway?The screwworm is actually a parasitic fly named for its larvae, which look like squirmy worms. Female screwworm flies lay eggs in wounds or other exposed tissues, such as the eyes, nose, or anal area, of mammals including livestock, dogs, and, less commonly, humans. Unlike maggots, which feast on dead and decaying material, screwworm larvae feed on the tissue of a living mammal host, then drop into the ground where the adult fly emerges after a week or up to a couple of months later, depending on the temperature and humidity. USDA programs eradicated screwworm flies in the 1960s, but they popped up again in the U.S. in the 1970s and most recently in 2016, in deer in the Florida Keys. Is screwworm contagious?According to the USDA, screwworm does not generally spread from animal to animal or from animals to people. Infestation requires flies to lay eggs in an animal host. If an infected animal does not get treated with anti-parasitic medicines or antibiotics, the larvae can cause sepsis and rupture blood vessels in the animal, killing it in as little as two weeks. Because animals like livestock are often in close quarters, eggs can potentially transfer from one animal to another, so Infected animals should be isolated from others until their wounds heal.Can I eat meat from cows infected with screwworm?The USDA says screwworm “is not a food safety issue” and that people cannot be infected by eating the meat of an infested animal. Can screwworm infect pets?Yes, pets like dogs and cats are just as vulnerable to screwworm infestations as cows and wildlife.How do I know if my pet has screwworm?If your pet has a wound, keep a close eye on how it's healing, says Anne Kimmerlein, global veterinary epidemiologist at Mars Veterinary Health in Sacramento. "If a wound starts growing rapidly, or looks like it's becoming infected and red with discharge, or if you see anything that looks like little white eggs at the site of the wound, these are all reasons to seek the care of a vet," she says. Some vets may not be able to distinguish maggots of other species from screwworm larvae. But vets can connect with experts at state agriculture departments who have more expertise in identifying screwworm for a more thorough assessment. "We as vets need to be aware of this and look out for it, but we really rely on experts to come in and help us make sure it's a true diagnosis and to help us manage the case going forward," she says. How can I protect my pet from screwworm?The risk for most pets is low, says Kimmerlein, but because screwworm is now in the U.S., pet owners should be more aware of the risk. "Certainly for people who live near areas where there are active cases, they definitely need to be a little more aware and cautious," she says. "But across the board, all pet owners should be monitoring their pets on a daily basis—inspecting them any time they come in from outside or anywhere they might have gotten a wound or even a tick bite. Those are all places flies might be attracted to lay eggs."Are there treatments for pets with screwworm?Yes, vets can prescribe certain anti-parasite medications, some of which are used to control fleas and ticks, to treat screwworm. The U.S. Food and Drug Administration granted emergency use authorization in June for a generic tablet that can kill screwworm larvae within hours in small pets, puppies, and kittens. Why is screwworm appearing now?The New World screwworm fly was eradicated from the U.S. more than half a century ago after agricultural scientists introduced sterile, lab-bred male flies that mated with females in the wild. They laid eggs that never hatched, leading to the eradication of the parasitic fly population (since the female flies only mate once). That success drove the field of genetic biocontrol, and remains one of the strongest successes in using genetics to manage pest populations over country-size regions, says Justin Overcash, a technical expert in regulation and policy at GeneConvene Global Collaborative, which is part of the Foundation for the National Institutes of Health.The reprieve was short lived, however, and agriculture experts say a combination of factors have contributed to the return of the species. Migration and transportation of herds from Central America and Mexico and warm temperatures reintroduced the screwworm fly to the U.S. in the 1970s, contributing to one of the worst outbreaks that infected more than 100,000 cattle in six states.The latest outbreak really began in 2022, says Philip Kaufman, professor of entomology at Texas A&M University and a leading expert on screwworm and pest management. While the USDA has been working with governments of countries in Central America and Mexico to contain them, screwworm flies have been progressively moving northward as temperatures get warmer and ranchers in Central America and Mexico seek additional grazing areas for their herds. “I really thought the fly would get here [in the U.S.] by August of last year, but it didn’t get here until June [2026], and that’s a testament to what the USDA is doing,” he says. What is the USDA doing to combat screwworm?Officials from the U.S., Mexico, and countries in Central America have been using surveillance methods to monitor the fly's movements northward and have been releasing sterile male flies there, which stop the reproductive cycle of the fly population. When cases appeared in the U.S. in early June, the USDA began to release millions sterile male flies in the U.S. as well. It also closed the border between the U.S. and Mexico to cow and horse importations. The problem is that there aren’t enough sterile males to outcompete the fertile males yet. Why is Texas criticizing the USDA’s strategy to control screwworm?State and federal authorities work together to contain infestations like screwworm, which could spread from state to state. But Sid Miller, Texas' agriculture commissioner, has been vocal in his opposition to the USDA's approach to controlling the flies, which he believes is insufficient. Miller says that the numbers (and history) show that sterilized males aren’t enough to contain screwworm outbreaks. “I am extremely concerned and have been since the outbreak started,” he says. “We have the tools to stop it, but we are not using the tools. What we are doing is the very definition of insanity because we are doing the same thing over and over and over and expecting different results.” Brooke TKHe says the USDA, working with Mexican authorities, have released about 100 million flies a week since the latest outbreak began. “Yet [screwworm] marched from southern Mexico 1,100 miles north into the U.S. to Texas and New Mexico,” he says.Kaufman notes that “we don’t have enough flies to stop it. We'll get there, but it will take a year or more to get there. What they are doing is buying time for Texas to get ready and us to build production plants [for the sterile flies].”What are the downsides of relying on sterile flies?In the 1970s, screwworm flies continued to flourish despite the release of sterilized males. Miller says one of the main reasons why the approach on its own is inadequate is because males aren't the only flies released via planes over acres of land where cattle graze; females are, too, since the manufacturing process can't select for just sterile males. The females and males that develop from the pupae end up mating, which leaves the males and females in the wild to continue mating and laying eggs. “That wastes some of their time,” says Kaufman. But he says that the Environmental Protection Agency is currently reviewing whether to test a new technique under emergency use that would allow scientists to select for males and eliminate females before the flies reach the pupa stage. “That would be a gamechanger, because it essentially doubles our production if we know all the flies are males,” he says. The process and cost of producing the sterile flies would also be more efficient, says Brinda Dass, senior technical expert at GeneConvene. "You're trying to have a lethal effect on females early in development, so you cut the cost of rearing and feeding them," she says. "You're looking at economies of scale to double production quickly without doubling the manufacturing apparatus."Miller sees another problem with the current use of sterile flies. Because the males are irradiated to become sterile, they are weaker than the wild males and less able to compete with them for mates, further reducing the chances of breaking the reproductive cycle. "The irradiated males are about 80% the size of the mature screwworm fly," he says. "So that’s why you have to have such overwhelming numbers.”Is there another potential approach?Miller advocates a more widespread combined approach to control the screwworm that starts with hanging insecticide-laced fly traps to capture and kill as many flies as possible, and then releasing sterile males to mate with any remaining females. In the 1970s, when Texas had 29,000 cases of screwworm, the state used several rounds of an insecticide placed in fly baits on patios, horse barns, and dairies for several weeks. “We went in and did an inventory survey and found that it killed 95% of the screwworm flies,” he says. “And now that the population was whipped down to almost nothing, we released sterile flies, and the sterile flies cleaned up the last little bit. The following year, in the spring screwworm season, we had 35 cases...so it works great.”In early June, Miller met with USDA director Brooke Rollins and her team at the White House. He says the White House is concerned about the environmental impact of the bait, which would kill not just the screwworm flies but also other blowflies, the family to which screwworm belongs. “I asked them, ‘What the hell is a good fly?’" Miller says. "I didn’t know we had good flies.” Miller proposed using the same facilities that are producing sterile male flies to produce fertile blowflies to replenish that population if their numbers dwindle as a result of the traps. The USDA continues to push back on the bait approach, noting that the pesticide used then is now banned as a carcinogen. Miller says other pesticides that are currently used in retail fly traps can be used, and continues to battle the USDA's exclusive focus on sterile flies. In emailed comments to TIME, Rollins said that Miller's criticism is "a very unserious comment from a perhaps unserious [agriculture] commissioner with just a few months left." (In March, Nate Sheets defeated Miller in the Republican primary for Texas agriculture commissioner.) "It is also a very dangerous suggestion. We're all on the same team, and we have to solve this together."Kaufman applauds Miller’s efforts to find alternative ways to control screwworm, but says some of the data don’t necessarily support the success of the bait traps. “The [scientific] papers paint a picture that it’s highly effective, but as a scientist reading the experiments, I can poke holes in almost everything they did,” he says. For example, he says there was a cold snap between the two bait programs that might have explained the reduction in the fly population. Still, he says, that doesn't negate the effectiveness of using traps, but he cautions against over-reading the results. Dass says it would be relatively straightforward to get an answer on how the combined approach compares to the sterile fly-release approach alone, using computer modeling, which could guide future approaches to outbreaks. "Texas or the USDA could run models and play with scenarios to come to a consensus about what makes sense scientifically," she says. "You could map out if you release over an area this large, during an outbreak with this many foci, what if you did this, then that, or what if you did things together. You could have a war-room scenario to map things out, all while you collect real-world evidence."Miller points out that the USDA and Mexico are heavily invested in the sterile-fly approach, with several fly-production facilities in Mexico, Panama, and Edinburg, Texas. Only the facility in Panama, which is jointly run by the U.S. and Panama, is currently producing about 100 million sterile flies a week. The Mexico facility, which had been producing fruit flies, was renovated and came online in late June and is expected to eventually produce another 60 to 100 million sterile screwworm flies, but hasn’t yet. And a new facility on Moore Air Base in Edinburg, which is expected to produce about 300 million flies a week, is not scheduled to open until November 2027. Miller says even if all were running at capacity, they wouldn’t produce enough flies to curb reproduction in the wild and reduce the screwworm fly population. “They are all in on the [sterile] flies,” he says. “They’ve got blinders on and won’t look at anything else. If they get 500 to 700 million flies, it may work. But that’s three years down the road.” For now, both the Texas Department of Agriculture and USDA are urging ranchers, pet owners, and veterinarians to report any cases of suspected screwworm so that local, state, and federal authorities can track its movement and target interventions such as sterile fly releases. Kaufman says that while he understands why ranchers would disincentivized to report cases—since a positive case could lead to quarantine and limited movement of their livestock—screwworm can be contained and any restrictions would likely be lifted quickly. "Yes, it's an inconvenience, but it's not a ban," he says of cattle transport. “If we weren’t doing sterile fly releases or stopping animal movement, this fly would be moving very rapidly around Texas."