As cases of the New World Screwworm rise in the U.S., state officials are working to prevent the parasitic fly from reaching Missouri.
Since June 3, there have been 15 reported cases of New World Screwworm in Texas and New Mexico. These pests’ larvae enter warm-blooded animals’ wounds and feed on their live tissue, leading to enlarged wounds, bacterial infections and death if left untreated.
A U.S. Department of Agriculture analysis published in 2025 found that an outbreak roughly the scale of the 1976 Texas outbreak could cost the Texas economy a loss of $1.8 billion.
The Centers for Disease Control and Prevention activated a Level 3 emergency response on June 11, its lowest emergency response level. There are no reported cases of New World Screwworm in Missouri, but officials worry about the threat posed to livestock.
“I think there's definitely some level of risk, so that's why detecting it early and getting it treated is very important,” Missouri state veterinarian Steve Strubberg said. “We have some very responsible livestock producers here in Missouri, and we expect them to notify us and help us eliminate it if it does arrive, but a big key on those producers too is if they practice really good biosecurity, it will most likely prevent it from ever arriving.”
The fly generally spreads to new areas through the movement of infested livestock. Craig Payne, director of veterinary extension at the University of Missouri, said that animals are now transported much farther distances than they were when the New World Screwworm was most active in the U.S. which could increase the area the screwworm spreads to.
Though there are no animals currently being transported to Missouri from very-high risk areas, Strubberg said Missouri does occasionally receive animals from those impacted areas, and animals come from many states on a daily basis. There are movement restrictions placed on animals from infested areas, and animals coming to Missouri from these areas require additional inspection and permission. In addition, health officials established 12-mile quarantines around confirmed cases.
The New World Screwworm can infect livestock, pets, wildlife and even humans, though human infestation is rare. Seth Meyer, director of the Food and Agricultural Policy Research Institute, said beef cattle may be at a greater risk for New World Screwworm because they are open range which can result in more injuries and less monitoring than livestock kept in enclosed areas. Missouri has the sixth largest cattle inventory in the U.S., according to USDA data.
“These animals will get cuts, and because they're out in an open range they'll get small wounds, et cetera, that can be infected or attract these maggots, and so I think that this becomes a real challenge,” Meyer said. “The challenge is not that you will lose the animal, and it's not that the animal won't, couldn't, can't be eaten afterwards, because it can, it's a wound, and you clean the wound, and you move on. It is a management issue, and the producers, the farm, the farmers, the ranchers have to increase their management of this.”
The screwworm’s reappearance in the U.S. comes after decades since the fly’s eradication in 1966, and the few outbreaks since have been quickly eliminated. The New World Screwworm population is controlled through the release of sterile flies, which helped eradicate the fly in the past. Since 2023, the New World Screwworm has made its way up through Central America and Mexico.
The USDA closed the Mexican border for livestock imports into the U.S. in November 2024 after New World Screwworm was detected in Mexico. The border reopened briefly but has remained closed since May 2025. Meyer said cattle imported from Mexico made up about 4% to 5% of total slaughter in the U.S.
“It isn't having an effect yet on domestic supplies of cattle, and I suspect that its ultimate impact on domestic supplies of cattle might be modest, but we are short Mexican cattle, and we are short domestic cattle already, so none of this is helpful in reducing beef prices,” Meyer said.
Ranchers may incur additional expenses from increased herd monitoring, treatment and animal movement restrictions. Beef prices have increased 12.9% since May last year, according to the U.S. Bureau of Labor Statistics. The cattle inventory is at its lowest since 1951.
“We have had a declining herd for several years, and even though we are feeding them to heavier weights, we are in a contraction phase of the cattle cycle,” Meyer said. “Even though profitability appears to be very large, we continue to be in a contraction phase of the cattle cycle, which is part of the reason why beef prices are so high.”
The Missouri Department of Agriculture, USDA, Missouri Department of Health and Senior Services, Missouri Department of Conservation, and the University of Missouri Extension have been working together to prepare for the threat of the New World Screwworm for over a year. Their main focus is on prevention, and in light of the U.S. screwworm detection, Missouri activated its NWS Response Plan.
The plan requires any animals moving into Missouri from a state with an infested zone to enter with a Certificate of Veterinary Inspection that states, “All animals were inspected and found free of evidence of NWS infestation and have not originated from or transited through a NWS infested zone,” according to the Missouri Department of Agriculture website. Domestic animals coming from an infested zone must meet requirements from the state of origin, receive an entry permit from the state veterinarian’s office and a Certificate of Veterinary Inspection that says, “All animals were inspected and found free of evidence of NWS infestation.”
Despite worries over the challenge infestation may cause for production, the screwworm does not impact the safety of the food supply. Even infested animals, after proper treatment, are able to safely enter the food supply.
Ryan Dierking, co-owner and operator of Idle Wind Farms in Union, said he is continuing to monitor his herd, but that he wasn’t sure how much Missouri producers would be affected in the near term. Newborn mammals and post-calving cows are especially vulnerable to infestation, but the spring calving season has ended in Missouri.
“I would say most producers across the state are done calving at this time of the year, and so you know what's primarily going to be affected is any animal that has an open wound, where if you know the screwworm were to get inside the borders of Missouri, would be the animals you'd want to pay attention to,” Dierking said.
In addition to cattle, the screwworm has been detected in Texas sheep and goats, along with a New Mexico dog. Sheep, goats, pigs, dogs, horses, cattle and more make up the list of more than 185,000 infested animals in Central America, along with more than 2,100 cases in people.
The USDA announced $105 million of funding for 40 projects focused on detection, control, eradication and making response times faster on June 16.
The New World Screwworm’s life cycle cannot be completed in dry soil below 46 degrees, and adult flies can’t survive in weather under 20 degrees. This means Missouri does not face the threat of the screwworm establishing a year-round presence, but maps based on historical data show that the flies might migrate to Missouri in the warmer months if they establish in southern states.
The Missouri Department of Agriculture advises ranchers to register their farms for a federal premises ID so the state can issue notifications about New World Screwworm. Prevention can include cleaning, treating and covering open wounds and delaying practices that can cause open wounds such as castration, dehorning, birthing or branding. Additionally, producers are advised to regularly monitor animals for signs of infection or maggots, and to report signs of infestation to a local veterinarian or the Missouri Department of Agriculture Animal Health Department.
