Human screwworm case in Maryland spotlights risk to U.S. livestock as flies push north

A Maryland resident who recently returned from travel to El Salvador was diagnosed last month with larvae of the New World screwworm, marking the first confirmed human screwworm infection in the United States in more than half a century. Health officials said the patient has recovered and there has been no sign of onward transmission, but public health and agricultural authorities are treating the case as a warning of a broader threat to livestock and trade.

The New World screwworm (Cochliomyia hominivorax) has been resurging through Central America since 2023 and into Mexico, driven in part by animal smuggling and shifts in climate that have expanded habitats where the fly can survive. Because female screwworms lay eggs in open wounds and the larvae feed on living flesh, infestations in cattle can escalate rapidly and devastate herds, prompting serious economic losses and prompting federal mobilization to prevent an incursion into U.S. livestock populations.

Screwworm flies are similar in appearance to common house flies but differ profoundly in biology and impact. Female flies are attracted to fresh wounds and natural openings and can lay up to about 200 eggs at a time; eggs typically hatch within 12 to 24 hours. The emerging larvae burrow into living tissue and feed for as long as a week before dropping to the soil to pupate and emerge as adult flies. Unlike many parasites, screwworm larvae consume living flesh, and a single untreated wound can attract successive waves of flies, multiplying damage and the risk of secondary infections.

Historically, the United States eliminated screwworm from the mainland by the mid-1960s through the sterile insect technique, or SIT, a method in which sterilized male flies are mass-produced in laboratory facilities, released by air, and mate with wild females that then produce no offspring. The technique exploits the species’ reproductive biology: female screwworms mate only once. The coordinated eradication campaign in the 1950s and 1960s, later extended through Mexico and Central America, created a biological “firewall” concentrated at the Darién Gap, a dense, sparsely populated stretch of rainforest on the Panama‑Colombia border.

From 1998, a U.S.-Panama program known as the Commission for the Eradication and Prevention of Screwworm (COPEG) maintained that barrier by releasing sterilized flies and by carrying out inspections and treatments for active infestations. The program has cost roughly $15 million a year and has included both aerial releases and ground-based suppression of active wounds.

That firewall, however, has been breached. Since 2023, detections of screwworm have been reported farther north in Central America and into Mexico, and by spring 2025 Mexican authorities reported detections in states as far north as Oaxaca and Veracruz. Mexican officials and industry groups have reported substantial economic impacts; the Mexican National Agricultural Council estimated about $1.3 billion in losses to the Mexican cattle export industry over the past year.

U.S. officials and agriculture industry leaders consider screwworm one of the most serious livestock threats because outbreaks can spread quickly among animals kept in close quarters on modern industrial farms. The U.S. Department of Agriculture has estimated a single-state outbreak could produce very large economic damage; one government estimate put a potential annual drag on Texas ranchers and the broader economy as high as $1.8 billion.

In response, the USDA has begun to ramp up capacity to produce sterile flies and to shore up prevention efforts. In May, the agency committed $21 million to renovate an existing facility in Metapa, Mexico, to churn out an estimated 60 million to 100 million sterile screwworms weekly for release in southern Mexico. Earlier this year the USDA announced an $8.5 million plan to build a plant in south Texas, and it has said it plans a larger production facility in Edinburg, Texas, with capacity several times greater than the renovated Metapa plant. Congress is considering the bipartisan STOP Screwworms Act to authorize and fund a new U.S. facility.

Veterinary researchers and entomologists say SIT remains the most proven, environmentally targeted tool for suppression and eradication, and it produces pest control without broad pesticide use. But experts caution that the context today is different from the mid-20th century. Climate change is shifting temperature and humidity patterns, potentially enlarging the range of habitats where screwworms can survive. Livestock production has consolidated and scaled up, increasing the speed at which an infestation could move through herds. At the same time, personnel and program changes within federal agencies have reduced some monitoring capacity; an earlier cut of roughly 15,000 USDA positions and the termination of a screwworm monitoring project have complicated rapid detection and response efforts.

There are additional tools under development. Genetic approaches that build on or improve the sterile insect technique show promise, and federal agencies say multiple regulatory pathways exist to expedite approval of therapies or novel control tools. As of now, however, there are no FDA‑approved drugs specifically to treat screwworm infections in humans or animals, so response relies on manual removal of larvae, wound care, and animal treatments available under veterinary guidance.

Officials say prevention remains the most cost-effective strategy. Maintaining and expanding sterile-fly production, bolstering inspection at borders, and rapid treatment of any detected infestations are central to that approach. The screwworm program has historically combined aerial releases with on-the-ground surveillance and treatment; specialists caution that SIT alone is insufficient unless active infestations are also suppressed.

The recent human case in Maryland underscores how international movement can introduce parasites even when the immediate public-health risk is low. Public health authorities reiterated that human infestations are typically treatable and rarely fatal when promptly identified, but the larger agricultural stakes have driven the current federal mobilization. For policymakers and industry, the immediate questions are whether scaled-up sterile‑fly production, renewed surveillance, and emerging genetic tools can be deployed fast enough to stop a northward expansion before it threatens U.S. herds and trade.

The screwworm resurgence has already forced significant investment and legislative attention; how effectively those measures translate into containment and prevention will shape the agricultural and economic consequences for the United States in the months and years ahead.