A flesh-eating fly is back in Texas livestock decades after America wiped it out

New World screwworm, a parasitic fly whose larvae feed on living flesh rather than dead tissue, has re-established itself in Texas livestock more than six decades after the United States eradicated the pest in 1966. The current outbreak began June 3, 2026, when a calf in Zavala County tested positive, and confirmed cases have since spread to 16 Texas counties and 38 premises, according to the Texas Animal Health Commission. Federal and state crews are running the same sterile-fly release program that wiped the insect out the first time, and quarantine zones around the earliest-affected premises are only now starting to lift as surveillance turns up no new detections.

A Fly the Country Eradicated in 1966

The screwworm fly, Cochliomyia hominivorax, was declared eradicated from the United States in 1966 using the sterile insect technique, a method that floods an area with lab-raised male flies sterilized by radiation so wild females produce no viable offspring. The pest never fully left the hemisphere, and cases climbed sharply in Central America starting in 2023, spreading north through Costa Rica, Nicaragua, Honduras, Guatemala, Belize and El Salvador. A cow tested positive at an inspection checkpoint in Chiapas, Mexico, in November 2024, and the infestation kept moving north from there until it crossed into South Texas this summer. The Texas Animal Health Commission, or TAHC, says the Zavala County calf that tested positive on June 3 has since recovered, and that individual case is now considered inactive even as the broader outbreak continues.

How the Larvae Attack Living Tissue

Adult screwworm flies lay eggs in open wounds or in body openings such as the nose, ears, navel or genitalia of warm-blooded animals. The eggs hatch into larvae armed with rows of backward-facing hooks that let them burrow deeper into flesh instead of feeding on the surface, which is what separates screwworm from the many fly species that only infest dead tissue. Left untreated, an infested animal can die within about a week. The parasite mainly strikes livestock but can also affect pets, wildlife and, in rare cases, people, and Texas health officials have asked anyone who spots a suspicious wound to contact a veterinarian immediately. Guidance from TAHC tells producers and pet owners to check animals for draining or enlarging wounds, maggots or egg masses, and lesions around the nose, ears, umbilicus or genitalia, since early treatment is what keeps a manageable infestation from turning fatal. The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, or APHIS, has stressed that the outbreak poses no threat to the food supply, since any infested animal would be caught during routine meat inspection before reaching commerce.

Case Count Climbs to 38 Premises Across 16 Counties

The outbreak has grown well beyond its original footprint. The Texas Animal Health Commission’s current tally puts confirmed cases at 38 premises across 16 counties, stretching from Zavala and Uvalde in the south up toward Andrews County near the New Mexico line, where an infested dog with a recent travel history to Mexico was confirmed in June. Every new detection triggers a 20-kilometer infested zone around the premises, along with quarantines, movement controls and stepped-up trapping. Animals inside an active zone cannot be moved out without a certified inspection confirming they show no signs of infestation, a requirement that has reshaped how ranchers manage herds across a wide stretch of the Hill Country and South Texas brush country this summer. Texas established a certified-inspector training program with Texas A&M AgriLife to expand the pool of people authorized to issue the movement certificates ranchers now need, since a single state veterinary staff could not keep pace with an outbreak spread across more than a dozen counties.

Sterile Flies Remain the Only Proven Weapon

Eradication still depends on the same sterile insect technique used in the 1960s. USDA activated a sterile fly dispersal facility at Moore Air Base in Edinburg, Texas, to get lab-raised pupae into the air over and around infested areas as quickly as possible, supplementing releases already underway near the Mexican border. USDA has described the response as an aggressive, sustained campaign involving roughly 75 people working the outbreak directly in Texas, plus hundreds more providing lab diagnostics, logistics, treatment supplies and aerial operations. Because sterile flies are still flying through the same monitored airspace as wild ones, USDA dyes the lab-raised pupae with a fluorescent marker that glows under ultraviolet light, letting inspectors quickly tell a released sterile fly apart from a wild one caught in a trap. Female screwworm flies mate only once in their life, so when they pair with a sterile male, the population’s ability to reproduce collapses over a few generations.

Trade Restrictions Begin to Ease as Zones Are Released

The outbreak has also disrupted cross-border livestock trade, since the U.S. paused imports of cattle, bison and horses from Mexico as a precaution while the response ramped up. USDA reopened the port at Douglas, Arizona, to cattle imports on August 24, 2026, the first step in what the agency has called a phased reopening of southern border ports that depends on Mexico hitting agreed milestones in its own eradication effort. Inside Texas, TAHC has begun lifting infested-zone orders in several of the counties hit earliest in the outbreak after inspectors confirmed no new wounds or larvae during follow-up surveillance, even as movement restrictions remain in force elsewhere in the state. Officials describe the fight as ongoing rather than won, with continued trapping, wildlife surveillance and rancher reporting expected to run for months before the pest can again be declared cleared from Texas. USDA has also launched a dedicated wildlife guidance page aimed at hunters and outdoor workers, since deer and other wild mammals can carry and spread the parasite even in areas where livestock owners are already on alert.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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