A three-week-old calf in Zavala County, Texas, became the first confirmed case of New World screwworm on U.S. soil during the current outbreak after federal inspectors found larvae burrowed into the animal’s umbilical area. The detection triggered an immediate federal and state response, and a second calf case soon followed in neighboring La Salle County. The outbreak marks the return of a parasite that the United States spent decades and billions of dollars eradicating, and it is now forcing ranchers, veterinarians, and health officials across southern Texas to confront a threat most assumed was long gone.
Why a single Texas calf has federal agencies on high alert
New World screwworm flies lay eggs in open wounds on warm-blooded animals. The larvae feed on living tissue, expanding the wound and potentially killing the host if left untreated. The United States declared the pest eradicated from its territory through a massive sterile-insect release program that ran from the 1950s into the 1980s, and the barrier has since been maintained in Panama and southern Mexico. A confirmed case inside U.S. borders means that barrier failed to hold.
The timeline raises pointed questions. Mexico’s Chief Veterinary Officer notified APHIS of a positive screwworm detection on Nov. 22, 2024, according to federal summaries of the cross-border alert. Between that notification and the Zavala County confirmation, the fly or infested animals crossed the border. Whether the parasite arrived through natural fly dispersal or through gaps in livestock import screening at the border is a question federal records have not yet answered publicly. APHIS import logs and case-onset dates could clarify which pathway is more likely, but no agency has released that comparison.
The distinction matters for ranchers. If the screwworm crossed on its own wings, the response centers on aerial sterile-fly drops and trapping. If it rode in on an animal that passed through inspection, the fix is procedural: tighter screening at ports of entry. Each scenario demands different resources and timelines, and each carries different implications for how long Texas producers may be dealing with sporadic cases.
Two Texas counties, two calves, and a dog
USDA’s National Veterinary Services Laboratories confirmed the Zavala County calf as the index U.S. case after larvae were recovered from the umbilical area of the three-week-old animal, a common entry point because the healing navel provides the moist wound environment screwworm flies seek for egg-laying.
The outbreak did not stay in one county. NVSL subsequently confirmed a new case in a calf in La Salle County, which borders Zavala to the southeast. That second confirmation signaled the fly population was not confined to a single ranch or pasture and that environmental conditions across a wider swath of brush country were suitable for the parasite to establish, at least temporarily.
The Associated Press also reported a case involving a dog, adding a non-livestock species to the outbreak’s profile and widening the concern to pet owners and wildlife managers. Companion animals, feral hogs, deer, and other wildlife can all serve as hosts if they suffer untreated wounds, complicating any attempt to draw a clean line around the affected area.
The Texas Department of State Health Services issued guidance telling residents to watch for wounds or lesions that do not heal and to report them promptly. Screwworm can infest humans, though it does so far less frequently than livestock. The state health advisory reflects the seriousness officials attach to even a small number of confirmed animal cases, and it underscores that the outbreak is being treated as a public health concern as well as an economic threat to the cattle industry.
USDA has directed producers in the affected region to report suspicious wounds through its dedicated screwworm channels. The agency is coordinating a multi-state surveillance effort and expanding sterile-insect releases across the border region, though it has not published quantitative data on how many sterile flies are being dropped inside Texas or how frequently flights are running over the new detection zones. Local veterinarians say they have been advised to treat any suspicious myiasis aggressively and to submit larvae for identification rather than assuming they belong to less dangerous fly species.
Gaps in the public record and what ranchers should watch
Several pieces of the story are still missing from the official record. No primary USDA or DSHS document has disclosed the pre-detection surveillance intensity or trap density in Zavala County before the index calf was found. Without that data, it is impossible to know whether the fly was circulating undetected for weeks or arrived shortly before discovery. Likewise, no agency statement has connected the index calf to a specific cross-border livestock shipment or identified the route by which the screwworm entered Texas.
The sterile-insect technique, the main weapon against screwworm, depends on flooding an area with enough lab-reared sterile males to outcompete wild males for mating opportunities. USDA has described adjustments to its suppression polygons and emphasized that it is ramping up aerial releases, but it has not released flight frequency data or sterile-fly volume figures for the new Texas zones. That information gap makes it difficult for outside analysts or affected producers to assess whether the response is scaled to the threat or to anticipate how long intensive operations will continue.
For livestock owners in southern Texas, the practical first step is direct, frequent inspection of animals. Calves, lambs, and other young stock with healing umbilical cords or castration sites are especially vulnerable. Ranchers are being urged by state and federal officials to check navels, branding sites, and any cuts or horn injuries for signs of maggot infestation, including a foul odor, tissue discoloration, and visible larvae.
When suspicious wounds are found, producers are advised to seek veterinary help quickly rather than attempting to remove larvae themselves and moving on. Effective treatment typically combines mechanical removal of the maggots, application of approved insecticidal products, and careful wound care over several days. Early intervention can save animals and also removes a breeding site that could sustain the local fly population.
Movement decisions are another pressure point. Even without formal quarantine orders covering large areas, ranchers in and around the affected counties are weighing whether to delay shipping calves or replacement heifers until they have had time to monitor for signs of infestation. Buyers farther north may begin asking for assurances that cattle sourced from the border region have been inspected and cleared.
Pet owners in the region face a different set of choices. Dogs that roam brushy pastures, work cattle, or accompany hunters are at higher risk of minor injuries that can attract screwworm flies. State guidance emphasizes keeping pets up to date on routine veterinary care, promptly cleaning any cuts, and seeking professional evaluation if a wound worsens instead of healing.
For now, the confirmed cases remain few, but the stakes are high. The last time screwworm was entrenched in the United States, it cost ranchers millions of dollars annually in lost animals and treatment expenses. The sterile-insect program that finally pushed the parasite south of the border has long been held up as a public health and agricultural success story. The Texas detections are a reminder that even successful eradication campaigns require constant maintenance – and that a single calf with a small, infested wound can signal a much larger challenge taking shape just out of sight.
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*This article was researched with the help of AI, with human editors creating the final content.
