Four confirmed cases of New World screwworm in Texas and one in a dog across the state line in New Mexico have put federal and state animal health agencies on high alert as the flesh-eating parasite reappears on U.S. soil for the first time in decades. The detections span multiple counties and species, with cases identified in cattle and at least one domestic dog. USDA Under Secretary Dudley Hoskins stated the agency had identified four confirmed detections of the parasite, and the geographic spread from the southern Texas border region northward into the Permian Basin signals that the threat is not contained to a single outbreak zone.
Why four Texas screwworm detections signal a widening threat
The first U.S. case surfaced in a bovine in Zavala County, a brushland county roughly 90 miles southwest of San Antonio. The CDC placed the date of the USDA-APHIS report at June 3, 2026. Within days, the case count climbed. A calf in La Salle County, one county southeast of Zavala, tested positive, along with a dog sample submitted by a veterinarian in Andrews County, far to the northwest in the Permian Basin oil patch near the New Mexico border.
That geographic jump is what makes the situation alarming. Zavala and La Salle counties sit along the same corridor used by cattle operations moving livestock between winter pastures in southern Texas and Mexico and summer grazing land farther north. Andrews County, by contrast, sits roughly 350 miles to the northwest. If the parasite traveled that distance through animal movements or fly dispersal, the affected zone is far larger than the initial cluster suggested. The timing of these detections during early summer, when warm temperatures accelerate the screwworm fly’s reproductive cycle, raises the stakes for ranchers across the southern Plains.
The dog case added another layer of complexity. USDA initially linked the sample to Andrews County because the submitting veterinarian practiced there. A subsequent clarification from the agency reclassified the case: the dog actually resides in Lea County, New Mexico, making it the first confirmed screwworm detection in that state. The National Veterinary Services Laboratories handled the confirmation. That same announcement also noted an additional confirmed Texas case, bringing the state’s total to four.
Confirmed cases, counties, and species in the APHIS record
The federal case record now includes five confirmed detections across two states. In Texas, the confirmed animals include a bovine in Zavala County, a calf in La Salle County, and at least two other cases confirmed by the National Veterinary Services Laboratories. The New Mexico case involves a dog in Lea County. USDA Under Secretary Dudley Hoskins stated the agency had identified four confirmed detections of New World screwworm, a count that referred to the Texas cases before the New Mexico reclassification added a fifth to the national tally.
The Texas Department of State Health Services issued public health guidance after the Zavala County calf was confirmed. The agency advised clinicians to watch for painful, foul-smelling wounds with visible larvae, the hallmark presentation of screwworm myiasis. Texas health officials also stated clearly that there is no human-to-human transmission of the parasite. People can become infested only through direct contact with screwworm fly larvae deposited in open wounds, and the risk is highest for those working closely with infested animals.
APHIS maintains a confirmed detections dashboard that tracks county, species, confirmation date, and case status. That tracker is the most reliable public source for updated numbers as the situation develops. The CDC’s situation summary separately frames the public health dimension, noting the myiasis risk to people who live or work near affected animals.
Unanswered questions about the northward spread
Several critical gaps remain in the public record. USDA has not released exact confirmation dates or laboratory accession numbers for the La Salle County and Andrews County cases, making it difficult to reconstruct a precise timeline of how quickly the parasite moved between counties. Without that timeline, the question of whether the detections reflect a single wave of fly dispersal or multiple independent introductions from Mexico stays open.
Border surveillance and livestock import data that could clarify the pathway have not been published in the APHIS announcements. The hypothesis that seasonal cattle movements from southern Mexico carried the parasite northward is plausible given the geography and timing, but testing it would require cross-referencing APHIS confirmation dates against Texas Animal Health Commission import permits and Mexican screwworm surveillance reports. None of those datasets are currently public in a form that allows outside researchers to reconstruct the chain of transmission.
Another unresolved issue is how long the parasite may have been present before the first confirmed detection in Zavala County. Screwworm infestations can be mistaken for routine wound infections, particularly in remote ranching areas where veterinary access is limited. If subclinical or misdiagnosed cases occurred in late spring, the fly population could already be several generations into a local life cycle, expanding the radius of risk beyond the counties named in federal releases.
Wildlife also complicates the picture. While the confirmed cases so far involve domestic animals, New World screwworm can infest deer, feral swine, and other free-ranging species that move across fences and property lines. No federal or state agency has yet reported confirmed wildlife cases linked to this outbreak, but the absence of reporting does not necessarily mean the absence of infection. Systematic wildlife surveillance would require coordinated trapping, necropsy, and larval identification efforts that have not been detailed in public statements.
Response measures and what ranchers are being told
In response to the detections, APHIS and state partners have activated elements of the screwworm emergency plan developed after the parasite was eradicated from the United States in the late 20th century. That plan emphasizes rapid detection, treatment of affected animals, and containment to prevent the establishment of a self-sustaining fly population.
Field veterinarians in the affected counties have been instructed to submit suspicious larvae to diagnostic laboratories rather than relying on visual identification alone. The National Veterinary Services Laboratories are providing confirmatory testing, which is critical because other fly species can cause superficially similar wound infestations. Accurate species identification determines whether a case is added to the federal screwworm tally and can trigger additional trace-back investigations.
Ranchers in South Texas and the Permian Basin are being urged to step up daily inspection of livestock, focusing on navels of newborn calves, castration and branding sites, and any traumatic wounds where adult flies might lay eggs. Recommended control measures include prompt wound cleaning, application of approved insecticidal dressings, and isolation of heavily infested animals until larvae are removed and the site is healing.
State agencies have also warned producers to be cautious when moving animals off affected premises. While no broad movement bans have been announced, officials are advising that animals be inspected and treated before transport to limit the chance of moving infested livestock into new regions. Auction markets and feedlots are being reminded to report any unusual clusters of myiasis cases to state veterinarians.
Public health risk and communication challenges
For the general public, health officials are emphasizing that the immediate risk remains low but not negligible. Most people have limited direct contact with livestock, and basic wound hygiene significantly reduces the odds of a fly successfully establishing an infestation. Nonetheless, residents in affected counties who handle animals, work outdoors, or hunt could be exposed if they encounter infested carcasses or live animals with untreated wounds.
Clinicians in Texas and New Mexico are being asked to take detailed travel and exposure histories when evaluating patients with unusual wound infections. In areas near the confirmed detections, physicians are encouraged to consider screwworm myiasis in the differential diagnosis when patients present with rapidly enlarging, malodorous wounds containing visible larvae. Early recognition allows for mechanical removal of larvae and reduces the risk of serious tissue damage.
Communication has emerged as a secondary challenge. The initial misclassification of the dog case as a Texas detection before its reassignment to Lea County illustrates how quickly evolving information can create confusion about where the parasite is present. Public health messaging now has to thread the needle between alerting residents and producers to a genuine emerging threat and avoiding unnecessary alarm in counties where no cases have been confirmed.
As the situation unfolds, the most concrete signals will come from updated entries on the APHIS confirmed case dashboard and any new federal announcements. Until more detailed epidemiologic data are released, key questions about how far the screwworm has spread and whether it can be pushed back across the border will remain unanswered, leaving ranchers and veterinarians to operate in a zone of heightened vigilance and incomplete information.
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*This article was researched with the help of AI, with human editors creating the final content.
