Health & Medicine · The Record
FDA authorizes dog screwworm drug as Texas reports cases in 15 counties
The August 13 emergency authorization describes an infestation as an unlikely event; the Texas Animal Health Commission's page, read four days later, counts 36 infested premises and a movement-restricted zone in parts of 23 counties.
The U.S. Food and Drug Administration issued an Emergency Use Authorization on August 13, 2026 for Simparica TRIO - sarolaner, moxidectin and pyrantel chewable tablets - for the treatment of New World screwworm infestations, or myiasis, in dogs and puppies. Four days later, the Texas Animal Health Commission's screwworm page reported 15 Texas counties with cases, 36 premises with cases, and a movement-restricted zone drawn across parts of 23 counties.
An emergency authorization is a more temporary instrument than a drug approval. The FDA release states that this one runs until it is revoked, or until the Secretary of Health and Human Services terminates the declaration that the potential public health emergency presented by screwworm justifies emergency use of animal drugs. That declaration came a year earlier: the HHS and FDA announcement carried by the Louisiana Department of Agriculture and Forestry said the authority would let the agency act quickly and "limit the spread of New World Screwworm," and described stopping the pest as a national security priority.
What the agency said
The August 13 announcement argued for readiness rather than response. Acting FDA Commissioner Kyle Diamantas, J.D., quoted in the release, said the agency keeps authorizing products so that veterinarians and pet owners have multiple treatment options "in the unlikely event animals under their care become infested with New World screwworm." He then offered the agency's assessment of where things stand: "To date, the U.S. has been successful in containing the spread of NWS." The stated hope was that it would remain that way, with the authorization held in reserve should the need arise.
The release put the risk to pets in geographic terms. Most dogs in the United States are at low risk of exposure because of where they live, it states, while dogs that live in, travel to or return from areas where officials have identified infestations - and certain countries south of the border - should be watched for visible maggots or foul odors in wounds or body openings. Screwworm has been detected only in specific areas of Texas and New Mexico, according to the same text. Timothy Schell, Ph.D., director of the FDA's Center for Veterinary Medicine, directed owners toward awareness rather than alarm: learn the signs, and raise concerns with a veterinarian.
The authorization's own limits are stated plainly, and they matter to a reader deciding what this changes. Simparica TRIO was approved in 2020 for heartworm, flea and tick-related indications; the emergency authorization covers treatment of screwworm myiasis only, and the release says the product is not authorized for prevention. FDA writes that it concluded, on the totality of the scientific evidence available, that it is reasonable to believe the product may be effective when used as authorized and that known and potential benefits outweigh known and potential risks. The drug remains prescription-only because, in the release's account, professional expertise is needed to monitor for and respond to neurologic adverse reactions; sarolaner belongs to the isoxazoline class, which the agency says is commonly used and safe for most dogs but has been linked to tremors, coordination loss and seizures.
What the record shows
The Texas Animal Health Commission maintains its own screwworm page, and its figures as retrieved on August 17, 2026 are specific. It lists total Texas counties with cases at 15 and total Texas premises with cases at 36. It also records an established Infested Zone in parts of 23 named counties, among them Bandera, Gillespie, Kerr, Pecos, Presidio, Tom Green, Uvalde, Val Verde and Zavala. Inside that zone the restriction is categorical: "all warm-blooded animals in an Infested Zone may not move out of the zone without prior authorization from the Texas Animal Health Commission (TAHC)."
The Commission dates the first Texas detection to June 3, 2026. The American Farm Bureau Federation's account of the earliest confirmations describes that index case as a three-week-old beef calf in La Pryor, in southwestern Zavala County, roughly 50 miles from the Mexico border, followed by a second calf a few miles away and a calf in neighboring La Salle County. Ten weeks later, the restricted counties include Tom Green and Gillespie in the Hill Country and Presidio and Brewster in far West Texas - hundreds of miles from the first detection.
The national tally sits alongside it. The Texas Standard's tracker reports that the U.S. Department of Agriculture has confirmed 44 screwworm infestations, 43 in Texas and one in New Mexico; the retrieved text does not date that figure, so it is reported here undated. The same tracker states that "movement restrictions now cover parts of 22 Texas counties, stretching from the Rio Grande Valley through West Texas and the Hill Country," one county short of the list on the Commission's page. Two records disagree by one county, and neither retrieved text explains the gap.
One detail bears directly on the word "unlikely." The Farm Bureau's list of the first confirmed U.S. cases includes "a dog in Lea County, New Mexico" - a dog infestation entered on the federal record roughly two months before the FDA authorized a dog treatment and described such an event as unlikely.
What that juxtaposition supports, as analysis, is narrower than it may look. Rising counts are not by themselves a measure of spread. PBS NewsHour's account of a USDA briefing on the Texas cases records the caution that after a first case is found, vigilance intensifies, and "when you are looking for something, you are more likely to see it" - surveillance effort and infestation both push a case count up, and neither record retrieved here separates them. The commissioner's sentence is also past-tense by construction: "To date" claims containment up to August 13, not after it.
What the comparison does support is a question about calibration. A federal release addressed to dog owners called an infestation in their animals unlikely on the same day a state agency was restricting the movement of every warm-blooded animal in parts of 23 counties, with a dog case already confirmed. Both statements can be accurate at once - most U.S. dogs live nowhere near the zone - but a reader in Kerr or Presidio County is being described by the wrong half of that sentence.
What to check next
Three things resolve on their own schedule. The authorization ends when FDA revokes it or when HHS terminates the underlying declaration, so its survival is itself a readable signal about how the government rates the threat. The Commission's county and premises totals and its list of restricted counties can be read against the figures above. And the discrepancy between 22 and 23 counties should close in one direction or the other.
One falsifiable marker, offered as this desk's own judgment from these records alone: on October 1, 2026, the Texas Animal Health Commission's page will still show an Infested Zone covering parts of 15 or more Texas counties. If it shows fewer, the containment claim made on August 13 will have earned support that the mid-August record did not yet give it.