FDA clears 14th emergency drug as screwworm outbreak widens in Texas

The Food and Drug Administration authorized a new over-the-counter drug on Thursday to help cattle producers fight New World screwworm, the flesh-eating fly larva that has re-established itself in Texas after a six-decade absence from the United States. The emergency clearance for Bimectin, a generic ivermectin injection, was the 14th such authorization the agency has issued against the pest since the outbreak reached Texas in June.

The FDA's emergency use authorization covers Bimectin injection, made by the Irish manufacturer Bimeda Animal Health, for cattle only. Producers can give it within 24 hours of a calf's birth, at castration, or as soon as a wound appears, to prevent screwworm larvae from infesting the tissue. The product is not authorized for lactating dairy cows or calves destined for veal, and treated animals must clear a 35-day withdrawal period before slaughter.

A widening outbreak

The Department of Agriculture had confirmed 47 domestic animal cases of New World screwworm as of Thursday. The parasite, once eliminated from American soil through one of the most celebrated triumphs of 20th-century veterinary science, reappeared in a three-week-old calf in Zavala County, Texas, on June 3. It has since spread across a wide swath of the Texas Hill Country and into South Texas, with a case also confirmed in a dog in Lea County, New Mexico, marking the first detection outside Texas.

The Texas Animal Health Commission now counts cases across 16 counties on 37 separate premises. To slow further spread, the agency has imposed movement restrictions on livestock in an "Infested Zone" covering 23 counties, from Val Verde and Terrell in the west to Kerr and Gillespie in the Hill Country. Under the current order, the commission says, "all warm-blooded animals in an Infested Zone may not move out of the zone without prior authorization." State officials modified the zone as recently as Wednesday, adjusting restrictions covering Crockett, Sutton and Val Verde counties, and again on August 5 for Pecos and Terrell counties — evidence that the boundary is still being redrawn as new premises test positive.

Sixty years undone

New World screwworm, whose larvae burrow into and consume the living flesh of warm-blooded animals rather than dead tissue, was declared eradicated from the United States in 1966 after a sustained federal campaign using the sterile insect technique — breeding and releasing billions of sterilized male flies so wild females' eggs never hatch. That method held the pest south of a biological barrier maintained near the Darién Gap in Panama for decades. The containment line broke down after the parasite was detected in Chiapas, Mexico, in November 2024 and pushed north through the country, generating more than 27,000 reported detections before reaching the Texas border this spring.

The Centers for Disease Control and Prevention also tracks the parasite as a human health concern in affected regions, warning that people with open wounds, particularly those spending extended time outdoors or near livestock, can develop myiasis — a painful, foul-smelling infestation that requires prompt medical treatment and should never be handled by removing larvae at home.

The government's response has leaned heavily on expanding the range of approved treatments available to producers, since sterile-fly dispersal alone cannot immediately stop transmission once flies are established on the ground. The Animal and Plant Health Inspection Service, which coordinates the federal eradication campaign alongside the FDA, maintains a running dashboard of confirmed detections by state, county and species as new cases are verified in USDA's veterinary laboratories.

Who is affected

Thursday's authorization is meant to give ranchers a cheaper, more accessible option. Bimectin is a generic version of Ivomec, the brand-name ivermectin product first authorized for screwworm prevention in February. Because it is sold over the counter without a prescription, producers across the quarantine zone and beyond can buy it directly rather than waiting on a veterinarian.

"By authorizing both generic and pioneer products, the Agency is ensuring producers aren't dependent on a single manufacturer or product to protect their herds," said Timothy Schell, director of the FDA's Center for Veterinary Medicine.

The stakes extend well beyond the affected counties. Texas produces more cattle than any other state, and industry analysts have warned that a sustained outbreak could raise producers' labor and veterinary costs enough to further squeeze an industry already contending with a shrunken national herd. Economists have said that if the parasite forces herd reductions on top of existing supply pressures, the added cost is likely to reach consumers through higher beef prices at grocery stores already coping with elevated food costs.

Ranchers in the infested zone have had to change routine practices — timing branding, dehorning and castration more carefully, and checking animals more frequently for open wounds — because any fresh wound is an opportunity for a screwworm fly to lay eggs.

What happens next

USDA has invested heavily in restoring the sterile-fly infrastructure that defeated the parasite the first time. A new dispersal facility opened in Edinburg, Texas, in February to expand the release of sterilized flies along the border, and the department has committed roughly $750 million toward a larger production plant in Texas expected to come online next year, capable of producing hundreds of millions of sterile flies a week. In the meantime, federal officials have also moved to partially reopen limited cattle imports from Mexico at select border ports under stricter inspection protocols, a step that has drawn mixed reactions from ranchers wary of reintroducing risk before the domestic outbreak is contained.

For now, containment depends on the tools available to individual producers. FDA officials have signaled more emergency authorizations are likely as manufacturers submit additional products, following the same pattern that produced 14 authorizations in roughly seven months. State and federal agencies say they will continue to adjust quarantine boundaries as weekly case counts are confirmed, with the Texas Animal Health Commission's zone maps subject to further revision as surveillance continues into the fall.