Screwworm cases double in two weeks

By Breyana Segura - Chief Staff Writer | July 7, 2026

Tuesday presentation shows 21 Texas counties affected

Nearly 60 years after it was almost entirely eradicated in South Texas, a new infestation of the deadly screwworm has raised alarms among farmers and ranchers as well as rural households over a surge in reported cases.

Agriculture specialists and local producers gathered at the Frio County Extension Office Tuesday morning to receive urgent briefings on the rapid escalation of the New World screwworm (NWS) infestation in South and Central Texas.

Hosted by the Texas Animal Health Commission, the July 7 presentation revealed there are presently 12 infestation zones in Texas, affecting 21 counties.

Data released by the US Department of Agriculture reveals documented infections have surged to 32 cases since the initial detection on June 3, effectively doubling the 16 cases that had been reported June 22.

Of the 32 documented cases, 18 remain active, the federal agency reported.

The current outbreak began with a single confirmed case in Zavala County on June 3, followed by a steady rate of one to two cases per day through June 12.

Although case numbers leveled for approximately a week, positive detections began rising rapidly over the next 14 days.

The peak of the present spike occurred on June 24, when officials documented infections in five sheep and one cow.

To date, those six cases represent the highest single-day infection total of the current outbreak.

The geographic footprint of the parasite has also expanded into Medina County, prompting immediate regulatory action.

Following the confirmation of a positive case in a Medina County cow, regional authorities issued a public advisory establishing a strict containment perimeter.

“Due to the detection of NWS in Medina County, areas of Bandera, Medina, and Uvalde counties are designated as infested Zone 9,” the notice read.

Federal data indicates livestock and domestic animal infections are on the rise, with more than 96 percent of all nationwide cases currently concentrated in Texas.

While the majority of the positive detections have occurred in cattle, the parasite has demonstrated its threat to several species. Detections have been made in cattle umbilical cords, castration sites, and on the skull at dehorning. Detections have been confirmed in the eyes of sheep and goats, and two domesticated dogs have also tested positive for the larvae, underscoring the risk to household pets as well as commercial livestock, according to the health commission. One infected dog was a working hound on a ranch in Pecos County.

In response to the mounting threat to livestock, federal and state authorities have intensified surveillance and elimination efforts along the international border as the NWS continues its migration from Mexico.

To combat the spread, the USDA executed a mass release of over 100 million sterilized flies across Texas and Mexico during June. This included a targeted drop of 80,000 sterile flies in Zavala County immediately following the initial confirmation of the parasite.

The time-tested biological control method relies on releasing sterile male flies into the wild to mate with wild females. Due to female screwworm flies only mating once in their lifespan, mating with a sterile male ensures the female will produce no offspring, causing a localized population collapse.

The livestock situation in South and Central Texas remains highly fluid, the health commission reported, because the NWS lifecycle can cause systemic tissue damage and death in warm-blooded animals if left untreated. Local agriculture extension agents are urging producers to maintain rigorous animal inspection schedules.

With the establishment of infested Zone 9 and intensified federal border interventions and a facility opening in Mexico on June 27, the coming weeks will be critical in determining whether the USDA’s massive biological suppression efforts can successfully halt the parasite’s northern trajectory.