Screwworm Update: Cases Now at 19, Quarantine Zones Expanding Across Texas

Three new cases were confirmed this week in Terrell County. Eleven Texas counties are now designated infested zones. And the sterile fly program won't show results for several more reproductive cycles. Here's where things stand as of June 24.

By Vet Candy Editorial  |  June 24, 2026  |  Outbreak Watch — Food Animal & Large Animal

The New World Screwworm outbreak in the United States continues to grow. As of June 24, 2026, confirmed cases have reached 19 — 18 in Texas and one in New Mexico — following three new detections this week in cattle in Terrell County, Texas.

This is a developing situation. We've been tracking it since the first confirmed U.S. case on June 3. Here's the current state of the outbreak, what the quarantine restrictions actually require of producers and their veterinarians, and a realistic timeline for when the sterile fly program is expected to begin bending the curve.

Current Case Count and Geographic Spread

The outbreak began June 3 with a confirmed case in a calf in Zavala County, Texas — the first U.S. case since eradication in the 1960s. Cases have since been identified across multiple counties in South and West Texas, primarily in cattle, goats, and other domestic livestock.

The 11 Texas counties now designated as New World Screwworm-infested zones are: Terrell, Pecos, Uvalde, Val Verde, Webb, Zavala, Edwards, Gillespie, Kerr, Kimble, and Sutton. One case remains confirmed in New Mexico.

The geographic pattern — spread across a broad arc of South and West Texas — reflects the fly's mobility and the density of livestock operations in the region.

Quarantine Restrictions: What They Require

The Texas Animal Health Commission has expanded quarantine boundaries in response to the new cases. The restrictions carry real regulatory teeth.

Livestock and warm-blooded animals cannot leave designated infested zones without inspection and written authorization from state animal health officials.

Movement certificates are required for any animals moving out of quarantined areas. These documents must include animal identification and treatment protocols specifically designed to prevent parasite spread.

Unauthorized movement carries penalties and, more critically, risks spreading the infestation to new areas — which state officials warn would significantly set back eradication efforts.

For veterinarians in or near affected counties: if you're signing movement paperwork for clients in these zones, verify current TAHC requirements before you certify. The quarantine map has been expanding, and a county that was unrestricted last week may not be this week.

A Refresher on Why This Parasite Is So Dangerous

New World Screwworm larvae are not typical wound flies. Unlike common myiasis-causing flies that feed on necrotic tissue, screwworm larvae actively burrow into living flesh — releasing enzymes that liquefy healthy tissue as they feed. The infestation is painful, rapid, and fatal if untreated.

Any breach in skin is a potential entry point: open wounds, navel stumps in newborns, dehorning sites, branding wounds, ear tag sites, tick bites, or mucous membranes. Cattle, goats, and other livestock are primary targets. Wildlife, pets, and occasionally humans can also be affected.

Early clinical signs — unusual wound odor, animal distress disproportionate to wound size, visible larval movement in a wound — warrant immediate examination and reporting.

The Sterile Fly Program: Why Cases Are Still Rising

USDA and state officials have expanded sterile male fly release operations across affected areas, including the newly impacted parts of West Texas. But practitioners and producers need to understand why confirmed case numbers will likely keep climbing in the near term even as those releases ramp up.

The sterile insect technique works by population collapse over multiple reproductive cycles — it is not an immediate intervention. Sterile fly releases do not kill existing larvae already developing in animals. Any screwworm eggs already laid before sterile males competed for mating will still hatch, develop, and produce adult flies.

The screwworm life cycle is approximately three weeks. Officials expect it will take several complete reproductive cycles before local populations begin to decline meaningfully. That means weeks, not days, before case numbers stabilize — and new detections in the coming weeks are expected and do not necessarily indicate the sterile fly program is failing.

This is a critical piece of context to communicate clearly to producers in affected areas. Rising case counts right now do not mean the response isn't working. They mean the population hasn't yet turned over enough times for the sterile males to dominate mating.

What Veterinarians Should Be Doing Right Now

If you practice in or near any of the 11 quarantined Texas counties or in New Mexico: review your wound management protocols with clients, and make sure your team knows the clinical presentation to look for. Screwworm myiasis can progress rapidly — hours, not days, matter.

If you're certifying interstate health certificates for clients in affected areas: confirm current TAHC movement requirements before signing. Requirements are being updated as the quarantine expands.

If you suspect a case: report immediately to your state veterinarian and USDA APHIS. Screwworm is a reportable condition. Early detection is the single most effective tool for limiting spread beyond current quarantine zones.

We'll continue to update this coverage as case counts and quarantine boundaries change.

Official Sources

Texas Animal Health Commission — NWS Emergency Page: tahc.texas.gov/emergency/nws.html

USDA APHIS — Confirmed U.S. Case Map: New World Screwworm Confirmed Detections

USDA APHIS — NWS Program Overview: USDA APHIS New World Screwworm