New World Screwworm Outbreak in Texas and New Mexico: A Comprehensive Overview

Federal and state authorities are responding to a confirmed outbreak of New World screwworm (Cochliomyia hominivorax) in livestock and pets across Texas and New Mexico. This marks the first natural incursion of the pest into the United States since it was eradicated in the 1960s. As of the most recent reports, the U.S. Department of Agriculture (USDA) has confirmed multiple cases, with response efforts involving quarantines, sterile fly releases, and expanded disaster declarations.

Timeline and Confirmed Cases

The first case of the current U.S. outbreak was confirmed on June 3, 2025, in a three-week-old calf in La Pryor, Zavala County, Texas, approximately 100 miles southwest of San Antonio. Subsequent cases have been confirmed in the following locations and species:

• Zavala County, Texas: A second case in a one-month-old calf, located approximately 5.6 miles from the first case.
• La Salle County, Texas: A calf.
• Andrews County, Texas: A dog from Lea County, New Mexico, which had visited Andrews County.
• Gillespie County, Texas: A goat.
• Lea County, New Mexico: A dog that had not recently traveled to Mexico or Texas, prompting local investigation.

As of the latest reports, the USDA has confirmed 15 cases in total, including cattle, sheep, goats, and one dog. No human cases have been reported in this outbreak. The USDA's National Veterinary Services Laboratories in Ames, Iowa, confirmed the diagnoses.

Background and Origin

New World screwworm is a parasitic fly whose larvae (maggots) feed on the living tissue of warm-blooded animals, including livestock, wildlife, pets, and occasionally humans. Female flies lay 200–300 eggs at a time in open wounds or natural body openings (mouth, eyes, nose, reproductive areas). If untreated, infestations can be fatal within 7–14 days.

The pest was eradicated from the United States in the 1960s using the sterile insect technique (SIT), which involves releasing radiation-sterilized male flies to mate with wild females, preventing reproduction. This method contained the species south of the Darién Gap in Panama for decades.

The current outbreak originated in Panama in early 2023, spreading to Costa Rica, Nicaragua, and Honduras. Mexico reported its first case in late 2024. Over 185,000 animal cases and 2,100 human cases have been reported in Central America and Mexico, with 10 human deaths reported by the CDC. Researchers have linked the spread to multiple factors, including climate change expanding the fly's habitat and illegal cattle trafficking by organized crime groups in Central America, which bypasses health screenings.

The USDA had been monitoring the pest's northward spread since late 2024 and began dropping sterile flies in South Texas in February 2025.

Response Measures

Quarantine and Movement Restrictions

• The USDA established a 20-kilometer (12-mile) "infested zone" in Zavala County and part of Uvalde County, Texas, restricting animal movement without inspection.
• Quarantine zones have been established around each confirmed case.
• California imposed interstate movement restrictions on livestock from affected areas.
• Canada temporarily halted imports of cattle, horses, and other livestock from Texas on June 6, 2025.

Sterile Fly Releases

• The USDA has been releasing sterile male flies in South Texas since February 2025. As of early June, over 130 million sterile flies have been dispersed in Texas.
• Approximately 100 million sterile flies are being brought weekly from a facility in Panama.
• A new fly-breeding facility in Metapa, Mexico, is expected to begin operations in July 2025.
• A $750 million sterile fly production facility in Edinburg, Texas, is under construction, with an initial timeline for completion in fall 2027. Governor Greg Abbott has requested completion by May 2026.

Government Actions

• Texas Governor Greg Abbott expanded a statewide disaster declaration on June 6, authorizing use of all state resources and personnel, including from university systems, to accelerate response efforts.
• The Texas emergency command center operates 24/7 with specialists from Texas A&M University and state personnel.
• The CDC designated the outbreak a Level 3 public health emergency (the lowest level).
• The USDA temporarily relocated its Iowa lab team to Kerrville, Texas, and launched a $100 million innovation initiative.
• President Donald Trump appointed John Bellinger, a Texas A&M University Board of Regents member, as senior adviser for screwworm preparedness.
• The USDA appointed a military commander to accelerate resource gathering.

Border and Trade Restrictions

• The U.S. closed its southern border ports to live cattle imports from Mexico in May 2025.
• Mexico also closed its border to U.S. livestock in response.
• The USDA deployed sniffer dogs and Tick Rider patrols to monitor border crossings.

Economic Impact

The USDA estimates a widespread outbreak could cause $1.8 billion in annual damage to Texas's economy and potentially increase beef prices. The U.S. cattle industry is valued at $113 billion, with Texas as the top state at $17 billion. Officials state the screwworm does not infest meat, fruits, or vegetables and is unlikely to affect beef production directly.

Official Statements

USDA Secretary Brooke Rollins stated that the border closure delayed the fly's arrival in Texas by a year and criticized Mexico's internal animal movement controls. She said the administration has provided all needed resources and denied that staffing cuts affected the response.

Texas Agriculture Commissioner Sid Miller criticized the federal response as "slow, bureaucratic, and incomplete" and urged use of the Screwworm Adult Suppression System (SWASS), a pesticide-bait combination. The USDA stated SWASS uses chemicals that are carcinogenic and would also kill sterile flies, calling it environmentally problematic and no longer viable.

Texas State Veterinarian Bud Dinges urged animal owners to check their animals and respect quarantine zones.

Governor Greg Abbott described the condition as highly treatable if acted on immediately and stated, "We have beaten this before and we will do it again."

Guy Loneragan, dean of Texas Tech University School of Veterinary Medicine, stated that authorities have been preparing for over 18 months and emphasized that treatment options are available: "It's not a time to panic."

Ecologist Jeremy Radachowsky (Wildlife Conservation Society) called for joint government action to stop illicit cattle trafficking in Central America.

Guidance for Pet Owners and Livestock Keepers

The USDA and state authorities advise the following:

Symptoms in Animals

• Visible discomfort or pain at wound sites
• Discharge with a foul, putrid odor
• Biting or licking at wounds
• Restlessness, aggression, or lethargy

Treatment

Screwworm infestation is treatable if caught early. Treatment involves:

• Removal of visible larvae by a veterinarian
• Application of larvicides or insecticides
• FDA-approved antiparasitic medications (chewable tablets or topical solutions for dogs and cats)

Prevention

• Daily inspection of pets and livestock for wounds or body openings
• Use of flea, tick, and mite preventatives
• Fly control measures indoors and in animal housing
• Extra monitoring for animals near the U.S.-Mexico border or traveling to/from Latin America

Reporting

• Suspected cases: Contact a local veterinarian and the Texas Animal Health Commission
• Stray animals with symptoms: Contact local animal control
• Wildlife with symptoms: Report to Texas Parks and Wildlife Department or Texas Animal Health Commission; do not approach within 25 yards
• A 24-hour screwworm hotline and a website with a map of reported cases have been established

Important: Screwworm infestation is not contagious between animals through direct contact, but multiple animals with open wounds in the same area require additional monitoring.