New World screwworm confirmed in U.S.

Despite more than a year of heightened surveillance along the Southern Border and continued efforts to keep the New World screwworm (NWS) out, the devastating pest has officially returned to the U.S.

On June 3, federal officials confirmed the first case of NWS on the northern side of the U.S.-Mexico Border in a three-week-old calf in Zavala County, Texas, marking the first domestic detection in decades.

Since then, additional cases have been confirmed in Texas and New Mexico, bringing the total number of active cases in the U.S. to six as of June 10, according to the U.S. Department of Agriculture’s (USDA) official NWS website. 

These include a second infected calf confirmed about 5.6 miles away from the original detection; two cattle in La Salle County, Texas; a goat in Gillespie County, Texas and a dog in Lea County, N.M.

A destructive parasite

Scientifically known as Cochlimoyia hominivorax, NWS is a parasitic fly whose larvae feed on living flesh. 

Unlike other flies whose larvae consume dead or decaying tissue, screwworm larvae burrow into healthy tissue, creating deep wounds which can become life threatening if left untreated.

According to USDA, the pest can affect livestock, pets and wildlife and, in rare cases, even humans, and infestations are commonly found in fresh wounds, new brands, dehorning sites, newborn navels and natural body openings like the nose, ears and genitalia. 

Officials emphasize, while NWS is a serious animal health issue, it is not a food safety concern.

“The U.S. food supply remains safe,” USDA reiterates in multiple press releases issued since June 3. “Screwworms do not infest meat, fruits, vegetables or other food products. Any affected animal would be identified during USDA Food Safety and Inspection Service inspection, and no contaminated product would enter commerce.”

Preparing for arrival 

While recent detections in the U.S. have prompted an aggressive response, officials note this was not an unexpected development.

In a June 5 USDA press release, USDA Under Secretary for Marketing and Regulatory Programs Dudley Hoskins states, “Many models projected this pest would reach the U.S. last year, but thanks to USDA’s phenomenal work and our cooperation with state, federal and industry partners, we’ve held it off until now when we are far more prepared to respond. USDA has not wasted any time in this fight, we have defeated NWS before, and we will do it again.”

Over the past year, USDA prepared for the pest by expanding border surveillance efforts, deploying thousands of traps and increasing monitoring of livestock and wildlife populations.

Specifically, a June 9 Farm Progress article written by Policy Editor Joshua Baethge explains USDA deployed more than 8,000 surveillance traps and examined roughly 59,000 fly samples and more than 19,000 wildlife specimens since January 2025.

“In addition, USDA has increased mounted patrols and trapping efforts along the border,” Baethge adds. “Dogs have also been trained to detect flies in companion animals crossing the border.” 

Continued efforts underway 

Following the first U.S. detection on June 3, USDA and the Texas Animal Health Commission (TAHC) quickly implemented response measures outlined in the USDA NWS Response Playbook, which went online in October 2025.

This includes establishing a 20-kilometer infested zone around confirmed detections; implementing movement controls and quarantines; heightening surveillance efforts; expanding wildlife monitoring; increasing trapping along the border and outside of the dispersal zone and conducting targeted outreach to local producers, veterinarians and communities.

Additionally, a USDA Animal and Plant Health Inspection Service (APHIS) strike team was immediately deployed to Texas after the first cases were confirmed, and the agency positioned veterinary personnel, entomologists and mobile laboratory facilities in the region to accelerate testing and response efforts.

The department reports, “USDA and TAHC continue to lead an aggressive unified response, with a total of 75 people actively responding on the ground and hundreds of additional people around the country providing laboratory diagnostics, logistics, treatment distribution, air operations, outreach, operational planning and resource support for the response. APHIS and TAHC will continue to surge additional trained personnel as needed to ensure an effective response.”

USDA maintains the sterile insect technique (SIT) is still the most effective tool in combatting NWS.

Through SIT, male screwworm flies are bred, sterilized and released into affected areas. Females mate with sterile males, producing no offspring and causing populations to dwindle over time.

“As part of the established NWS eradication strategy, USDA began releasing sterile flies over the area on June 4. These missions will disperse two million sterile screwworms twice a week to disrupt the pest’s life cycle,” states a June 5 USDA press release. “In addition to aerial releases, USDA is shipping another four million sterile flies per week to Texas. These will be deployed using 24 ground release chambers, strategically placed in and around the detection zone to maximize impact.”

Currently, North America’s only sterile fly production facility is based in Panama and has the capacity to produce 100 million sterile flies per week.

As part of preventative measures taken last year, USDA invested $21 million to increase production at a fruit fly facility in Metapa, Mexico to an additional 60 to 100 million sterile flies per week, which is expected to begin later this month.

USDA also broke ground on a $750 million sterile fly production facility at Moore Air Base in Edinburg, Texas in April. The facility is forecast to be completed by the middle of 2027, and when complete, it is expected to produce another 100 million sterile flies per week.

In addition to SIT, officials say future innovations may further strengthen NWS eradication efforts. 

Researchers at the newly opened Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas are in the process of developing the “NovoFly” system, which would produce only sterile male flies, as the current process produces both males and females.

This could potentially double effective production without increasing facility size, USDA notes.

Industry reaction and impact

In the wake of domestic NWS detections, ag organizations have expressed support for the USDA’s quick response while encouraging the department to engage in continued communication with producers. 

The U.S. Cattlemen’s Association (USCA) commended USDA and TAHC for their vigorous preventative efforts leading up to the outbreak.

“Their early activation of personnel in the region; close coordination with state and local partners and continued focus on prevention, early detection and rapid response have bought producers and animal health officials critical time to prepare for a situation just like this,” USCA states. 

When it comes to economic impact, industry analysts including Patrick Linnell of CattleFax and Dr. Derrell Peel of Oklahoma State University agree the pest will likely create management challenges for producers in impacted areas, but they don’t expect to see widespread cattle loss thanks to preparedness efforts and available treatment options.

“The ‘good’ news in all of this is it is not a broader market issue,” Peel says. “NWS will affect relatively few animals and is not something that will affect cattle supply or beef production.”

“Additionally, no major trade impacts are warranted or expected relative to NWS,” Peel continues. “However, the reaction of trade partners is unpredictable. Canada has announced a temporary ban on livestock from Texas, which is seen as a largely symbolic political move as it will affect a very small volume of trade. Although there is significant bilateral cattle trade between Canada and the U.S., very few Texas cattle are typically involved. Canada is well north of the climate boundary for NWS and the threat is minimal.” 

Linnell points out movement restrictions are another factor producers should watch closely.

“Federal guidance currently adopts a highly localized approach, calling for a 72-hour hold on infested zones around cases with a 12.4-mile radius,” he explains. “States are encouraged to adopt federal guidelines but may adopt stricter strategies.”

Overall, he says, “It is frankly not clear whether the market impact in the U.S. is positive, negative or neutral. Either way, the safe bet is volatile, especially as the futures complex digests new and evolving information.”

“The immediate market reaction following the confirmation has been positive, aside from an initial sharp flush in the futures complex which lasted literally only two minutes,” he continues. “Fundamentally, NWS should not alter market dynamics from what is known today.”

Both analysts further emphasize NWS is not expected to significantly affect beef supplies or food safety. 

What producers need to know

For producers, animal health officials stress early detection is the most effective defense against NWS and encourage them to inspect livestock daily, especially newborn animals and those with recent wounds. 

Officials note fresh navels, brands, dehorning wounds and reproductive tissues should receive special attention.

According to USDA, common signs of NWS include enlarging wounds, draining lesions, maggots or egg masses, unusual animal behavior, signs of discomfort and lesions around body openings.

The department recommends promptly treating all wounds with approved insecticides, maintaining a strong parasite control program and working closely with a local veterinarian on prevention and treatment options.

USDA also reminds producers NWS is a federally reportable foreign animal disease, so anyone suspecting an infestation should immediately contact a veterinarian or state animal health official.

In Wyoming, suspected cases should be reported to the Wyoming Livestock Board at 307-777-7515 during business hours or 307-840-1389 after hours.

Moving forward, officials are confident surveillance and containment efforts will hold strong and that the U.S. has the tools and resilience to eradicate NWS once again.

 “Our producers, veterinarians and state officials are among the very best in the U.S. It is critical to stay vigilant and stay informed,” Texas Gov. Greg Abbott tells Drovers in a June 8 article. “We prevented and eradicated this pest before. We can do it again.”

Hannah Bugas is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.