“Not if, but when.” New World screwworm detected in Zavala County, Texas, calves

On June 3, 2026, the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) confirmed the detection of a New World screwworm (NWS) in the umbilical area of a three-week-old calf in Zavala County, Texas. This is the first case of NWS infestation in the U.S. since the pest crossed the sterile fly barrier zone in Panama and began spreading northward into Central America and Mexico in 2023. According to the Texas Animal Health Commission, the NWS case was confirmed by the National Veterinary Services Laboratory (NVSL) in Ames, Iowa.

On June 5, a sample collected from a 30-day-old calf in Zavala County was also confirmed to be NWS. This second confirmed case is located within the existing infested zone, where animal health officials are actively working to identify any additional flies in the area.

NWS is a serious pest affecting warm blooded animals, including livestock, pets, and wildlife. Humans and birds can also be affected by screwworm. Unlike other fly larvae, the maggots of NWS burrow into the flesh of living animals. An untreated screwworm infestation will always result in the death of the animal.

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On June 5, USDA reported that an APHIS strike team is already on-site in Texas. This team includes veterinary medical officers and animal health technicians – experienced experts who serve as USDA’s boots on the ground response force. APHIS is fully positioned to surge additional trained personnel as needed. 

Additionally, NVSL has deployed an entomologist to the USDA Agricultural Research Service laboratory in Kerrville, Texas, to expedite confirmation processes by positioning NVSL diagnostic assets in the area, a USDA press release stated.

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“APHIS has also deployed two dedicated NWS response cargo trailers. Each 24-foot trailer serves as a mobile preparedness and field operations center. Once unloaded, the trailers convert into operational and laboratory space, providing USDA personnel with the equipment and environment necessary to mount a rapid, efficient response,” said the release.

“The National Veterinary Stockpile is actively procuring additional tools to treat and prevent NWS. USDA is working with federal and regulatory partners, and within federal purchasing processes, to secure more treatment options that are flexible, safe, and effective.

“The National Veterinary Stockpile has relocated NWS treatment supplies to Texas to better support affected producers in the infested zone. If you or anyone you know needs access to these treatment supplies, they are now available and can be obtained by reaching out directly to the Texas Animal Health Commission,” according to the USDA.

USDA reported that the release of sterile flies began over the area where NWS was detected on June 4. These missions will disperse two million sterile screwworms twice a week.

“In addition to aerial releases, USDA is shipping another four million sterile flies per week to Texas,” stated the press release. “These will be deployed using 24 ground release chambers, strategically placed in and around the detection zone to maximize impact.”

Boots on the ground

Susan Storey Rubio is a seventh generation Texas rancher. She recalls riding on her grandmother’s cattle in the 1970s when she was a child and teenager to check for screwworm infestations and animals that required doctoring.

“I am the fifth generation living and working on this ranch in LaSalle County, Texas. We’ve had it in the family for over 100 years,” she said. “The old medication we used was called  ‘Smear 62’ and it was the worst smelling stuff. Of course, an animal infested with screwworm would already smell terrible because of the infection and the rotting flesh.”

“I grew up spending most of my time on my mother’s mother’s ranch in Zavala County, not far from here. [Today’s] confirmed screwworm case was in that county.”

Rubio suspects that recent storms with 60-70 mile per hour winds helped push the NWS flies north.

Rubio has heard claims that screwworm will only move on cattle, which are untrue, she said.

“How does a fly get 45 miles across the border with no bootlegged Mexican cattle?” she asked. “It has wings. It’s a fly, and flies will fly.”

In the past, screwworm was identified as far north as Canada, and the last case of NWS in Texas was confirmed in 1981, Rubio said.

The Texas Animal Health Commision veterinarians, along with the entire TAHC team, are providing support to Texas ranchers, Rubio said.

“We have an excellent TAHC vet in my region; the whole team is good. This particular case was handled out of a different region, but it was handled timely. Instead of a couple of days, we were given the notice of confirmation in a couple of hours. It was given priority and sent to the lab on a private plane.”

While every rancher does things a little differently, Rubio said they would pull bulls to only calve in December, January and February for the foreseeable future, rather than run bulls year round with their cows.

“I remember my grandparents used to be calving in January and February, and I thought it was so cruel because it was so cold,” she said. “This was done by design because of screwworm.”

This past winter was so temperate it didn’t even slow the rattlesnakes down much, Rubio said.

As per a conference call on June 4, Rubio said USDA was planning to start dropping two million flies twice a week on the “hot zone” where the NWS case was located.

“Last week they were dropping 50 million sterile flies per week in the area where they projected screwworm to hit,” she said. “Now they’re dropping four million sterile flies per week where it is.”

TAHC is providing veterinary assistance and any medicines and supplies producers need, but USDA has not provided any subsidies for Texas ranchers preparing to deal with screwworm, Rubio said.

“We’re all responsible for ourselves. We stocked up on recommended medicines and preventions. We’re fighting the fight for the government.”

NWS is not just a cattle problem. Rubio said nothing is being done for wildlife.

“You can’t gather them and can’t doctor them,” she said. “It’s prohibited to feed medicated feed to wildlife because it is dosed by weight. All it does is make the fly immune to that medication anyway. We also have a fever tick problem, which ivermectin is used for. Cattle fever ticks can also become immune to it, then we would have two huge problems.”

More sterile flies are needed to protect wildlife populations, Rubio believes. Some people are looking at putting up feeding stations with approved medications that would rub onto the necks of deer that come to feed, but medicating wildlife against NWS is not a simple solution to a complex problem.

“Wildlife metabolizes things differently than livestock,” Rubio said. “Medication can easily get lost in the hair coat of a whitetail deer, and ivermectin doesn’t metabolize as in them as it does with cattle, horses, sheep or goats.”

Any warm-blooded animal or bird is susceptible to NWS infestations. Along with her concern for the livestock industry, Rubio is apprehensive for the impact NWS will likely have on several endangered and watch listed species in Texas, including the Texas Big Horn Sheep.

“A screwworm can detect the bite of a mosquito, a bee sting or a tick bite,” Rubio said. “It will find it and lay up to 500 eggs, sometimes several females will lay eggs in the same spot, and within 10-12 hours the larvae will hatch and begin feeding.”

Rubio is fighting “tooth and nail” to get approval from the USDA for M3 Agricultural Technologies sterile insect production methods for aiding in the production of sterile NWS flies. The ag technology company, based out of Phoenix, Arizona, has used X-ray technology to sterilize other insects, and has the capability to use the same technique on NWS larvae. Rubio noted that during the previous outbreak of NWS in the United States, X-ray sterilization was proven to work, although at the time it was not cost effective compared to gamma ray sterilization.

“X-ray is now cheaper and just as effective,” she said. “I toured the M3 company and saw their machines and they damn sure know what they are doing. They are CDC approved and the Department of Energy is backing them and saying they have a viable concept. The Departments of Health and Human Services and Homeland Security are also favorable.”

Rubio would like to see USDA funding provided to M3 Technology so they can produce sterile NWS flies to help combat their northward movement. She described how a group of people privately funded the original sterile fly production facility at Moore Field, now Moore Air Base, at Edinburgh, Texas.

“A group of ranchers put their money together and got it built in 120 days,” she said. “I have a group of land owners and people who are concerned and donors who want to fund M3’s technology. If the government will not fund it, we will fund it, just like in the 50s. Texans are going to take care of Texas, and we do it best.”

The petition can be viewed here: https://na4.documents.adobe.com/public/esignWidget?wid=CBFCIBAA3AAABLblqZhDIcXhck_OheNFYu3slym2d8acMVQOMTGP0wxVG1MwBpkPh46i_9BDygPrbnjMSCmA&fbclid=IwY2xjawSQaEhleHRuA2FlbQIxMABicmlkETFFVmJLaTh2aEFGek9wazczc3J0YwZhcHBfaWQQMjIyMDM5MTc4ODIwMDg5MgABHiAQs95cppAGZR3WNZh1qs7mCj7O2fXyhGrpUT6CcDflGJvtqr1QrOxTJvAI_aem_-bFnGBUlfempKEnyY_nkUw and anyone is welcome to sign.

As per the CDC, Rubio said 2,075 human cases of NWS infections have been reported in Mexico, along with 10 human deaths due to screwworm.

For the record, Rubio said Texans want the border to stay closed. The economic impact on the Texas livestock industry would be devastating. Rubio is aware of suspected cases of NWS in Frio County and Uvalde County, although they remained unverified as of press time.

“Assuming we get a full infestation of screwworm, I’m told we can expect a $20 billion loss,” she said.

The Texas Animal Health Commission will inspect livestock prior to movement out of the vicinity of the location where NWS was detected. For more information, see: https://www.tahc.texas.gov/regs/pdf/2026-06-3-NWS-Zone01.pdf

Life Cycle

Understanding screwworm control requires an understanding of the life cycle of the NWS fly. Male screwworm flies mate with many females in their lifespan. A female only mates once but retains the male’s sperm for fertilization of multiple batches of eggs. She can lay nearly 3,000 eggs in her lifespan, in batches of 200-500 at one time. The female finds a place where there is a wound or fresh blood to lay her eggs. The wound can be as insignificant as a tick bite. 

“They’re not looking for a cow pie or a rotten hay pile, they want blood,” Dr. Robert “Max” Thornsberry said. “The skin has got to be broken before they can enter the body, they can’t chew through skin. But it doesn’t take much, just a tiny scratch or poke. One adult female lays thousands of eggs and dies.”

Eggs are laid in shingle-like layers. When they hatch, the maggots start to feed.

“He has hooks on his mouth that allow him to dig into the bloody tissue and as he eats he goes deeper and deeper. The maggots breathe through their rear end so all their heads are buried in the wound; they don’t need air down there. When you look into a wound you will see all their butts sticking up and see the two nostrils on all the maggots.”

The NWS maggots feed for two to three weeks until they are around half an inch long. Then when they reach a certain point of maturity, they back out of the wound, drop on the ground and begin to pupate. The emergence depends on the season; in cold weather they can stay in the ground for months, but in tropical conditions they emerge very quickly, within a few weeks.

“They build a hard black or brown shell around themselves called a pupa. You’ve seen these around your manure pile; they look the same as a blow fly or house fly. They crawl down in sand or dirt to pupate, and a metamorphosis takes place just like a butterfly.

When the pupa has developed into a fly, it breaks open, the fly comes out, and once its wings are dry, “they go where they want to go,” Dr. Thornsberry said.

“Their life cycle is not very long, just a few weeks, but a female will lay thousands of eggs in that length of time. Every generation repeats the same thing. The only way we have found to stop that life cycle is to sterilize the spermatozoa in the male fly using radiation. When he breeds, and the sterile sperm contact the female’s eggs, she can lay eggs but they will never develop.”

The system to develop sterile flies is well established, although complex and difficult. Maggots must be developed, given sand to burrow into, and pupa are sifted from the sand and exposed to radiation. The sterilized flies are then dropped from airplanes in boxes to release the sterile male flies.

“People don’t realize how many million sterile females are released along with the males. The sterile females aren’t going to do anything, but you want the males to go breed with every female he can find,” Dr. Thornsberry said. “It’s a matter of mathematics. The more you release, the better the odds.”

These flies are very adaptable, and although historically they were not active after a frost, they survive the winter in the ground as other flies do.

“It is almost incomprehensible when you realize how many billions of sterile flies need to be dropped,” Dr. Thornsberry said. “It is a complex issue to develop them. It takes tons and tons of raw meat to develop the pupae to the point when you can put them under radiation to sterilize them. Right now they are just hot spotting where they have had outbreaks; imagine trying to cover the entire geographical area of Mexico with sterile flies.”

Food safety

South Dakota state veterinarian Dr. Mendel Miller told the South Dakota Cattlemen’s Association: “New World screwworm is not a food safety problem. The normal precautions that are in place every day are there. Animals are inspected prior to slaughter. If there was a problem or something was found, that animal is diverted out and never enters the food chain. This larvae is so unique: it feeds on live tissue, so if it came in contact with meat somewhere down the line, it’s not going to lay eggs on that meat like a typical fly might because it’s not live tissue. It also doesn’t lay eggs on fruits and vegetables, so not only on the meat side, do we have a high degree of confidence it is safe, but all food is safe.”

Industry response

Texas Animal Health Commission is working closely with USDA and other state officials in a joint incident response to the confirmation of NWS in a Zavala County calf.  An infested zone has been established in the area, and animal movement restrictions are in effect to prevent further spread of the pest. The agency is working in coordination with the USDA to expand fly trapping and surveillance efforts to identify any additional flies in the area and investigate potential suspect cases in animals. Epidemiological efforts are underway to learn more about this case and prevent spread of the pest.

NWS are larvae or maggots of the NWS fly (Cochliomyia hominivorax), that cause the painful condition NWS myiasis. NWS flies lay eggs in open wounds or orifices of live tissue. These eggs hatch into dangerous parasitic larvae, or maggots, which burrow or “screw” into flesh with sharp mouth hooks. NWS primarily infest livestock, but can also affect any warm-blooded animal, including wildlife, pets, humans, and birds.  

“TAHC has been actively preparing for a resurgence of NWS for over two years,” said Dr. Bud Dinges, TAHC state veterinarian and executive director. “We are putting these preparations into action, and we encourage all animal owners and caretakers to continue to remain vigilant for the presence of larvae in animal wounds and immediately report any suspicions.” 

Targeted releases of sterile NWS flies are being expedited to eliminate any reproducing populations in the area associated with the detection. When combined with enhanced surveillance, movement restrictions, prompt treatment of wounds, and education and outreach efforts, the sterile insect technique is a proven and effective tool for controlling and eradicating this pest. 

In addition to frequently monitoring animals, producers should implement enhanced management practices including keeping open wounds clean and covered. Animals infested with NWS should be immediately treated with an appropriate method under the direction of a veterinarian. If you suspect NWS in your livestock or domestic animals, contact the TAHC immediately and do not move your animals. A representative will advise you on current collection protocols for submitting suspicious maggots. Contact the Texas Parks and Wildlife Department (TPWD) for wildlife infestation information and the Texas Department of State Health Services (DSHS) for human infestation information. 

Learn more about New World screwworm at screwworm.gov and get current Texas updates and resources at: https://www.tahc.texas.gov/emergency/nws.html.

R-CALF USA CEO Bill Bullard issued the following statement in response:

“This confirmation marks a serious moment for American livestock producers and requires an immediate, coordinated response to protect animal health, the national cattle herd, America’s livestock producers, and our nation’s food supply.

“We now switch from prevention to eradication. Producers need to be intensely vigilant in monitoring their livestock and reporting suspected cases to their veterinarian. We must push this pest back to Mexico and then continue efforts to push it back to Central America.”

National Cattlemen’s Beef Association (NCBA) CEO Colin Woodall responded to the announcement from the U.S. Department of Agriculture (USDA) that one case of New World Screwworm has been detected in a calf in South Texas:

 “NCBA and our state partner organizations have been working for more than a year to combat the incursion of the New World Screwworm, and we are doing everything possible to protect the interests of American cattle producers. We appreciate USDA and other agencies for working rapidly to bolster critical resources that protect producers, including building a domestic sterile fly facility, increasing fly surveillance, and expediting the availability of necessary drugs and treatments for cattle. We defeated this harmful pest before in the 1960s by investing heavily in sterile fly dispersal and NCBA will continue using every tool in the toolbox to ensure Screwworm is eradicated in the U.S. once again.”

Texas & Southwestern Cattle Raisers Association President Stephen Diebel today responded to a confirmed case of New World screwworm detected in an umbilical cord lesion on a three-week-old calf in Zavala County, Texas:

“This is a serious animal health threat, particularly for Texas, but it is not a food safety issue and ranchers and landowners should not panic. For the past 18 months, we have worked alongside our state and federal partners to prepare for the possibility of a positive New World screwworm case, and that preparation now positions us to respond quickly and effectively.

“Texas & Southwestern Cattle Raisers Association has worked alongside the U.S. Department of Agriculture (USDA), Texas Animal Health Commission (TAHC) and Texas Parks and Wildlife Department (TPWD) to increase surveillance, sterile fly production and dispersal capacity. We have identification, treatment and reporting protocols in place. We are exceptionally grateful for the leadership and response of our partners on this issue.

“Texas ranchers and landowners may be on the front lines of this challenge, but they are not facing it alone. TSCRA continues to provide information and resources and ensure there is a voice speaking on their behalf and ensuring their interests are represented.

“We ask ranchers and landowners to regularly put eyes on their livestock and wildlife, maintain a strong relationship with their veterinarian and report suspicious wounds or infestations immediately. Early detection, rapid response and limiting animal movement when necessary remain our strongest tools for containing and eradicating New World screwworm.”

The U.S. Cattlemen’s Association (USCA) is closely monitoring the announcement from the U.S. Department of Agriculture regarding the suspected detection of New World Screwworm (NWS) in Texas and the ongoing confirmatory testing now underway at the National Veterinary Services Laboratories in Ames, Iowa.

USCA commends USDA’s Animal and Plant Health Inspection Service (APHIS) and the Texas Animal Health Commission (TAHC) for their swift response to this potential detection and for their extensive work on-the-ground to hold off the reemergence of New World Screwworm through 2025 and into the first half of 2026. 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 welcomes USDA Secretary Rollins’ commitment that the Department will remain in constant communication with industry stakeholders as this situation develops. Timely, transparent updates from USDA, APHIS, and TAHC are vital to ensure that cattle producers have the information they need to safeguard their herds, implement any recommended biosecurity measures, and coordinate effectively with animal health officials.

USCA’s Animal Health Committee is actively engaged on this issue and is in close contact with federal and state animal health officials as the situation evolves. USCA’s priority remains the protection of animal health, the integrity of the U.S. cattle herd, and the long-term viability of domestic livestock production.