Breckenridge Texan

Veterinarian columnist says we can keep this threat away from people, pets and livestock.

Editor’s Note: This column originally appeared in The Dallas Morning News, where Glenn Rogers is a contributing columnist.

By Glenn Rogers

Evil looking, worm-like creatures literally screw and twist their way into live flesh, creating excruciating pain, damage, a putrid stench and even death.

This is not the plot to some B-rated horror movie, this is the real-life description of the New World Screwworm’s catastrophic effects on the bodies of warm-blooded animals, including humans.

Screwworms have been in the news lately. In August, a case of human screwworm infection was confirmed in Maryland in a patient who had recently traveled to El Salvador. And other factors indicate heightened risk of screwworms in U.S. animals, especially in Southern states as more cases emerge in Latin America.

History and science

Screwworms are not really worms, but the larvae of blue-green flies. After landing on a wound, these flies collectively lay thousands of eggs. The eggs hatch in 12 to 24 hours and out pop swarms of wormlike larvae (maggots) which burrow deep into a wound and surrounding tissue. After engorging on live flesh for up to a week, the maggots fall out of the wound, pupate in the soil and emerge as adult flies, seeking out their next victim. The entire cycle is concluded in around 21 days but is shorter in warm climates.

The scientific name for screwworm flies is Cochliomyia hominivorax, a name I still remember from veterinary school. In English, hominivorax translates to “man eating” or “consuming humans.”

Historical records for screwworms in the U.S. date back to 1825 and many accounts of devastating losses were reported throughout the 19th and 20th centuries. But an innovative solution arrived in the 1960s with the remarkable discovery and implementation of the Sterile Insect Technique (SIT).

SIT works because female screwworm flies mate only once. Scientists discovered they could sterilize thousands of male flies using radiation, then release them to mate with females.

Developed by Drs. Edward Knipling and Raymond Bushland, the SIT was the key to virtual screwworm eradication in the United States. Although brief flare-ups occurred in the 1970s, the United States has remained largely free of screwworms, saving the livestock industry billions. The eradication efforts continued south of the Mexican border and by 1997, screwworms were wiped out all the way to Panama. It’s one of the greatest success stories in the history of animal health.

A reemerging threat

Critical to the effort to control the spread of screwworms was the continuous release of sterile flies in the Darien Gap between Panama and Columbia. Unfortunately, this barrier was breached in 2022, and screwworm flies have been advancing toward the U.S. at an alarming rate, now only a few hundred miles from the border.

Texas is among the states at greatest risk due to warm temperatures and moist environments ideal for screwworm growth and expansion.

Adult screwworm flies are not long-distance travelers, and the most significant source of spread is through movement of larval-infested animals. Recently, the USDA suspended live cattle, bison and horse imports in response to the outbreaks in Mexico.

Since the mid-1960s, there have been several changes in human and animal population dynamics, workforce availability, institutional knowledge, and the potential for mass misinformation, which combine to present major obstacles should screwworms enter the United States.

The human population in the U.S. has nearly doubled since 1960, and with it, the susceptible pet population. The human population of the Rio Grande Valley in Texas has also increased dramatically. The American Veterinary Medical Association reports overall pet populations reached 163.5 million in 2024, a 45% increase from 1996.

There has also been growth in wildlife populations, particularly deer and feral swine, since the 1960s. Wildlife can serve as a major reservoir and spreader of screwworms, further complicating eradication efforts.

That’s not just a risk for livestock that may be infested, but for hunting and related industries. In Texas, the total economic contribution from deer hunting alone is about $9.6 billion annually (https://www.beaumontenterprise.com/txoutdoors/article/survey-shows-texas-whitetail-hunting-generates-18595586.php).

Unfortunately, there are currently no available drugs in the U.S. specifically approved by the FDA for the treatment of New World Screwworms in animals, though some unapproved products, including avermectins, have been effective. The FDA can grant emergency use authorizations for those products, and veterinarians are allowed to prescribe extra-label uses within FDA restrictions and under veterinary supervision.

The most important steps for prevention and treatment are close surveillance, early detection, removal of larvae, and wound management.

Humans are susceptible

Human cases are rare and most often a result of foreign travel to endemic areas where the flies infest a wound. But increasing rates of human migration, especially illegal migration that doesn’t come through orderly screenings, as well as rises in homelessness and drug addiction, could exacerbate the potential for screwworm infestation for those with inadequate access to health care and early wound treatment.

In my opinion, and after conversations with numerous livestock veterinarians, entomologists and state agriculture officials, it’s not a matter of if but when we start to see more cases.

The good news is that preparations are being made now. The USDA recently announced it will provide $750 million to build a sterile fly production facility at Moore Air Force Base in Edinburg. This facility will produce 300 million sterile flies weekly to augment the 60 million to 100 million produced weekly in Mexico and the current 110 million produced in Panama. Unofficial projections of two to three years for completion of the Edinburg facility may be too late to prevent re-introduction, but it will be welcome and this massive number will be required to push the flies back to Panama.

More good news is that Screwworms will not compromise food safety in the U.S. Every animal used for human consumption is inspected and no animals with screwworms will enter the food supply.

Screwworms can be re-eradicated with the tools we have available. Livestock producers, pet owners and all Texas citizens should commit to timely reporting of potential cases. Early detection can alleviate the spread, potentially saving lives.

To report a possible screwworm infestation, call the Texas Animal Health Commission hotline at 800-550-8242.

Glenn Rogers is a rancher and veterinarian in Palo Pinto County. He served in the Texas House of Representatives from 2021 to 2025, representing Stephens County. He is a Dallas Morning News contributing columnist.