Return of New World Screwworm

New World screwworm has returned to the United States, and the concern among wildlife managers and livestock owners is immense. For wild sheep conservationists, it immediately raised alarms because at the time of this writing, confirmed detections in Texas reached Terrell County, only one county removed from Elephant Mountain Wildlife Management Area, Black Gap Wildlife Management Area and the heart of decades of desert bighorn restoration work. Although no cases have been documented in wild sheep, the parasite is now closer to some of the continent's most important desert bighorn restoration areas than it has been in decades, prompting heightened attention among wildlife biologists and conservation organizations.

The Wild Sheep Foundation (WSF) was among the first wildlife conservation organizations to publicly address the screwworm issue. 

In 2025, WSF voiced support for expanded efforts to contain New World screwworm, recognizing that while much of the public discussion centered on cattle, it also represents a wildlife conservation issue. Its chapters and affiliates have also been following developments as New World screwworm has returned to the United States.

"I am cautiously optimistic for our bighorn herds," said Clay Brewer, President of the Texas Bighorn Society.  Brewer noted that while New World screwworm has historically affected the Southwest, it has generally been more persistent in less arid regions than the Trans-Pecos. Even so, he emphasized that no one is taking the situation lightly and that herds are being monitored as action is taken on several fronts. At the same time, Brewer said he is immediately concerned about whitetails and other wildlife species across much of the rest of Texas.

State wildlife officials also recognize the broad implications for wildlife.

“New World screwworm can have devastating impacts on free-ranging wildlife populations and rural communities," said Dr. David Yoskowitz, Executive Director of the Texas Parks and Wildlife Department (TPWD). "TPWD, in partnership with our state and federal partners, will work diligently to respond to suspected cases, and to urge the public to monitor and report signs of an infestation in warm-blooded animals.

That concern has already influenced wildlife agencies beyond Texas. A confirmed New World screwworm infestation in a dog in Lea County, New Mexico, prompted wildlife officials there to accelerate preparations sooner than expected.

"We thought we'd have a few weeks from the time it would be confirmed in Texas until it reached New Mexico, which clearly wasn't the case," said Katie Piecora, bighorn sheep biologist with the New Mexico Department of Wildlife.

Although New Mexico's nearest desert bighorn herd lies roughly 170 miles west of Lea County near Alamogordo, Piecora said elk, deer, pronghorn and other wildlife closer to the detection currently face the greatest concern. Even so, she emphasized that bighorn sheep will receive close attention. "We'll be monitoring the bighorns closely," she said.

A young of the year New Mexico Rocky Mountain bighorn scales a foothill near Taos, New Mexico. A single case of screwworm has been observed in a dog in New Mexico but so far it has only been found more than 100 miles from the nearest bighorn herd. State wildlife officials are closely monitoring the situation. Photo by Chester Moore

Following recommendations from New Mexico State Wildlife Veterinarian Dr. Elin Crockett, wildlife personnel have increased surveillance by adding wound and maggot inspections to wildlife capture protocols while asking hunters and field staff to carefully examine harvested animals and report suspicious wounds or larvae for testing.

New Mexico's response illustrates the growing concern among wildlife agencies across the Southwest. Surveillance has increased and action plans are being updated, but many wildlife professionals acknowledge that significant challenges remain if New World screwworm continues advancing north before additional control capacity comes online.

To understand why wildlife managers are taking those precautions, it helps to understand what makes New World screwworm unlike almost every other fly in North America. Unlike most maggots, which feed on dead tissue, screwworm larvae attack living flesh.

"The New World screwworm fly feeds upon live flesh," said longtime biologist and whitetail expert Larry Weishuhn who dealt with the issue head-on early in his career.  "It lays eggs in an open sore or wound, and those eggs turn into flesh-eating larvae." Unlike ordinary maggots that help break down dead tissue, screwworm larvae burrow into healthy flesh. As they feed, wounds enlarge, infections worsen and animals can quickly become debilitated if they are not treated. "What happens is that the animal is eaten from the inside out," Weishuhn said.

Perhaps most troubling is the parasite's lack of discrimination.

"That screwworm larva eats on any warm-blooded animal, and that includes birds as well as mammals," Weishuhn said. "It could be everything from a whitetail to cattle to goats to bighorn sheep." Historical records show the parasite occurred well beyond South Texas, affecting livestock and wildlife across Arizona, New Mexico and other southern states. According to USDA historical records, livestock producers across the Southwest lost an estimated $50 million to $100 million annually before eradication efforts succeeded.

Wildlife paid a price as well.

One of the parasite's most devastating impacts occurred among newborn animals. Healing umbilical cords provided ideal sites for female flies to lay eggs, allowing larvae to burrow into living tissue only hours after birth.

"We really did not have any whitetail deer to speak of in a lot of parts of Texas and elsewhere because there was such a small amount of reproduction each year or survival of that reproduction," Weishuhn said.

Ironically, one reason so few Americans understand New World screwworm today is because the campaign to eliminate it worked so remarkably well.

In 1966, the United States was officially declared free of indigenous New World screwworm following one of the most successful insect-eradication programs in history. Researchers discovered that female screwworm flies typically mate only once during their lifetime, leading to development of the Sterile Insect Technique. Millions of laboratory-reared male flies were sterilized and released into the wild. When wild females mated with sterile males, no offspring were produced, causing screwworm populations to collapse.

Dr. Roel Lopez of Texas A&M University describes the strategy as creating a biological barrier. "The sterile fly technique basically created a biological wall," Lopez said. "For the last 50, 60 years, there's been a biological wall of flies."

For many Americans, the story seemed over. It wasn't.

Just six years after the United States declared victory, New World screwworm returned. According to USDA officials, screwworm-infested livestock were confirmed in California, Arizona, New Mexico, Texas, Oklahoma and Arkansas during the 1972 outbreak after flies crossed back into the United States from Mexico. Texas alone recorded approximately 90,000 confirmed cases, a dramatic reminder that declaring eradication within U.S. borders did not mean the threat had disappeared.

The 1972 outbreak reinforced an important lesson. The parasite had not disappeared. It had been pushed beyond America's borders, where continued surveillance and rapid response were required to keep it from regaining a foothold. Additional incursions followed during the next decade, and Texas continued dealing with infestations until 1982. That hard-earned lesson would become even more relevant more than three decades later.