When an Old Threat Returns

By Ann Asher

Then there are the diseases and pests that younger generations have only read about.

For decades, New World screwworm belonged in that second category. Older ranchers remembered the days when a simple branding wound or a newborn calf’s navel had to be watched closely because it could attract one of the livestock industry’s most destructive pests. Veterinarians still taught students how to recognize it, but for many producers, screwworm was something that had been conquered long before they ever worked their first set of cattle.

This summer, that changed. On June 3, 2026, the U.S. Department of Agriculture confirmed New World screwworm in a calf in South Texas, marking the parasite’s return to the United States after decades of successful eradication. While Oklahoma has not reported any confirmed cases, the discovery has ranchers across the Southern Plains paying close attention. The concern isn’t just where screwworm is today, but where it could be tomorrow.

Unlike the common maggots that feed on dead tissue, New World screwworm larvae feed on living flesh. Adult female flies are drawn to fresh wounds and natural body openings, laying hundreds of eggs at a time. Within hours, the larvae hatch and begin burrowing into healthy tissue, enlarging the wound as they feed. Left untreated, an infestation can become life-threatening.

There are some livestock problems that today’s ranchers know by heart. Pinkeye. Foot rot. Horn flies. Internal parasites. Most producers have dealt with them at one time or another.

The list of potential hosts is long. Cattle, horses, sheep, goats, pigs, dogs, wildlife and, in rare cases, people can all become infested. Even relatively small injuries—a wire cut, a tick bite, a branding wound, a castration site, a dehorning wound or a newborn calf’s healing navel—can provide an opportunity for the fly.

Fortunately, today’s producers have one advantage their grandparents often did not: they know what they’re looking for.

A wound that continues to grow instead of heal should never be ignored. Bloody or watery discharge, a strong foul odor, swelling, pain, visible larvae and unusual behavior such as excessive licking, head shaking or isolation from the herd all warrant a closer look. While these signs can indicate several different conditions, they should also prompt producers to consider the possibility of screwworm and contact their veterinarian immediately.

The return of screwworm has also reminded many people of one of agriculture’s greatest success stories.

During the mid-1900s, New World screwworm cost American livestock producers millions of dollars each year. Rather than relying solely on insecticides, scientists developed what became known as the Sterile Insect Technique. Millions of sterile male flies were raised and released into affected areas. Female screwworm flies mate only once during their lifetime. When they mated with sterile males, no offspring were produced. As generation after generation failed to reproduce, screwworm populations steadily declined until the pest was eliminated from the United States and later pushed farther south through Mexico and much of Central America.

That same strategy is once again leading the response.

Federal and state animal health officials have increased surveillance in affected areas, established movement controls where necessary and resumed sterile fly releases to prevent the pest from becoming established. Veterinarians, livestock markets and producers are all part of the early detection system, with every suspicious case helping officials determine where additional monitoring may be needed.

For Oklahoma producers, the message is one of awareness rather than alarm.

As of this writing, no cases have been confirmed in Oklahoma. Even so, Oklahoma shares a long border with Texas, and livestock routinely move throughout the region for sales, breeding, shows and grazing. That makes vigilance especially important during fly season.

Routine herd checks have always been one of the best management tools available, and they are even more valuable now. Fresh wounds should be treated promptly and monitored until they heal. Newly branded or processed cattle deserve extra attention, as do newborn calves, horses with injuries, sheep after shearing and any animal recovering from surgery or trauma. Wildlife can also serve as hosts, making complete prevention impossible, but early detection can make all the difference.

If producers suspect screwworm, they should avoid simply treating the wound and moving on. Instead, they should contact their veterinarian or the Oklahoma Department of Agriculture, Food and Forestry so the case can be evaluated and, if necessary, samples collected. Rapid reporting is one of the most effective tools available for limiting the spread of the pest.

Perhaps the most encouraging part of this story is that agriculture has faced this challenge before—and won.

The same science that eliminated New World screwworm from the United States decades ago is once again being deployed. Researchers understand the insect’s life cycle. Veterinarians know what to look for. Producers are better connected than ever through Extension services and state animal health agencies.

Screwworm may have returned to the United States, but so have the tools that defeated it the first time.

For Oklahoma livestock producers, the best response is also the simplest: keep an eye on your animals, don’t ignore wounds that aren’t healing, and report anything that doesn’t look right. When it comes to screwworm, finding one case early is far better than finding many too late.