When an Old Threat Returns

By Ann Asher

Today’s ranchers know some livestock problems by heart: pinkeye, foot rot, horn flies and internal parasites. Most producers have dealt with each of them at one time or another.

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 remember 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 a problem their grandparents had conquered long before they 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. Oklahoma has not reported any confirmed cases, but ranchers across the Southern Plains are watching closely and wondering where the pest could turn up next.

Common maggots feed on dead tissue. New World screwworm larvae feed on living flesh. Adult female flies are drawn to fresh wounds and natural body openings, where they lay hundreds of eggs at a time. Within hours, the larvae hatch and burrow into healthy tissue, enlarging the wound as they feed. Left untreated, an infestation can kill the animal.

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

Today’s producers have one advantage their grandparents often did not: they know what they are looking for.

Never ignore a wound that keeps growing instead of healing. 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. These signs can point to several different conditions, but they should also prompt producers to consider screwworm and call their veterinarian right away.

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. Instead of relying on insecticides alone, scientists developed what became known as the Sterile Insect Technique. They raised millions of sterile male flies and released them into affected areas. Female screwworm flies mate only once in their lifetime, so a female that mated with a sterile male produced no offspring. As generation after generation failed to reproduce, screwworm populations declined until the pest was eliminated from the United States. Officials later pushed it farther south through Mexico and much of Central America.

Officials are using that same strategy today.

Federal and state animal health officials have increased surveillance in affected areas, set up movement controls where needed and resumed sterile fly releases to keep the pest from becoming established. Veterinarians, livestock markets and producers all serve as part of the early detection system, and each suspicious case they report helps officials decide where more monitoring is needed.

For Oklahoma producers, awareness matters more than alarm.

As of this writing, no cases have been confirmed in Oklahoma. Even so, Oklahoma shares a long border with Texas, and livestock 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. Treat fresh wounds promptly and monitor them until they heal. Newly branded or processed cattle deserve extra attention, as do newborn calves, injured horses, freshly sheared sheep and any animal recovering from surgery or trauma. Because wildlife can also carry screwworm, no producer can prevent every case, but early detection can limit the damage.

If you suspect screwworm, do not treat the wound and move on. Contact your veterinarian or the Oklahoma Department of Agriculture, Food and Forestry so they can evaluate the case and collect samples if needed. Rapid reporting is one of the most effective ways to limit the spread of the pest.

Agriculture has beaten this pest before. Officials are deploying the same science that eliminated New World screwworm from the United States decades ago. Researchers understand the insect’s life cycle, veterinarians know what to look for, and producers have closer ties than ever to Extension services and state animal health agencies.

Screwworm has returned to the United States, and 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. Finding one case early is far better than finding many too late.