New World Screwworm Fly in U.S.

The United States Department of Agriculture (USDA) recently confirmed that New World Screwworm (NWS) has been found in a calf (bovine) in Zavala County, Texas.

NWS is a species of parasitic fly. USDA’s Animal and Plant Health Inspection Service (APHIS) explained that adult flies mate, and the female lays eggs on a wound or body opening. Eggs hatch into larvae (maggots) that burrow into the wound and feed on the living tissue of warm-blooded animals. After about 7 days of feeding, larvae drop to the ground, burrow into the soil, and pupate. An adult screwworm fly emerges from the soil after 7-54 days, depending on temperature and humidity. Female flies mate after 3 days, while males can mate within 24 hours of maturation, and the cycle continues.

USDA said that NWS was eradicated from the United States decades ago. NWS is endemic in Cuba, the Dominican Republic, and countries in South America. Using sterile insect technique, NWS was eradicated from the United States in 1966, and a small outbreak from the Florida Keys in 2017 was successfully eliminated. In recent years, NWS has moved northward through Central America and Mexico and was confirmed in Texas on June 3, 2026.

USDA-APHIS stated, “New World Screwworm (NWS, Cochliomyia hominivorax) is a serious pest that affects livestock, pets, wildlife, and less commonly, people and birds.” They said that adult screwworm flies are about the size of a common housefly or slightly larger.

The presence of this parasitic fly presents a great concern to livestock producers, particularly those in southern Texas. USDA reported that the pest can easily become deadly for wildlife and livestock as these open wounds fester and grow.

“USDA maintains intensive surveillance along the U.S.-Mexico border to detect screwworm and ensuring rapid response if it enters the United States,” stated USDA-APHIS.

USDA-APHIS further explained that NWS would not spread rapidly in the United States. “Screwworms thrive in tropical and subtropical climates,” they said. “They do not tolerate prolonged periods of very dry hot or very cold weather.” They said that rapid spread in a country is mainly due to humans moving infested animals over large distances.

While human infestation is uncommon, the U.S. Centers for Disease Control (CDC) stated, “If you travel to these areas [where flies are present], have an open wound and spend a lot of time outdoors, you may be at greater risk of becoming infested with NWS.”

The USDA is currently taking several actions to slow and halt the spread of New World screwworm, including the release of sterile flies, intense surveillance, and trapping.

USDA-APHIS stated that the U.S. food supply is safe. Screwworms do not infest meat, fruits, vegetables, or other food sources. USDA’s Food Safety and Inspection Service (FSIS) ensures that the nation’s commercial supply of meat, poultry, and egg products is safe and properly labeled. Under the Federal Meat Inspection Act (FMIA), FSIS inspection personnel must inspect all eligible animal species unless they are exempt or covered by a state inspection program.

Any evidence of screwworm infestation in an animal would be identified during these inspections, and any contaminated product from an affected animal would not be allowed to enter the food supply.

For more information, see screwworm.gov or https://www.cdc.gov/new-world-screwworm/about/index.html.