A flesh-eating New World screwworm was confirmed in a calf in Texas this week, news that could seriously damage the U.S. livestock industry and raise already high beef prices.

The parasitic larvae of the screwworm fly typically enter an animal through an open wound and feed on their flesh, eventually killing the animal if left untreated, according to the U.S. Department of Agriculture. It is rare for people to contract screwworm, and they do not pose any food safety issues.

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Weather and climate have kept screwworms from entering the U.S., but climate change is extending its range northward, according to the National Institutes of Health. Warmer climates would also increase cattle tick populations and their range, which would also increase the outbreak potential for screwworm.

The parasite's presence in the U.S. could be the result of a warming climate. Screwworms are cold-sensitive — they can't survive unless low temperatures are regularly above freezing, year-round, according to Drovers, the oldest livestock publication in the U.S. Ideal conditions for adult flies are temperatures between 77 and 86, with relative humidity between 30-70%.

If the pest spreads through U.S. livestock, it could cause up to $1.8 billion in economic losses and further reduce the U.S. cattle herd, which is already at its lowest level in 75 years, Reuters reported. Infestations can be cured, but they require removing larvae from wounds and disinfecting them, which requires a lot of work, time and money.

The USDA announced Wednesday that it confirmed the presence of a screwworm in a calf in Zavala County, near the Texas-Mexico border. The agency sent a response team to the site and stopped movement of animals in the area. The USDA already releases 4 million sterile screwworm flies in the area weekly, and that number will be increased. That strategy helped eradicate screwworms during the last outbreak because the sterile male flies mated with wild females, which produced infertile eggs.

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"Protecting our livestock industry is a national security issue of the utmost importance, and USDA is wasting no time in taking action,” said Dudley Hoskins, Under Secretary for Marketing and Regulatory Programs. “USDA invested heavily in the tools needed to eliminate (New World screwworm) ever since cases started increasing in Central America and Mexico. The United States has defeated this pest before, and we will do it again.”

The U.S. government has kept the Mexican border closed to livestock imports for over a year to keep screwworms from reaching the country. Mexico has confirmed more than 27,000 cases since November 2024, including more than 2,000 active cases, according to Reuters. The last outbreak in the U.S., in the 1960s, devastated the local wildlife and cost ranchers millions of dollars.

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