Flesh-eating screwworm is confirmed in the U.S., officials say

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A New World screwworm has been detected in a 3-week-old calf in Zavala County, Texas, the U.S. Department of Agriculture's Animal and Plant Health Inspection Service said Wednesday.

The case is the first detection of the New World screwworm, which is a fly, in the United States since 2017.

New World screwworm larvae "burrow into the flesh of living animals, causing serious damage to livestock and economic losses," the USDA noted in a press statement about the detection in Texas.

Larvae were identified in the young calf's umbilical area, the statement said, adding that there are no further detections to date.

The New World screwworm, which is a fly, was eradicated in the United States in 1966, and a small outbreak in the Florida Keys was eliminated in 2017, according to the USDA. The NWS is endemic in Cuba, the Dominian Republic, and countries in South America, the agency said.

"USDA invested heavily in the tools needed to eliminate NWS 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 USDA said.

The USDA and Texas officials are working to contain and eradicate NWS from the country, including establishing a 20-kilometer (12-mile) infested zone around the detection and enforcing quarantines, movement controls, and surveillance in the area.

"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, undersecretary for marketing and regulatory programs at the USDA.

"USDA invested heavily in the tools needed to eliminate NWS ever since cases started increasing in Central America and Mexico," Hoskins said. "The United States has defeated this pest before, and we will do it again."

In December, the Food and Drug Administration granted conditional approval to the topical solution Exzolt Cattle-CA1, which is used to prevent and treat New World screwworm infestations, and which is produced by Merck Animal Health, a division of Merck & Co.

The price of Merck shares was up more than 3.5% in early trading on Thursday.

Sen. Ted Cruz, R-Texas, in a statement on X, said," The confirmed detection of New World screwworm in Texas is serious."

"My team and I are closely tracking the situation. USDA and State of Texas are taking immediate action to ensure our food supply remains safe, support our livestock producers, and eradicate the pest," Cruz said. "Texas has beaten this threat before and remains the backbone of America's cattle industry."

— CNBC's Ryan Ruggiero contributed to this article.