Mexico’s ministry of agriculture on Sunday confirmed a case of New World screwworm among cattle in Nuevo León, a state in northern Mexico. It is the northernmost case of the livestock infestation, which was eradicated from the United States in the 1960s but has resurged in Mexico in recent months.
The case involved an 8-month-old cow that had been moved to a feedlot in Nuevo León from an outbreak-affected region in Mexico. Of the 100 cattle in the shipment, only one animal carried the parasitic infestation, according to a statement from the ministry of agriculture.
Sabinas Hidalgo, a small city in Nuevo León where the case was detected, is less than 70 miles away from Mexico’s border with the United States. Before Sunday, the northernmost case of screwworm in cattle was reported in July in the state of Veracruz, in eastern Mexico, 370 miles from the border.
In a social media post on Monday, Mexico’s secretary of agriculture, Julio Berdegué, said that the Mexican government was monitoring the case and carrying out measures agreed upon with the American government. He added that he had spoken on Sunday with Brooke Rollins, the secretary of the U.S. Department of Agriculture.
Mexican authorities caught the infestation early, they said. Screwworm flies lay eggs in open wounds; on hatching, the larvae burrow into wounds and feed on living flesh, causing extensive damage. After maturing, they drop to the ground, burrow into the soil and emerge as adult screwworm flies.
The cattle shipment from Veracruz was thoroughly inspected for wounds and maggots, and all the animals were treated with ivermectin, an antiparasitic drug, as a precaution, according to Mexican authorities.
Ms. Rollins called the case a “national security priority” in a statement released by the U.S.D.A. She said that the American government had given Mexican officials opportunities and resources to counter the parasite, but would continue to combat the parasite before it made its way to the United States.
As the outbreak has spread across Central America, the U.S.D.A. has relied on mass-producing sterile flies to contain the flies’ range. The agency spent millions of dollars ramping up the production of sterile flies in a breeding facility in Panama and increasing awareness and checkpoints in Central America. This year, the agency announced that it would invest nearly $30 million more to renovate a sterile fly production facility near the Mexico-Guatemala border and build a dispersal facility in Texas.
Since July, both governments have been actively monitoring thousands of fly traps in states along the border, and no flies have been detected so far.Authorities on both sides of the border have committed to investing in new tools to combat the parasite.
In her daily news conference on Monday, President Claudia Sheinbaum of Mexico said that a team from the U.S.D.A. had recently been in Mexico to look at measures being put in place by the Mexican government. The team found “that we’re doing everything in our power to prevent further contamination from the New World screwworm, and they are about to issue their ruling shortly,” she said.
Earlier this year, after the detection of cases in the Mexican states of Oaxaca and Veracruz, the U.S.D.A. closed southern ports of entry to livestock trade. In July, Ms. Sheinbaum said that the decision to halt cattle imports was “exaggerated,” after the detection of the single case in Veracruz. American ports remain closed to Mexican imports of cattle, bison and horses.
“The important thing here is that there was an agreement with the U.S. government: that the decision to open or close the border would not be discretionary, but would be based on a series of technical indicators,” Ms. Sheinbaum said on Monday. The team from U.S.D.A. that was recently in Mexico will give their ruling in a few days, she said.
Alexa Robles-Gil is a science reporter and a member of the 2025-26 Times Fellowship class, a program for journalists early in their careers.
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