Sonora confirms first case of New World screwworm parasite

The Mexican state of Sonora, south of Arizona, has confirmed its first case of a flesh-eating parasite in a cow just days before the border is set to reopen to cattle.

The United States has barred cattle from Mexico from crossing the border for more than a year to protect against the spread of the New World screwworm parasite.

Now, Mexico has confirmed the first case of the parasite in the state of Sonora more than a year and a half after the screwworm first entered Mexico in late 2024. The case was reported in a cow in the southern region of Sonora, about 10 miles from the border with the Mexican state of Chihuahua.

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“From the first moment we activated the entire international force to attend to this case,” Juan González Alvarado, Sonora’s state agriculture minister, said in a statement. “The priority is to contain it, avoid spread and work until it’s eradicated, with the participation and coordination of authorities and producers.”

The border is set to reopen to cattle on Monday, starting with the Agua Prieta-Douglas crossing from Sonora into Arizona.

The U.S. Department of Agriculture did not immediately respond to a question about whether the reopening would still take place as scheduled. Texas has confirmed more than 40 cases of New World screwworm since the parasite first appeared in the United States earlier this summer.

The state of Sonora said that 360,000 sterile New World screwworm flies had been released in the area around the state’s first case. The USDA, as well as its counterpart in Mexico, considers sterile fly releases the most effective method for eradicating the parasite.