Mexico reports second screwworm case in two days

Sterilized screwworm flies sit in a basket in Metapa de Domínguez, Mexico, Friday, Oct. 17, 2025. (AP Photo/Fernando Llano)

Mexican authorities reported a case of the New World screwworm parasite late on Thursday, the second case reported in two days as Mexico works to contain an outbreak that has kept the U.S.-Mexico border closed to Mexican livestock.

The parasite was detected and treated in a goat in the State of Mexico, which borders capital Mexico City, the ministry said, adding that the 20 other animals at the site tested negative and were given preventive treatment.

On Dec. 31, the ministry reported a case in a six-day-old cow calf in the northern state of Tamaulipas, also the only positive case on site.

Mexico has so far reported a total of 13,106 cases since November of 2024, according to government data through Dec. 31, 2025. Of those cases, 671 were active currently.

The southern border state of Chiapas has the most confirmed cases, followed by Oaxaca, Veracruz and Yucatan.

Screwworms are parasitic flies whose females lay eggs in wounds. Their larvae burrow through the flesh of animals, eventually killing their host if left untreated.

Mexico has been working to contain an outbreak of the screwworm, which has spread northward from Central America, rattling the livestock and beef industries of both the U.S. and Mexico.

Because of the outbreak, the U.S. has kept its southern border largely closed to Mexican livestock since May.

Although U.S. officials say the pest has not yet breached the U.S. border, it threatens US$1.8 billion in damage to Texas’ economy alone, according to a U.S. Department of Agriculture estimate.

Reporting by Raul Cortes Fernandez and Ana Isabel Martinez; additional reporting by Cassandra Garrison, Editing by Chizu Nomiyama, Reuters