On Nov. 22, 2024, the Chief Veterinary Officer of Mexico notified the United States Department of Agriculture’s Animal and Plant Health Inspection Service of a positive detection of New World screwworm in Mexico. The NWS was found in a cow in the southern Mexico state of Chiapas, at an inspection checkpoint close to the border with Guatemala. NWS are fly larvae that infest living tissue of warm-blooded animals, causing infection. 

“Given the northward movement of NWS, APHIS has in recent months stepped up its efforts in Central America to partner with impacted countries to push this pest out of newly affected areas,” said Dr. Rosemary Sifford, chief veterinary officer of the United States. “With this latest find in Mexico, we will further intensify this work to protect American agriculture and reestablish the barrier in Central America.”

Given the severity of the threat from NWS, APHIS is restricting the importation of animal commodities originating from or transiting Mexico effective immediately and pending further information from Mexican veterinary authorities on the size and scope of the infestation. Live cattle and bison are the animal commodities impacted by this restriction. A complete list of regions APHIS recognizes as affected by screwworm, as well as more detailed information on trade restrictions, are available on the USDA APHIS Animal Health Status of Regions website. 

NWS maggots can infest livestock and other warm-blooded animals, including people. They most often enter an animal through an open wound and feed on the animal’s living flesh. While they can fly much farther under ideal conditions, adult flies generally do not travel more than a couple of miles if there are suitable host animals in the area.

APHIS is working with partners in Mexico and Central America to stop the spread of NWS into the United States and asking all producers along the southern border to watch their livestock and pets for signs of NWS and immediately report potential cases. 

Screwworm infestations are difficult to detect at first. APHIS advises checking pets and livestock for draining or enlarging wounds, and signs of discomfort, as well as screwworm larvae or eggs. Screwworm eggs are creamy and white and are deposited on or near the edges of superficial wounds. Those suspecting an animal of having screwworm infection should contact their veterinarian.

Since 2006, APHIS has collaborated with Panama to maintain a barrier zone in eastern Panama, working to prevent the northward movement of NWS from South America to NWS-free areas in Central and North America. In the last two years, screwworm has spread north of the barrier throughout Panama and into Costa Rica, Nicaragua, Honduras, Guatemala and now Mexico. This increase is due to multiple factors including new areas of farming in previous barrier regions for fly control, and increased cattle movements into the region. APHIS is releasing sterile flies through aerial and ground release at strategic locations throughout Central America.

APHIS will continue working with partners in Mexico and Central America to eradicate NWS from the affected areas and to reestablish the biological barrier in Panama.

Source: USDA's APHIS