While the U.S. and Mexico may still be hashing out the tariff situation , the countries are again united on one common enemy: the New World Screwworm.

A parasite, the New World Screwworm, is actually a fly. Female screwworm flies lay eggs in wounds or body openings of mammals, including cows, deer, birds, pets, and even humans, that can lead to a sometimes fatal infection.

Here in the U.S., the Agriculture Department eradicated the NWS parasite in 1966 by releasing sterile male flies and pupae to eventually lead to infertile screwworm flies, according to the USDA's Animal and Plant Health Inspection Service . But a risk of it returning remains because the parasite is found in both Central America and South America, and it could spread north into Mexico.

In November 2024, America halted imports of Mexican livestock after the country alerted the U.S. to a NWS detection in a cow at a inspection checkpoint close to the Mexico border with Guatemala. Subsequently, the USDA increased sterile fly deliveries and supported additional surveillance programs, which led to livestock imports restarting in February 2025.

But recently, relations became strained in April when Mexico authorities began curtailing 7-day flights of USDA planes dispersing sterile flies and imposing import duties on "critical aviation parts, dispersal equipment, and sterile fly shipments," U.S. Agriculture Secretary Brooke Rollins said in a letter to Esteban Moctezuma Barragán, Mexico's ambassador to the U.S. Ambassador, dated April 26, 2025.

Mexico avoided another U.S. livestock ban last week by reaching a deal to allow additional flights and to waive custom duties, Rollins said April 30.

“I am happy to share Mexico has continued to partner in emergency efforts to eradicate the New World Screwworm," Rollins said in a press release. “This pest is a devastating threat to both of our economies, and I am pleased to work together with Mexico in good faith to protect the livelihoods of our ranchers and producers who would have been hurt by this pest."

What is the New World Screwworm?

Adult screwworm flies are similar to – or slightly larger than – the common housefly, but they have orange eyes, a metallic blue or green body, and three dark stripes along their backs, according to the USDA's Animal and Plant Health Inspection Service (APHIS).

The parasite gets its name from how the larvae's resulting maggots, which as they feed on an animal will burrow, or screw, into the wound or opening, creating "extensive damage by tearing at the hosts’ tissue with sharp mouth hooks," APHIS says.

This story was updated to fix an inaccuracy.