REDFIELD, Iowa (DTN) -- Progress has been made in critical areas of Mexico following the border closing due to the northern spread of New World screwworm (NWS). That led U.S. Secretary of Agriculture Brooke Rollins to announce on Monday a risk-based port reopening for cattle, bison and horses as soon as July 7.

Imports of live cattle, bison and horses from Mexico were halted on May 11 when NWS was found within 700 miles of the U.S. border. The border was previously closed in November 2024 when the first case of NWS was found in southern Mexico and was reopened on Feb. 1, 2025.

Since the ports were closed on May 11, more than 100 million sterile NWS flies were dispersed each week. Since the NWS flies only mate once, releasing sterile males to mate with the females helps decrease the population.

On June 18, Rollins announced the construction of an $8.5 million facility to be located in South Texas for sterile fly dispersal, which is expected to be completed by the end of 2025 and will be able to disperse sterile flies in northern Mexico as well.

PHASED APPROACH TO REOPENING

USDA's Animal and Plant Health Inspection Service (APHIS) experts said in a news release that they, along with their counterparts in Mexico, plan to increase NWS surveillance, detection and eradication efforts during the phased reopening of the ports, starting with Douglas, Arizona, as soon as July 7.

If no adverse effects arise following each port opening, other ports will be opened in the following timeline: Columbus, New Mexico, on July 14; Santa Teresa, New Mexico, on July 21; Del Rio, Texas, on Aug. 18; and Laredo, Texas, on Sept. 15.

The announcement of these reopenings came after no notable increases in reported NWS cases were found in Mexico and no northward movement has been seen over the past eight weeks.

"At USDA we are focused on fighting the New World Screwworm's advancement in Mexico," Rollins stated in the release. "We have made good progress with our counterparts in Mexico to increase vital pest surveillance efforts and have boosted sterile fly dispersal efforts. These quick actions by the Trump Administration have improved the conditions to allow the phased reopening of select ports on the Southern Border to livestock trade. We are continuing our posture of increased vigilance and will not rest until we are sure this devastating pest will not harm American ranchers."

CONTINUAL PROGRESS HELPS WITH OPENING

Continual progress in NWS control in Mexico will help keep the U.S. ports opened. Mexico is also beginning renovation of its sterile fruit fly facility in Metapa this week with an expected completion by July 2026. This facility will produce 60 million to 100 million sterile NWS flies weekly. This will help get to the 500 million flies needed each week to reestablish the NWS barrier at Darien Gap between Panama and Columbia.

Animals crossing the U.S. border from the south will still be required to follow importing protocols, including treatment, observation and quarantine.

NWS are fly larvae that infest living tissue of warm-blooded animals, causing infection. According to the Animal and Plant Health Inspection Service (APHIS), these maggots will burrow into a wound, feeding as they go, doing extensive damage by tearing the host's tissue with sharp mouth hooks. The wound becomes larger and deeper. NWS can cause serious, even deadly, damage to the animal.

The adult screwworm flies are about the size of a common housefly with orange eyes, a metallic blue or green body, and three dark stripes along their backs.

Affected mammals and birds show signs of irritated behavior, head shaking, smell of decay, evidence of fly strike and presence of fly larvae in wounds.

USDA eradicated NWS from the United States in 1966 using the sterile insect technique, and it was used successfully again in 2016 in the southern Florida Keys when found in deer.

For more, see "USDA Facility Will Help Combat NWS" here: https://www.dtnpf.com/….

Jennifer Carrico can be reached at jennifer.carrico@dtn.com

Follow her on social platform X @JennCattleGal

(c) Copyright 2025 DTN, LLC. All rights reserved.