A newly confirmed case of New World screwworm (NWS) in northern Mexico is renewing concern among U.S. cattle producers and policymakers, as the parasitic fly continues to inch closer to the U.S.-Mexico border.
As reported by Drovers, on Dec. 27, Mexico’s National Service of Agro-Alimentary Health, Safety, and Quality (SENASICA) reported a case of NWS in a 6-day-old calf with an umbilical lesion in the municipality of Llera, located in the state of Tamaulipas. The location is approximately 197 miles south of the U.S.-Mexico border, and a reminder that NWS is still a high threat to the U.S.
Critical Timing with Calving Season Approaching
NWS, which was eradicated from the U.S. in the 1960s through an extensive sterile fly program, poses a serious threat to livestock. The larvae infest open wounds, feeding on living tissue and often leading to severe injury or death if untreated. Calving season is considered a particularly vulnerable period due to natural points of entry such as navels and birthing injuries.
Seth Meyer, director of the Food and Agricultural Policy Research Institute (FAPRI) and former chief economist for USDA, says the new case raises a tremendous amount of concern as USDA remains vigilant on keeping NWS out of the U.S. But Meyer says the growing proximity of NWS complicates already difficult decisions for cattle producers at calving season, which is a critical time of the year.
“There are concerns not just from a consumer standpoint, but also about whether southern producers are willing to retain heifers during calving season if there’s a risk of fly exposure,” he says. “Calving is a point of access for these animals, and that risk matters.”
Those decisions could have longer-term implications for herd expansion and cattle supplies, Meyer notes. If producers decide the risk is too great and opt against retaining replacement heifers, it could tighten supplies further down the road.
“That’s the last thing you want,” Meyer says. “You don’t want people giving up on retaining heifers and turning away from herd rebuilding.”
The Possibility of Reopening the Southern Border
The U.S. most recently closed its southern border to Mexican cattle imports in May of 2025 due to the rapid spread of NWS in Mexico. There were additional closures and reopenings in July 2025 as the situation evolved ultimately halting trade again to protect U.S. livestock.
Here’s a timeline so far:
- November 2024: NWS was first detected in southern Mexico, leading to initial border closures and trade disruptions.
- Feb. 1, 2025: A temporary ban was lifted after agreements for inspections.
- May 11, 2025: U.S Secretary of Agriculture Brooke Rollins ordered an immediate suspension of imports due to NWS spreading closer to the border.
- July 2025: A phased reopening began but was halted again after new NWS cases were found farther north, leading to another immediate closure of southern ports to protect American livestock.
Considering the cattle just south of the border are being vigilantly monitored and inspected, the bigger threat of NWS crossing the Southern border could be through wildlife. Still, as NWS gets closer, USDA is keeping the border closed and remaining cautious.
When could the U.S. reopen the border? That’s exactly what Farm Journal asked economists in the latest Ag Economists’ Monthly Monitor and the responses were extremely mixed. It’s important to note the survey was sent out prior to the most recent detection of NWS.
- 33% of economists say USDA could reopen the border in February 2026
- 25% say it could happen April through June
- 17% think the border could reopen July through September
- And 17% were unsure.
For policymakers, the situation adds another layer of complexity as they balance animal health, trade and producer confidence. While officials stress that there is no immediate threat to the U.S. herd, the latest detection underscores the importance of surveillance, rapid response and continued cooperation between U.S. and Mexican animal health authorities.
As Meyer puts it: “There are a lot of balls in the air right now,” and preventing NWS from crossing the border remains a critical priority for the livestock industry on both sides.
Follow Farm Journal’s extensive coverage of the ongoing NWS situation.