Parasitic fly's northward march through Mexico threatens US agriculture and raises concerns about potential $1.8 billion economic impact
The United States has suspended all imports of cattle, horses, and bison at the southern border to prevent the spread of New World Screwworm (NWS), a devastating parasitic fly rapidly advancing northward through Mexico. USDA Secretary Brooke Rollins announced the immediate implementation of the ban on Sunday, citing the urgent need to protect America's livestock and food security from a pest that once took decades and billions of dollars to eradicate from US soil.
Deadly Parasite Poses Significant Threat to Animals and Humans
The New World Screwworm, scientifically known as Cochliomyia hominivorax, creates devastating infections by laying eggs on open wounds of living animals. Unlike common maggots that consume dead tissue, NWS larvae burrow into and feed on living flesh, causing severe damage that can kill a full-grown cow within one to two weeks.
"The last time this devastating pest invaded America, it took 30 years for our cattle industry to recover," Rollins stated in her announcement. "This cannot happen again" Fox News1.
The parasitic fly primarily targets livestock but can also infest pets, wildlife, and birds. Though rare, human infections have been documented. Mexico reported its first human case in April 2025—a 77-year-old woman from Chiapas state—while Nicaragua has confirmed at least 30 human cases and Costa Rica has reported 28 cases in recent months CIDRAP2.
In humans, symptoms include painful, non-healing wounds, visible maggots, foul odor, and potential secondary bacterial infections that can cause fever and chills. Medical professionals warn that any suspected infestation requires professional treatment and should never be self-managed USA TODAY3.
History Repeats: The Return of an Eradicated Threat
This resurgence marks a concerning reversal of decades of successful eradication efforts. The United States eliminated NWS in the 1960s, followed by Mexico in the 1970s and much of Central America in the early 2000s. This was achieved through an innovative approach of releasing sterile male flies that would mate with females, preventing viable reproduction and creating a protective barrier zone NPR4.
The strategy involved a long-standing collaboration between the US and Panama, which since 1994 has involved dropping millions of sterile flies over a jungle area at the Panama-Colombia border to contain the parasite in South America CIDRAP2.
However, in 2023, NWS detections surged dramatically in Panama and subsequently spread to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and eventually reached Mexico in November 2024. Recent detections at remote farms in Mexico's Oaxaca and Veracruz states place the pest approximately 700 miles from the US border CIDRAP2.
Economic Stakes and Previous Impact
The potential economic damage from an NWS outbreak is substantial. A 1976 outbreak in Texas affected more than 1.4 million cattle and hundreds of thousands of sheep and goats. USDA analysis estimates that a similar outbreak today would cost the Texas economy $1.8 billion NPR4.
This isn't the first time the US has taken such precautionary measures. A previous ban was implemented in November but lifted in February after both nations agreed on new containment protocols. The reinstated suspension will continue on a month-by-month basis "until a significant window of containment is achieved," according to the USDA NPR4.
Officials Emphasize Safety Over Politics
Secretary Rollins has been careful to frame the suspension as a public health and agricultural safety measure rather than a political action.
"The protection of our animals and safety of our nation's food supply is a national security issue of the utmost importance," Rollins stated. "This is not about politics or punishment of Mexico, rather it is about food and animal safety" CIDRAP2.
The USDA emphasized that despite working with Mexico "in good faith" to control NWS, there has been "unacceptable northward advancement" of the parasite, necessitating stronger containment measures CIDRAP2.
Expert Insights on Containment Strategy
The USDA's Animal and Plant Health Inspection Service (APHIS) is coordinating with U.S. Customs and Border Protection to implement the import restrictions. Under the current policy, livestock already in the process of being imported will still undergo inspection by port veterinary medical officers Fox News1.
The historical success of the sterile insect technique suggests this method may again be crucial for controlling the current outbreak. However, the speed of the northward progression indicates that additional interventions and international coordination will be necessary to reestablish the protective barrier zone CIDRAP2.
Future Implications and Monitoring
Secretary Rollins has committed to reopening the border for livestock trade once increased surveillance and eradication efforts show positive results Fox News1. However, the timeline remains uncertain given the challenging nature of pest eradication across international borders.
The situation highlights the delicate balance between trade interests and biosecurity, especially considering that a small wound as minor as a tick bite can attract screwworm flies, making even routine cattle movements potentially risky USA TODAY3.
Agricultural officials will continue monitoring the situation while working with Mexico to strengthen surveillance and eradication efforts. The suspension serves as a stark reminder of how rapidly invasive species can reemerge and threaten agricultural systems even after successful eradication.
As climate change and global trade continue to alter traditional pest distribution patterns, will this outbreak serve as a wake-up call for strengthening international biosecurity cooperation, or will it become another chapter in the ongoing battle against agricultural pests?
