Mexican officials are intensifying efforts to combat a significant resurgence of the New World screwworm, a parasitic fly whose larvae feed on the flesh of livestock, posing a severe threat to cattle health and the agricultural economy. The outbreak, first identified in November 2024, has since spread to 30 of Mexico’s 32 states, leading to over 34,000 reported cases and causing major disruptions to cattle industries in both Mexico and the United States.
The screwworm was largely eradicated from the region by the early 2000s through the use of sterile insect techniques, but factors including increased migration and illegal animal trafficking have contributed to its return. This has prompted border restrictions on Mexican cattle imports to the U.S., which were only partially lifted last month as authorities continued to manage the threat.
In response, a key sterilization plant in Metapa, a town near the Mexican border with Guatemala, has been refurbished to produce sterile screwworm flies as part of a biological control program aimed at reducing pest populations. The method involves breeding large numbers of flies, sterilizing males through radiation, and releasing them into the wild. When sterile males mate with wild females, no viable offspring are produced, gradually diminishing the population.
The Metapa facility, which was repurposed from a 1970s plant originally designed for Mediterranean fruit fly control, resumed operations in late June and aims to reach a production capacity of 100 million sterile flies per week by November. Mexican authorities invested approximately $30 million, with an additional $21 million contributed by the United States, to upgrade the plant’s infrastructure, which includes sealed ventilation systems and in-house production of larval food. The plant operates continuously, maintaining conditions optimal for larval development.
Despite this ramp-up, experts have said that combined sterile fly production from the Metapa site and the established facility in Panama remains insufficient to fully contain the outbreak. It is estimated that 500 million sterile flies per week are necessary to reverse the spread effectively. The U.S. Department of Agriculture plans to open an additional plant in Texas by early 2027, with full capacity projected by the end of 2028, though this timeline leaves a gap in containment capabilities in the near term.
Veterinarians and cattle ranchers on the front lines are facing daily challenges. In Chiapas, the state hardest hit by the screwworm, ranchers like Francisco Javier Rodriguez report multiple infections among their livestock. The screwworm larvae infest open wounds and orifices, feeding on live tissue and causing severe damage that can be fatal if untreated. While preventive measures such as deworming medications are in use, close inspection and treatment of infected animals are labor-intensive and necessary.
Experts, including researchers at the College of the Southern Border, suggest several possible causes for the resurgence, ranging from lapses in sterile insect program oversight to migration-related factors facilitating the flies’ movement across previously impenetrable biological barriers like the Darién Gap between Panama and Colombia.
Mexican President Claudia Sheinbaum, who initially opposed the cattle import restrictions, has emphasized the importance of the sterile fly program and expressed cautious optimism about eventual eradication, pointing to historical precedent where the screwworm was successfully eliminated from the region. However, some veterinary professionals urge a scale-up of efforts, cautioning that current responses may be too limited and slow to meet the outbreak’s severity.
Until wider containment is achieved, livestock owners in affected areas continue to grapple with the economic and animal health impacts of the infestation, highlighting the need for sustained and coordinated binational efforts to address this ongoing threat to the cattle industry.