Efforts to combat an outbreak of New World screwworms across Central America have intensified as the U.S. prepares to open a new facility for breeding sterile flies near its border with Mexico.
Hundreds of millions of flies released from airplanes might sound like a nightmare scenario, but experts assert that this strategy serves as a crucial defense against a flesh-eating threat making its way toward the southwestern United States.
The outbreak of New World screwworms, which are larvae of a fly known for infesting the wounds of warm-blooded animals and slowly consuming them, has spread across Central America since early 2023. Countries including Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and El Salvador have recorded infestations after nearly two decades without significant problems.
The fly’s arrival in southern Mexico in November spurred U.S. agricultural officials to act, leading to the closure of several border-area trading ports for cattle, horses, and bison. The U.S. previously eradicated screwworm populations from 1960 to 1970 through a strategy of breeding sterile male flies and releasing them from planes to mate with wild females, effectively suppressing reproduction.
As the threat approaches the U.S., officials hope to apply the same method again, despite challenges posed by the need for more sterile flies. Currently, only one facility in Panama produces these flies, and hundreds of millions more are required to manage the outbreak effectively. On June 17, 80 U.S. lawmakers highlighted this shortfall in a letter, prompting the U.S. Department of Agriculture (USDA) to announce plans for a “fly factory” near the Texas-Mexico border.
Understanding the screwworm danger underscores the urgency of such measures. The parasitic larvae of the Cochliomyia hominivorax fly, unlike other blowflies, target living animals, explained Dr. Phillip Kaufman, head of Texas A&M University’s entomology department. These maggots inflict severe wounds, mainly on livestock such as cows and horses. Even humans and pets can occasionally fall victim.
“After mating, the female fly finds a living host, lands on its wound, and lays 200 to 300 eggs,” Kaufman said. These hatch within 24 hours, burrowing into the host’s tissue. After several days of feeding, they drop to the ground to pupate and later emerge as adult flies, noted Thomas Lansford, deputy executive director of the Texas Animal Health Commission.
Since 2023, over 35,000 screwworm infestations have been reported, with cattle comprising a significant portion of affected animals, according to the Panama–United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG). Treatment involves wound cleaning and antiseptic measures, but the flies can easily spread if untreated, threatening ranchers’ livelihoods.
Rancher Stephen Diebel, first vice president of the Texas & Southwestern Cattle Raisers Association, emphasized the necessity of daily inspections to prevent infestations and warned of the considerable economic impact they pose. Furthermore, there are no vaccines or effective repellents available, necessitating preventive measures like avoiding wounds on livestock during warm months.
Addressing the spread is complicated by the diverse ways the flies travel, ranging from livestock trading to affecting wildlife such as deer and rodents, Diebel noted.
The core of the strategy to combat screwworms is breeding sterile flies. In facilities, fly pupae undergo gamma radiation that disrupts DNA, rendering male flies infertile without harming people or animals, explained the USDA. Female flies, mating once in their 20-day lifespan, lay unfertilized eggs, leading to population decline.
While specifics on dispersal methods in the U.S. remain unconfirmed, sterile flies are typically released from planes over rural areas, keeping them out of urban environments, Kaufman stated.
Currently, COPEG produces and disperses about 100 million sterile flies weekly across affected regions in southern Mexico and Central America. The new U.S. facility plans to be set at Moore Air Base in Texas, with an estimated cost of $8.5 million, while lawmakers anticipate the factory itself could cost $300 million. Renovating an old Mexican fly factory will also require $21 million.
Despite high costs, rancher Diebel views this investment as essential for protecting the multibillion-dollar livestock industry. “Offsetting $300 million for facility costs against $10 billion in potential economic impact is a straightforward trade-off,” he said, highlighting the importance of domestic production for efficient sterile fly distribution.
Following the USDA’s June 18 announcement, plans to reopen livestock trading ports in Arizona, Texas, and New Mexico are underway, reflecting progress in efforts to curb the screwworm spread in Mexico.
As the U.S. strengthens its defenses against screwworm infestations, the agricultural community anticipates further updates on advancements from COPEG and other involved agencies.