Mexico’s Desperate Battle to Curb the Screwworm Infestation

The New World screwworm is a parasitic fly that devastates livestock and wildlife. Picture a facility churning out millions of these insects weekly, where the larvae feed on a foul concoction of cellulose, egg, milk, and bovine blood designed to replicate flesh.

This process generates a pungent, sickly-sweet stench of decaying tissue that permeates the air and lingers indefinitely.

“You acclimate to it,” noted Dr. José Luis Quintero, who operates one of these laboratories in Mexico. “It is simply part of the occupation.”

Mexican authorities have been urgently addressing the country’s first significant screwworm infestation in decades. This outbreak has severely impacted the already stressed cattle industry across both Mexico and the United States. Although the parasite was largely eradicated in the region by the early 2000s, a convergence of factors has enabled its unwelcome return, closing the U.S. border to Mexican cattle imports last year and driving up American beef prices.

In Metapa, a town situated along the Mexican-Guatemalan border, a recently renovated facility has emerged as Mexico’s primary weapon against the parasite. Since the first case was detected in November 2024, the outbreak has resulted in over 34,000 cumulative cases across 30 of Mexico’s 32 states. The economic toll, encompassing both livestock losses and eradication efforts, is estimated to reach billions of dollars.

Inaugurated in late June, this facility is the sole plant in Mexico dedicated to producing sterile flies. The methodology involves breeding massive populations of the insects, sterilizing them via radiation exposure, and subsequently releasing them into the wild. Wild female flies then mate with these sterile males, gradually diminishing the overall population.

The objective is to reach a production capacity of 100 million sterile flies per week by November, enabling their distribution—beginning in Texas and northern Mexico—to contain the outbreak.

The livelihoods of countless individuals hinge on this multinational effort to eradicate a fly capable of wreaking extreme havoc.

Adult females deposit roughly 200 eggs per batch into the open wounds or orifices of warm-blooded animals. Upon hatching, the larvae burrow into the wound, consuming the host’s tissue—hence the name screwworm. Without treatment, infections can prove fatal to animals within a week.

“This is indeed a deeply concerning issue,” stated Hunberto Gomes Velasques, who oversees the sterile fly plant in Metapa, acknowledging the intense pressure of the deadline.

Last month, the United States initiated a phased reopening of its border to cattle imports, though officials warned that the screwworm problem would persist for the foreseeable future.

Mexican President Claudia Sheinbaum, who initially opposed the border closure and attended the Metapa plant’s inauguration, stated that officials cannot definitively predict how long eradication will take, though she noted the facility will aid in containing the spread.

“It was eradicated previously, which is why experts believe we can achieve eradication a second time,” she remarked.

Experts estimate that full eradication could take up to two decades. When asked about the timeline, Dr. Quintero pointed to historical precedents: The fly was a severe scourge for livestock in the region during the early 20th century. However, the United States was declared free of wild screwworms in 1966 and combated a severe outbreak the following decade. By 1991, Mexico had also achieved freedom from the parasite. Since then, aided by a U.S.-funded sterile fly production plant that opened in Panama in 2006, efforts have kept the flies suppressed in the Darién Gap.

Cases surged in Panama and Costa Rica in 2023, subsequently spreading steadily northward through Mexico and eventually into the United States. The Darién Gap, a narrow jungle land bridge connecting Panama and Colombia, had served as the biological barrier that historically kept the screwworm at bay.

Experts suggest several reasons for the screwworm’s return. Pablo Liedo, a researcher and entomology expert at the College of the Southern Border, posited that lapses or insufficient attention from authorities regarding sterile insect programs may have contributed.

He also noted that the hundreds of thousands of migrants who traversed the Darién Gap toward the U.S. border during the Biden administration might have contributed to the latest outbreak.

“It was a highly enclosed jungle,” Mr. Liedo explained.

The flies may have hitched rides on animals transported by humans, he suggested. He further pointed to illegal animal trafficking throughout the region that evades inspections, a persistent complaint among U.S. officials.

To combat the new screwworm crisis, Mexican authorities determined that the fastest approach—achieving operational status within a year—was to retrofit a Metapa plant originally built in the 1970s to breed and sterilize the Mediterranean fruit fly. The Mexican government funded the upgrade with $30 million, while the United States contributed an additional $21 million.

Mr. Gomes Velasques explained that the plant features a sealed ventilation system to prevent fly escapes, alongside a sterilization process for waste. New equipment has been installed to produce the larvae’s food on-site. The facility maintains rooms heated to 101 degrees Fahrenheit with 80 percent relative humidity for larval incubation. Furthermore, the plant will operate 24 hours a day, 365 days a year.

In July, the facility was still ramping up initial production, leaving many gleaming new rooms vacant. However, in one room, trays of pupae—a stage in the flies’ lifecycle—awaited processing, while a constant hum emanated from six nearby cages housing nearly 300,000 flies.

Mr. Gomes Velasques noted that the plant was producing approximately 10 million sterile flies per week at the time. These insects are transported to northern Mexico, where they are dispersed via aircraft.

Combined with the Panama site, experts asserted that this output was insufficient to combat the current regional outbreak. They estimated that a minimum of 500 million sterile flies per week was necessary. The U.S. Department of Agriculture indicated that a new plant in Texas would open in early 2027, but would not reach its full capacity of 300 million sterile flies per week until the end of 2028.

Eduardo Farfán, vice president of a state veterinarians association and a veterinarian in Pijijiapan—a small Mexican town three hours from the Guatemala border—said the Mexican government was “trying to block out the sun with a finger.” He argued the response has been neither ample nor swift enough.

“Can it be eradicated? Yes. But it is a matter of having more resources,” he stated. “The worm consumed our time.”

Until then, frontline veterinarians like Mr. Farfán and ranchers like Francisco Javier Rodríguez must battle the screwworm daily. Chiapas remains the most affected Mexican state, while northern Mexico along the U.S. border has experienced relatively fewer cases.

On a recent afternoon, Mr. Farfán accompanied Mr. Rodríguez, 38, to his land to inspect Coneja, a three-day-old calf that had already been infected.

The flies are difficult to combat due to their numbers, mobility, and small size. They can detect minute amounts of blood, such as a scratch on a dog’s ear or a tick bite on a horse. For ranchers, inspecting all their animals is necessary but tedious work. (They can also use deworming medication preventively.)

Although Mr. Rodríguez stated he had applied a disinfecting solution after the calf’s birth, the site where the umbilical cord was attached had not yet scarred over, allowing screwworms to burrow in.

Mr. Rodríguez restrained Coneja while she lay on the back of a truck. As Mr. Farfán applied a blue antibacterial powder to the wound, he used tweezers to extract 20 worms.

“There are likely many more,” Mr. Farfán remarked.

Of his 50 animals, Mr. Rodríguez stated that nine have been infected by screwworms over the past year, mostly calves. While none have died, some cattle required weeks to recover as the infection diminished their appetites and milk production.

“I have to inspect them every day,” he said.

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