In September, Mexican officials confirmed an outbreak of the lethal flesh-eating parasite in Nuevo Leon, located less than 70 miles (113 kilometers) from the Texas border.

The outbreak was identified in an 8-month-old cow along the busy Monterrey to Laredo highway. Cocliomia hominivorax – the New World screwworm – has resurfaced for the first time in decades, threatening its eradication status achieved in the United States over fifty years ago.

This discovery raised concerns among U.S. ranchers and veterinarians who vividly recall the destruction this voracious insect once wreaked. The screwworm fly, a metallic blue-gray, appears relatively ordinary until its larvae begin to burrow into the living flesh of animals.

“The adults resemble the blowflies commonly found in my lab in North Carolina,” remarks Professor Max Scott, an entomologist at North Carolina State University who has dedicated his career to pest research. “The difference is that this blowfly is an obligate parasite. The female must lay her eggs in a living host.”

Within hours of being laid in a wound or natural opening, the eggs hatch into maggots that burrow deep into the tissue. After several days of feeding, the larvae drop to the ground and pupate, often leaving the host deformed or dead.

Each female can lay hundreds of eggs, and untreated infestations often attract other species of flies, exacerbating the host’s decline.

In livestock, screwworms can result in significant weight loss, skin damage, and even fatalities, representing a multibillion-dollar challenge for the livestock industry in South America.

A Terrible Past

The New World screwworm once plagued regions in the southern United States, Mexico, and the Caribbean. It wasn’t until the mid-20th century that an unexpected mix of genetics, radiation, and air transport helped regain control over the species.

Post-World War II, scientists at the United States Department of Agriculture (USDA) pioneered germ-free insect technology, a groundbreaking method for genetic control. The approach was both simple and radical—breed millions of screwworm flies in captivity, sterilize them using gamma rays, and release them into the wild.

Females mate only once, so even if they pair with a sterile male, they won’t produce offspring, leading to a population collapse.

In the 1950s, experiments on Curaçao confirmed the method’s effectiveness. By the early 1960s, screwworms had been eradicated from Florida. The initiative spread west and south, ultimately eliminating the fly from the continental United States, Mexico, and much of Central America by the early 2000s.

The eradication campaign was costly, reportedly exceeding $750 million. However, it saved the livestock industry billions in losses annually and allowed U.S. cattle production to thrive in the following years.

A small production facility in Panama, jointly managed by the U.S. and Panamanian governments, maintained permanent barriers, releasing 100 million sterile flies weekly along the Panama-Colombia border and regularly developing new strains for longevity.

This system functioned effectively for two decades until it faltered. By 2023, infection cases began to re-emerge in Panama. Within two years, the parasite spread to Costa Rica, Nicaragua, and Honduras, now reaching northern Mexico.

What Went Wrong?

The specific reasons behind the barrier’s collapse remain unclear, but Scott noted “warning signs indicating trouble along the border.”

Farmers failing to adhere to protocols when transporting livestock may have further facilitated the spread of infection, explaining the surge in cases beyond the natural movement of fly populations.

Scott pointed out that the Panama facility is currently overburdened, producing around 110 million sterile flies weekly. In contrast, during the initial eradication efforts in Mexico, local facilities generated between 500 million and 700 million units weekly. “We needed that volume,” he noted.

This shortfall means an insufficient supply of sterile flies to manage the ongoing outbreak.

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The Enemy at the Gate

As of now, there is no evidence that the screwworms have crossed into the U.S. The USDA has deployed approximately 8,000 traps in Texas, Arizona, and New Mexico but has not detected any flies. Nevertheless, vigilance is high.

“The risks are substantial,” Scott cautions. “The best chance to curb the outbreak was further south, near the Isthmus of Tehuantepec. Reports of cases further north complicate containment given the current production capacities.”

In response, the U.S. government initiated a “five-pillar plan,” which includes constructing two new fly dispersal facilities in Texas. One facility at Moore Air Force Base can spray 100 million sterile flies weekly, while plans for a larger production plant in southern Texas aim to triple that capacity.

The USDA is also investing $100 million into innovative pest control technologies, exploring options from electron beam and X-ray sterilization to genetically modified flies.

A significant resurgence of screwworms could be catastrophic for U.S. livestock producers. An outbreak may incur billions in animal losses, quarantines, and trade restrictions. When the parasite re-emerged in Florida in 2016 from Caribbean-imported deer, it infected 145 animals, taking nearly a year to eradicate.

Human infections are rare but can be severe. The parasite’s scientific name, Cocliomia hominivorax, translates to “man-eating fly.” Most cases affect travelers to infested regions, leading to infections from wounds and nasal passages.

“This is not a major public health issue,” Scott explained. “However, caution is advised when visiting affected areas.”

Modern Genetic Upgrades

Scott believes that the old sterile fly method remains viable, especially when enhanced by new biotechnology. His lab at North Carolina State University has spent years developing genetically engineered screwworm strains that produce only males, significantly increasing efficiency.

Traditional sterile insect techniques require releasing both males and females, resulting in many males mating with sterile females.

This means that very high doses of radiation are needed to fully sterilize females, Scott noted. In contrast, his team’s method yields only males. They can then focus on sterilizing these males specifically, allowing researchers to maintain a higher quantity of healthier flies unlikely to reproduce.

These male-only stocks were field-tested in Panama in 2018 but were not deployed because existing methods were deemed sufficient. As the parasite moves northward, regulatory approval for genetically modified screwworms might become essential.

Moreover, researchers are investigating “gene drive systems” utilizing CRISPR technology to bias genetics, ensuring that most offspring carry infertile or male-only genes. In theory, this could drastically reduce the number of flies needed for suppression.

“We hope funding will be available to develop more effective technologies,” Scott stated. “The goal is to reduce releases from 100 million sterile flies to potentially just 10 million. This would make genetic control far more feasible.”

For the time being, strategies remain focused on aerial dispersal, targeted capture, and cross-border collaboration. Yet, scientists are increasingly concerned about the screwworm’s capability to migrate northward faster than anticipated.

“Given the current fly population, we sincerely hope to prevent them from entering the United States,” Scott warned. “But uncertainties remain.”

At present, the border remains intact. With each new case emerging towards the north, the focus shifts from whether the screwworms will invade the U.S. to whether they will be prepared upon their arrival.

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