Decades of Combat: The United States Faces a Renewed Battle Against the Flesh-Eating Screwworm

For over 50 years, the United States has invested hundreds of millions of dollars to keep the flesh-eating screwworm at bay. Unfortunately, it’s back.

Subscribe for ad-free access to this article

Enjoy unlimited access to exclusive content and articles.

While screwworms can infest any warm-blooded animal, they pose a significant threat to livestock, often resulting in fatalities among cattle. The ethical implications of intentionally eradicating this species have prompted debate among environmental bioethicists.

“Some species warrant consideration for complete eradication, and the screwworm is among them,” commented Gregory Koebnick, a senior research fellow at the Hastings Center for Bioethics.

Recently, the Department of Agriculture confirmed the presence of New World screwworms in calves in Texas. This marks the first occurrence of natural infestation in U.S. cattle since 1982, as reported in a recent study. A second case was subsequently identified, approximately 6 miles from the initial infection, as documented in tweets by researchers. This discovery represents a troubling resurgence of the species, reigniting the U.S.’s decades-long battle against screwworms.

Experts suggest that the U.S. will adopt a similar strategy to the one employed in the late 1950s, during which an aggressive, collaborative approach aimed at the screwworm population was initiated. The strategy involves mass-producing sterile males that, when released, render natural mating attempts futile since female screwworms mate only once.

“This strategy is effective,” stated Chad Cross, a professor of parasitology at Texas Tech University’s College of Veterinary Medicine. “It has proven successful in the past, and I believe it will continue to be effective.”

He also pointed out that the new cases in Texas serve as a critical reminder of the urgent need to prevent further outbreaks.

The screwworm fly, classified as a black fly, thrives in southern regions of the U.S. These flies are drawn to infected, neglected wounds, where females can deposit up to 300 eggs, leading to invasive larvae development.

“The larvae emerge from the eggs and feed on the flesh of warm-blooded hosts,” explained Philip Kaufman, a professor of entomology at Texas A&M University. “They pose a threat not only to domestic animals but also to wildlife, and even humans.”

Screwworm larvae possess specialized hooks that allow them to penetrate animal flesh and burrow deeper. Infection manifests as open, rotting sores on the skin, often accompanied by an unpleasant odor, which can attract other fly species. If left untreated, the infection is likely to lead to death unless proper intervention occurs, including insecticide and antibiotic treatments.

Infections in humans are rare but extremely painful.

“You are essentially consuming tissue, whether it is muscle, fat, or skin,” Kaufman stated. “It’s highly probable that you would become aware of this issue.”

Currently, screwworms are primarily found in the southernmost areas of the United States but can adapt to milder climates provided they remain warm enough.

“They could survive in the Midwest during favorable weather conditions, but they can’t endure the winter,” Kaufman remarked. “South Texas and South Florida will remain warm enough to sustain their survival.”

These regions were previously the main habitats for screwworms until aggressive elimination efforts began in the 1960s, which involved constructing factories and spray fields in Florida, Texas, and Central America to produce and release sterile flies.

These sterile flies are irradiated and released in large quantities to saturate outbreak areas. Their mass presence prevents wild female screwworms from successfully mating, thereby curtailing egg-laying and reproduction.

The approach yielded success, and when infections reached zero in 1982, the U.S. continued operations in Mexico and Central America, driving the screwworms further south.

“It wasn’t until around 2004 that their effectiveness extended past the Panama Canal,” Kaufman remarked.

However, the U.S. and its partners gradually reduced investments in facilities that produced and distributed sterile insects in locations where screwworms had been eliminated.

“As new factories were opened further south, those in the north were shut down, leading to the closure of the Texas facility followed by those in Mexico and Nicaragua, leaving only the Panama factory operational,” Kaufman explained. “That aging plant is now showing signs of wear.”

For two decades, the Darien Valley, an inaccessible rainforest bordering Panama and Colombia, marked the northern limit of screwworm distribution. However, in 2023, outbreaks began spreading northward, initially to Panama and Costa Rica, and subsequently to Mexico and the United States.

“The crucial question now is: why have they re-emerged?” Kaufman ponders. “No one really knows.”

Regardless of the cause, the U.S. is investing again. The USDA is constructing a $750 million facility in Texas expected to produce around 300 million sterile screwworms weekly, tripling current production levels and aligning with capabilities established in the 1960s. However, the facility is not expected to be operational until late 2027, requiring time to ramp up to full capacity.

Until then, the likelihood of outbreaks remains significant. Screwworm infestations could inflict roughly $1.8 billion in losses each year to the Texas economy alone, encompassing livestock deaths, veterinary care, treatment, and additional labor costs, according to USDA estimates for 2024.

The ultimate goal is the complete eradication of the screwworm fly from the U.S. and Central America. Some researchers argue for considering the species’ total elimination. In 2024, a consortium of bioethicists, conservation biologists, and scientists debated whether refining sterilization methods and employing genetic modifications to introduce lethal genes into the gene pool of screwworms could lead to extinction, as discussed in a paper published in Science magazine last year.

The authors contend that screwworms cause immense suffering to livestock under human care. The infections they inflict upon animals are slow to develop and agonizing for both livestock and humans, raising questions about the environmental value or benefits provided by this species.

Koebnick, the bioethicist, noted that while the group includes individuals who cherish biodiversity and advocate species conservation, they ultimately concluded that there may be instances where eradication is justifiable.

The researchers were predominantly speculating. Currently available genetic modification techniques geared towards screwworm eradication have not been tested at a large scale and are not yet viable for implementation. “If such options were available, it would represent a significant decision with potential long-term consequences,” Kebnick explained.

“These methods are not yet ready for deployment,” he added.