The U.S. Department of Agriculture’s recent announcement of the return of the screwworm has sparked widespread concern due to the serious threat this pest poses to livestock across North America.
Known as the New World screwworm, this parasitic fly feeds on the living tissue of warm-blooded animals, effectively consuming its hosts from the inside out. Considered one of the world's most dangerous transboundary pests, its re-emergence after decades of absence raises pressing questions: What is behind its return? Has climate change played a role? And could humans also be at risk?
Larvae That Feed on Living Flesh
الأكثر قراءة
Screwworms are the larvae of a parasitic fly. What makes them particularly dangerous is the female fly’s habit of laying eggs in open wounds or mucous membranes of warm-blooded animals. Once the eggs hatch, hundreds of larvae use their sharp mouth hooks to burrow into and feed on living tissue.
If left untreated, infestations can kill the infected animal. The pest typically spreads when infected animals move from one region to another.
Livestock are among the most vulnerable hosts due to routine husbandry practices. According to Lee Haines, Associate Research Professor of Biological Sciences at the University of Notre Dame, common procedures such as dehorning, shearing, or transporting animals can create cuts and abrasions that provide ideal sites for flies to lay their eggs.
Livestock producers warn that the danger is not limited to major wounds. Even minor injuries, including tick bites, may be enough to initiate an infestation.
Without early detection and treatment, the larvae can prove fatal. Although effective treatments exist, the pest reproduces rapidly and can spread across vast distances through its hosts, particularly wildlife.
The Return of the Screwworm
What concerns experts today is not merely the destructive nature of the larvae, but the fact that they have returned after decades of apparent eradication.
During the first half of the twentieth century, screwworms were a seasonal scourge that caused substantial losses to U.S. livestock producers. However, American authorities successfully eradicated the pest from the country in 1966, according to the U.S. Department of Agriculture.
The parasite resurfaced in Central America in 2023 before spreading into Mexico. More recently, an infected calf was identified in Texas near the Mexican border, according to Food Manufacturing magazine.
This raises a critical question: How did a pest eradicated nearly six decades ago make a comeback, and did climate change create favorable conditions for its resurgence?
From Eradication to Resurgence: What Brought the Pest Back?
Experts attribute the return of the screwworm to a combination of factors, including animal movement, geographic expansion of the fly’s habitat, and challenges in disease containment.
Lee Haines explained that climate change is a major driver behind the spread of pests that thrive in warm environments. Rising temperatures are expanding the fly’s habitable range, while cold spells that once eliminated populations in marginal areas have become less frequent and less severe. As a result, the screwworm now has greater opportunities to survive and spread.
Economic Losses and Human Health Risks
The threat posed by screwworms extends far beyond animal health. Their return jeopardizes the U.S. cattle industry, which is valued at approximately $113 billion.
Between 2023 and early June this year, more than 171,700 animal infections were recorded. The impact has not been confined to wildlife or livestock. According to the U.S. Centers for Disease Control and Prevention (CDC), more than 2,070 human cases have been reported across Central America and Mexico.
Screwworm larvae can cause painful, foul-smelling wounds in both animals and humans. If infestations are not detected and treated promptly, they may result in extensive tissue destruction and, in severe cases, death where adequate medical care is unavailable.
Nevertheless, human infections remain relatively uncommon and are generally treatable. The greatest threat continues to be the devastating impact on livestock populations.
Between massive livestock losses, significant economic consequences, and potential public health risks, the return of the screwworm serves as a stark reminder that some pests once thought defeated can re-emerge in a world shaped by climate change and increasingly interconnected movement of people and animals across borders.
