Return of the Flesh-Eaters

Summary of Return of the Flesh-Eaters

by WNYC Studios

42m•March 13, 2026
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Overview of Return of the Flesh‑Eaters

This Radiolab episode (Return of the Flesh‑Eaters) reports on the New World screwworm (a parasitic fly whose larvae eat living flesh), the mid‑20th century campaign that essentially eradicated it from much of North America using the sterile insect technique (SIT), and a frightening resurgence in Central America since 2023. The show covers the science and history (pioneered by entomologist Edward F. Knipling), the current outbreak and U.S. response, and a contemporary ethical debate about whether modern genetic tools (gene drives) should be used to permanently eradicate species like the screwworm.

History: screwworms and the sterile insect technique (SIT)

  • The organism: New World screwworm (fly whose maggots infest wounds and mucous membranes of warm‑blooded animals, including humans), causing gruesome, often fatal infestations.
  • Early problem: Devastated livestock across the southern U.S.; young Edward F. Knipling (USDA entomologist) grew up pulling them from cattle and later worked on solutions.
  • The key observation: Female screwworms mate only once. Knipling realized that flooding wild populations with sterile males would produce matings that yielded no offspring and could drive populations down.
  • How SIT was implemented:
    • Sterilization using ionizing radiation (X‑rays) targeted at a precise life stage (to knock out testes while minimizing lethal mutations).
    • Mass‑rearing factories produced sterile males in huge numbers (early successful trial: Curaçao, mid‑1950s).
    • Aerial releases: chilled flies loaded into planes and dispersed across landscapes.
  • Results:
    • By coordinated national and international campaigns (U.S. → Mexico → Central America), screwworm was driven south to a sterile barrier at the Panama–Colombia gap.
    • By the 2000s authorities declared much of North and Central America (up to the Panama–Colombia border) screwworm‑free.
    • Maintenance of a sterile barrier in Panama has been relatively affordable (~$15 million/year) versus the economic losses a re‑establishment would cause (estimates of >$1 billion saved per year).

The recent resurgence (2023–present) and current response

  • Since 2023, Panama recorded a dramatic jump in cases (from a few dozen per year to thousands). The fly has been found moving north through Costa Rica, Nicaragua, Honduras, Guatemala and into Mexico.
  • Human impact: Over 1,000 human cases have been reported in the current outbreak; infestations can occur from tiny wounds and mucous membranes (eyes, nose, ears, urogenital). While not a pandemic‑level human public‑health threat, it is a severe burden on vulnerable populations and healthcare systems.
  • How the barrier may have been breached: possible causes include weakened production/maintenance of sterile flies (pandemic disruptions), a manufacturing strain less effective at competing with wild males, or simple movement of infected animals/people/products (smuggling of infected livestock is suspected).
  • Current actions:
    • USDA, CDC and regional authorities are re‑mobilizing: traps along the U.S.–Mexico border, horseback patrols, rebuilding mass‑rearing facilities in Texas and Mexico, and plans to produce hundreds of millions of sterile males (targets cited around ~500 million/week).
    • Large expenditures (hundreds of millions) are anticipated to stem the spread.

The ethical debate: eradication vs. containment; gene drives

  • Gene drives explained briefly:
    • Genome editing can be used to insert a trait (e.g., female‑lethal) that a gene drive then biases to be inherited at far greater than Mendelian 50%, allowing rapid spread through a wild population.
    • A successful female‑lethal drive could, in theory, drive screwworms to global extinction.
  • The ethical question: Is it acceptable to deliberately cause the permanent extinction of a species—however harmful?
  • Workshop and expert views:
    • A Hastings Center convening of ethicists, ecologists, geneticists and entomologists discussed candidate species for gene‑drive eradication (mosquitoes, screwworms).
    • Arguments in favor:
      • Screwworms cause intense suffering to humans and animals; ecological role appears limited (a minor pollinator and not a critical prey species).
      • Historical eradication in North/Central America didn’t cause obvious ecological collapse—some empirical support that removal is low‑risk.
      • Alternatives (treating the disease/infestation) are limited: the fly’s flesh‑eating behavior is intrinsic and not addressable without removing the vector.
    • Arguments against / cautions:
      • Potential unknown ecological knock‑on effects and the hubris of irreversible intervention.
      • Moral concerns about participating in the sixth mass extinction and the intrinsic value of species.
      • Risk management: proponents note “undo” options (preserving stock/cultures frozen in labs) could enable future re‑introduction if needed.
    • Outcome: many participants—reluctantly—found screwworm to be among the better candidates for elimination; nevertheless, debate continues and decisions would be controversial.

Notable details and human elements

  • Edward F. Knipling’s SIT work was later honored (Golden Goose Award) as a government‑funded science success despite earlier political ridicule.
  • The reporting includes vivid, sometimes grisly descriptions of patient scenes (people removing larvae from nasal passages) and practical details of mass‑rearing operations (putrid larval diets, rearing rooms, smelly airplanes).
  • The episode stresses both scientific ingenuity and social/political complexity in applied public‑health entomology.

Key takeaways

  • The sterile insect technique is a proven, historically successful method that eradicated screwworm from much of North and Central America.
  • Screwworm is back in Central America in force (post‑2023) and has been moving toward (and into) Mexico; the U.S. is mobilizing a large response to prevent re‑establishment.
  • Gene drives could permanently eliminate screwworms, but the option raises serious ecological and ethical questions about deliberately driving a species to extinction.
  • Decisions will require weighing animal and human suffering, ecological risk, feasibility of alternatives, and the political and moral implications of intentional extinction.

Resources and further reading (from the episode)

  • Reporting by Sarah Zhang in The Atlantic on the outbreak and response.
  • Sam Kean’s podcast episode about screwworms (The Disappearing Spoon).
  • Radiolab episodes referenced: the episode’s CRISPR/gene‑drive coverage and related Golden Goose stories.
  • Official public‑health and agricultural resources (USDA, CDC, local ministries of health/agriculture) for up‑to‑date maps, precautions, and guidance if you are traveling or living in affected areas.

If you want, you can use the show’s episode description to find direct links to Sarah Zhang’s piece, Sam Kean’s episode, and official USDA/CDC pages for screwworm information.