What are Screwworms? Explaining the Flesh-Eating Parasite

A chilling medical alert has surfaced: the New World screwworms, parasitic flesh-eating larva, has been confirmed in a human in Maryland, the first such case in the U.S. in decades. The incident spotlights not only rare human infections but also the parasite’s deeply destructive life cycle and threat to livestock, agriculture, and public health.

What Are Screwworms? Anatomy of a Flesh-Eating Fly

Screwworms are the larvae of the New World screwworm fly (Cochliomyia hominivorax), a species notorious for burrowing into living flesh. These maggots feed aggressively, causing serious tissue damage, a process called myiasis. Female screwworm flies lay eggs in wounds or natural openings in warm-blooded animals, including humans. Once hatched, larvae penetrate and burrow deeper into the flesh, feeding voraciously for several days before pupating in the soil.

Life Cycle at a Glance:

1. Eggs are deposited in open wounds.
2. Larvae emerge and feed on live tissue for 3–7 days.
3. They drop to the ground to pupate, emerging as adult flies within about 20 days.
   A single female can lay up to 3,000 eggs and travel up to 200 km in its lifetime.

First U.S. Human Case Confirmed in Decades

On August 25, 2025, the U.S. Department of Health and Human Services confirmed the first travel-associated human case of screwworm myiasis, a Maryland resident who recently returned from El Salvador (reports initially cited Guatemala), marking a rare return of this dangerous parasite to U.S. public health focus.

• The CDC confirmed the infection, and the patient received treatment; fortunately, screwworm infestations in humans are uncommon and treatable when detected early.
• U.S. officials emphasize that public health risk remains very low, though surveillance and precautions are being heightened.

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Agricultural and Economic Stakes

Screwworms are particularly destructive to livestock. The U.S. eliminated NWS in the 1960s through an innovative sterile insect technique (SIT), releasing sterilized male flies to disrupt breeding. That success extended to quelling a small outbreak in Florida’s Keys in 2017.

However, recent resurgence in Central America and Mexico has triggered major response efforts:

• The USDA is constructing a sterile fly production facility in South Texas, aiming to produce hundreds of millions of sterile males weekly.
• Authorities have also suspended livestock imports from Mexico, and experts warn a widespread resurgence could cost billions, potentially $1.8 billion in Texas alone and more broadly up to billions for the U.S. economy.

Signs, Prevention & Control

Symptoms of Screwworm Myiasis involve smell of decay, obvious maggot infestation in wounds, irritation, and tissue destruction. Prompt medical and veterinary attention is critical.

Prevention Checklist:

• Inspect and clean wounds immediately, especially in travelers returning from endemic areas.
• Carefully monitor livestock birth wounds and treat immediately, navel infections in newborns are high-risk entry points.
• Report any suspected cases to the CDC or USDA for prompt mitigation.

A Rare Human Danger, Major Agricultural Threat

Screwworms remain a rare direct threat to people, but their potential for devastating livestock losses cannot be overstated. The recent Maryland case serves as a stark reminder of the parasite’s persistence and reach, and renews urgency for vigilance, research, and coordinated response across health and agricultural sectors.

Understanding screwworms, their biology, risks, and preventive strategies, is essential for maintaining both public and animal health in an interconnected world.