The New World Screwworm (Cochliomyia hominivorax, literally "man-eater") is a parasitic fly whose larvae feed on the living flesh of warm-blooded animals. Unlike most maggots, screwworm larvae burrow into healthy tissue around a wound, enlarging it and drawing more flies. Left untreated, an infestation can kill a full-grown steer. The U.S. eradicated it in 1966 and beat back a Florida Keys outbreak in 2017 Verify — but in June 2026 it returned to the mainland.
What it is
The screwworm is a true parasite of the living. Its life cycle runs in a tight loop:
- Egg → a female lays clusters of eggs at the edge of an open wound.
- Larva (maggot) in the wound → larvae hatch and burrow into healthy flesh, feeding as they go and enlarging the wound.
- Drops to soil → mature larvae fall to the ground.
- Pupa → they pupate in the soil.
- Adult fly → a new adult emerges to mate and find the next wound.
The name is literal: the larvae screw themselves into flesh, feeding head-down. That behavior — attacking living tissue rather than dead — is what makes this fly so dangerous.
How infection happens
Any break in the skin is an entry point: tick bites, castration and branding sites, dehorning wounds, the navel of a newborn — even mucous membranes of the nose, eyes, and genitalia. Warning signs include:
- A wound that enlarges instead of healing.
- Draining, foul-smelling discharge.
- Visible maggots deep in the wound.
- Animal distress — restlessness, isolation, headshaking.
What it infects
Screwworm threatens virtually any warm-blooded animal:
- Cattle and livestock — the highest-stakes target. Signs, prevention & response in cattle →
- Pets and humans — dogs, cats, and, rarely, people. Screwworm in pets & humans →
- Wildlife — deer and other wild mammals, which can carry it across the landscape.
The 2026 U.S. outbreak
In June 2026, the New World Screwworm crossed back into the mainland United States for the first time in decades, with confirmed cases in Texas and New Mexico. The situation is changing week to week.
How it's being stopped
The screwworm has been beaten before — with biology, not pesticide. The Sterile Insect Technique floods an area with sterile male flies so wild females mate without producing offspring, collapsing the population. It works best when responders know exactly where the outbreak is, early.
What you can do
Everyone near the outbreak has a role:
- Report suspected cases to your state animal health authority. How to report →
- Protect your animals if you run livestock. For ranchers →
- Support the response. Fund early detection →
Still have questions? The short answers to the most common ones — human risk, pet risk, symptoms, reporting — are on our FAQ.