New World Screwworm & Human Health

The New World screwworm is not a worm at all. It is the larva (maggot) of a type of fly. These larvae can enter open skin or soft tissue of warm‑blooded animals—like cattle, deer, pets, wildlife, and sometimes people.

Female flies lay their eggs on open wounds or in moist areas like the eyes, nose, mouth, or genital areas. When the eggs hatch, the larvae burrow into living flesh, which can cause serious harm. After feeding, they fall to the ground, grow in the soil, and later become adult flies.

To stop the spread, experts release sterile male flies, which prevent new screwworms from being born. This method has been very successful for decades at eliminating screwworms

Human cases are rare, but they can happen. Knowing the signs and how to prevent infection helps protect people, pets, livestock, and wildlife.

The screwworm spreads through myiasis, a condition where fly larvae enter human or animal tissue. It cannot spread from person to person.

You may be at higher risk if you:

• Travel to places where screwworms are common, such as South America, Cuba, Haiti, or the Dominican Republic, or to areas with an active outbreak
• Work closely with livestock or wildlife
• Have open cuts, sores, or wounds from injuries, surgery, or medical conditions
• Have a weakened immune system, which can slow wound healing
• Are unhoused or sleep outdoors in areas where screwworms are active
• Experience challenges such as poor housing, poor hygiene, malnutrition, disability, or alcohol abuse

Watch for:

• Wounds or sores that don’t heal and get worse over time
• Painful or bleeding skin sores
• Feeling movement inside a wound or in the nose, mouth, or eyes
• Seeing maggots in or around a sore
• A bad smell coming from the wound

A bacterial infection may also develop, causing fever or chills.

You can lower your risk by:

• Keeping all wounds clean and covered
• Wearing long sleeves, long pants, and socks to reduce exposed skin
• Using an EPA‑registered insect repellent
• Treating clothing or gear with 0.5% permethrin
• Sleeping indoors or in screened tents or shelters

The New World screwworm caused major problems in the United States in the early 1900s. It was recognized as a serious issue in 1933, though cases were recorded as early as 1842. The U.S. successfully eradicated the pest in the 1960s. Since then, it has sometimes reappeared in Mexico and Central America, underscoring the importance of ongoing monitoring.

Healthcare providers should know how to identify, treat, test, and dispose of suspected New World screwworm cases

• Specimen Handling and Submission

• Confirmed Detections of New World Screwworm | Animal and Plant Health Inspection Service

• NWS Zones Interactive Map

• CDC New World Screwworm Overview
• CDC Clinical Overview
• USDA New World Screwworm
• Oklahoma Department of Agriculture, Food and Forestry
• Oklahoma Department of Wildlife Conservation
• OSU Extension New World Screwworm