New World Screwworm Myiasis: Protecting Human Health 

The New World screwworm (NWS) is not new.  NWS was reported as a major problem in the United States (U.S.) in 1933, though historic records indicate that cases were first documented in 1842. NWS poses a particular threat to cattle and wildlife, especially in southern states. While rare, NWS can also infest human tissue. The U.S. eliminated NWS in 1966, but there is concern about it being reintroduced in Texas. Awareness about risks and prevention will help protect livestock, pets, wildlife, and people. 

NWS is not actually a worm. It is a fly larva (maggot) that infests the tissue or flesh of warm-blooded animals (e.g., cattle, deer, and wildlife), and potentially humans. Female NWS flies lay hundreds of eggs on open wounds or mucous membranes (eyes, nose, mouth, or genital areas), which hatch into larvae that then burrow into and feed on living flesh. After feeding, the larvae drop to the ground, burrow into soil, and later emerge as adult screwworm flies (Fig. 1).

As of January 2026, the U.S. Department of Health and Human Services has reported a single, travel-related human case in the U.S. after the affected person traveled to El Salvador and then returned to Maryland.

What makes this fly different? NWS larvae eat living tissue, not dead tissue like common maggots.

Myiasis is an infestation of fly larvae (maggots) in human and animal tissue. Myiasis is not an infestation shared from person-to-person.

Risk Factors for Contracting NWS Myiasis

• Traveling to regions and countries where NWS is consistently present, such as South America, Cuba, Haiti, and the Dominican Republic OR traveling to areas currently experiencing an outbreak
• Working closely with livestock and/or wildlife animals in an area with NWS activity
• Having open sores or wounds that may attract NWS flies—for example, from a scratch, cut, insect bite, surgery, or medical conditions such as peripheral vascular disease or diabetes, etc
• Having a weakened immune system that might lead to chronic wounds that could attract NWS (e.g., HIV, cancer treatment, advanced age, medications, etc.)
• Being unhoused or sleeping outdoors in areas experiencing NWS activity
• Other factors have also been identified as risks, such as physical and mental disability, malnutrition, alcohol abuse, poor hygiene, and poor housing conditions

Recognition: Signs and Symptoms of NWS Myiasis

• Unexplained skin lesions (wounds or sores) that do not heal and that worsen over time 
• Painful skin wounds or sores; bleeding from open sores 
• Feeling larvae movement within a skin wound or sore; or in the nose, mouth, or eyes 
• Seeing maggots around or in open sores 
• A foul-smelling odor from the site of the infestation  

Note: Secondary bacterial infections can occur, which include symptoms of fever or chills.

NWS Myiasis Prevention

• Keep open wounds clean and covered. 
• Wear loose-fitting, long-sleeved shirts and pants and socks to limit areas exposed to flies.  
• Use an EPA-registered insect repellent. 
• Treat clothing and gear with products containing 0.5% permethrin. 
• Sleep indoors or in screened enclosures (e.g., tents). 

New World Screwworm Resources

• Texas A&M AgriLife Extension Screwworm web page
• The Screwworm Coalition of Texas web page
• USDA Screwworm homepage
• USDA Infographic/Brochure (PDF)
• CDC Overview (Screwworm)
• CDC Overview (Myiasis)
• CDC Clinical Overview
• Texas Tech School of Veterinary Medicine
• Hall, M. J., Wall, R. L., & Stevens, J. R. (2016). Traumatic myiasis: A neglected disease in a changing world. Annual Review of Entomology, 61(1), 159–176.
• Taylor, L. (2025). New World screwworm: Nicaragua confirms 30 human cases of flesh-eating parasite. BMJ, 388, r355.
• U.S. Department of Health and Human Services. (2025, August 25). HHS and USDA confirm singular traveler-associated New World screwworm case; Precautionary and proactive surveillance ongoing.