What is New World Screwworm (NWS)?
New World Screwworm (NWS) is a species of parasitic fly that feeds on live tissue and can cause a painful condition called myiasis. Infestations happen when the larvae (maggots) from a NWS fly infest the tissue or flesh of warm-blooded animals (e.g. humans, dogs, cats, cows, birds). NWS flies are attracted to open wounds and will lay eggs in open wounds. The name screwworm refers to the feeding behaviors of the larvae as they screw (burrow) into the wound.
  New World screwworm larvae (maggot), highlighting the mandibles (mouth parts) at the end of the body (left side in the images). Images courtesy of Centers for Disease Control and Prevention |  The New World screwworm adult fly have orange eyes, a metallic blue or green body, and three darker stripes along their backs. Image courtesy of Denise Bonilla, U.S. Department of Agriculture |
How it Spreads
New World Screwworm infestations begin when a female lays eggs on open wounds or other parts of the body in live, warm-blooded animals. It primarily affects livestock but can also infest birds and humans. The female NWS fly lays eggs in open wounds or other parts of the body such as mucous membranes (nose, eyes, or mouth) or the umbilical cord of a newborn animal. Wounds as small as a tick bite can attract a female fly who may lay 200-300 eggs at a time and up to 3,000 eggs in the 10-30 day lifespan.
Signs and Symptoms
Humans
- NWS infestations are very painful and can cause severe wounds. If you have a NWS infestation, you may see larvae (maggots) around or in an open wound. Larvae could also be in your nose, eyes or mouth. Symptoms can include:
- Unexplained skin lesions (wounds or sores) that do not heal or go away
- Skin wounds or sores that worsen over time
- Painful skin wounds or sores
- Bleeding from sores or wounds
- Feeling larvae movement within skin wounds, sores, nose, mouth or eyes
- Seeing larvae around or in an open wound or sore.
- A foul smelling odor from the site of infestation
Animals
- When NWS larvae (maggots) infest an animal, the wounds can be small to large depending on damage caused by the larvae’s sharp mouth hooks. As more eggs hatch and more larvae feed on the tissue of an animal, the larger and deeper the wound gets. Some additional symptoms in animals are:
- Foul-smelling wounds with visible maggots
- Animals biting or licking at wounds
- Lesions in navels, ears, dehorning or branding sites
- Unusual restlessness or lethargy
Endemicity/Geography
NWS is native to the Western Hemisphere and endemic in several Caribbean countries such as Cuba, Haiti, and the Dominican Republic, as well as in South America. In 2023, NWS detections increased in Panama and since then it has spread north into Central America.
Risk in the U.S.
Before 1960, NWS was a major problem in the southern U.S. costing livestock producers in the southeast $10-20 million per year and costing producers in the southwest $50-100 million per year. It was eradicated from the U.S. in 1966 using the sterile insect technique where sterile male flies are released to mate with wild females. However, there have been occasional outbreaks in southern U.S. wildlife populations and sporadic cases of NWS in the U.S. from residents traveling to endemic areas. People who travel to areas where NWS is typically found are at a higher risk. Risk factors for people who travel to NWS endemic areas are:
- Having an open wound from a scratch, cut, insect bite or from a recent surgery.
- Having a weakened immune system (e.g., from HIV, cancer treatment, or medications that weaken your immune system).
- Having a medical condition that can cause bleeding or open sores such as from skin or sinus cancer.
- Sleeping outdoors.
- If you live, work or spend extended periods of time with livestock or other warm-blooded animals in these areas.
To see the most current updates about where NWS has been detected in Mexico and if it is present in the U.S., visit the USDA’s webpage.
Prevention and Control
How to protect yourself and livestock or domestic pets:
- Keep wounds or sores clean and covered
- Please see a healthcare provider if a wound or sore is not healing or is getting worse
- Use an EPA-registered insect repellent https://www.epa.gov/insect-repellents/find-repellent-right-you
- Treat clothing and gear with products containing 0.5% permethrin
- Sleep indoors or in rooms with screens
- Protect pets and livestock from wounds
- Make sure pets traveling internationally, especially to endemic areas or outbreak areas, are inspected for screwworm prior to returning home to the U.S.
- Protect pets and livestock from other wound-causing parasites such as ticks
- Treat the umbilical cords of newborn animals and all wounds with an approved insecticide in consultation with a veterinary professional
Information for Providers
- Larvae disposal/removal. It is important to conduct a thorough exam as larvae can be hidden inside the mouth, nose and ears. Physically remove all larvae and eggs from infested tissue(s) and place all larvae in a leak-proof container of 70% ethanol or other alcohol, enough to fully submerge the larvae and eggs. This will kill and preserve the larvae for specimen submission, image capture, or disposal. Do not dispose of any larvae or eggs directly in the trash or outside on the ground. To dispose of any larvae not being sent for testing, place them in a separate leak-proof container of 70% ethanol or other alcohol, then place the container inside a zip-top plastic bag, seal the bag, and then dispose of the bag in the trash.
- Treatment. FDA has approved and conditionally approved several medications to treat NWS myiasis (infestation) in animals, but no approved medication currently exists for use in humans. Please work with a veterinary and medical professional for the most up to date guidance and treatment options. Providing appropriate wound care to affected areas, including covering wounds will help prevent infection. Use standard precautions when treating wounds and handling larvae.
- Travel History. There is currently an outbreak of NWS in Central America and Mexico, please be suspicious of recent travel to that region or to countries in the Caribbean and South America where NWS is endemic.
- How/When to Test. After considering travel history to assess exposure risk and evaluating symptoms, please follow directions for larvae disposal/removal and make sure to use standard precautions during the removal process. Contact your local health authority for further guidance.
- NWS versus other larvae infections (botflies, etc.). There are several features that can help distinguish NWS vs other larval infections.
| Feature | New World Screwworm (NWS) | Other larval infections |
| Feeding Behavior | Feed on living tissue of warm-blooded animals | Most feed on dead or decaying tissue. Human botflies infest living hosts but do not consume living tissue as aggressively. |
| Wound Characteristics | Destructive wounds, deep and painful | More localized or superficial |
| Larval Location | Deep in tissue, larvae may not always be easily visible | More superficial close to skin’s surface |
| Larval Appearance | Tapered and small with dark tracheal tube | Human botfly larvae are larger with black spine in middle segments |
| Infestation Pattern | Mass infestation with hundreds of larvae that consume tissue and attract more flies to lay eggs | Single larva for human botflies, isolated skin lesions |
| Symptoms | Intense pain with severe debilitation | Localized pain and itching |
| Geographic Risk | Western Hemisphere: endemic in South America and Caribbean with outbreak in Mexico and Central America as of 2025 | Varies by species |
Reporting
Notification to Public Health in Nevada. Please contact your local health authority if you suspect NWS. Veterinary providers suspecting NWS should reach out to the Nevada Department of Agriculture by calling (775) 353-3709.
Carson City, Douglas and Lyon Counties
Carson City Health & Human Services (CCHHS)
(775) 887-2190
Website
Clark County
Southern Nevada Health District (SNHD)
(702) 759-1300
Website
Churchill, Mineral, Eureka and Pershing Counties
Central Nevada Health District (CNHD)
(775) 866-7535
Website
Washoe County
Northern Nevada Public Health (NNPH)
(775) 328-2447
Website
All Other Counties
Nevada Division of Public and Behavioral Health (DPBH)
(775) 400-0333
Website
Resources
About New World Screwworm | CDC
Clinical Overview of New World Screwworm | New World Screwworm | CDC
New World Screwworm Outbreak in Central America and Mexico | USDA