The New World screwworm (NWS) is endemic in the American tropics and subtropics which includes the countries of South America, Cuba, Haiti, and the Dominican Republic. However, Central America and Mexico are experiencing an outbreak of NWS with cases in both animals and humans for the first time in decades. ONLY ONE CASE has been seen in the U.S. and that was in a man returning from El Salvador. The infestation is a type of myiasis that occurs when NWS fly larvae (Cochliomyia hominivorax) infest the living flesh of warm-blooded animals—more commonly livestock and less commonly birds, pets, and humans. The screwworm flies are attracted to and lay their eggs on and in open wounds and mucous membranes. Note that what makes this infestation different from those we have all seen with the common housefly is that the screwworm eats LIVING TISSUE. This is the “Alien” version of maggot infestation. NWS larvae cause extensive damage by tearing into the host’s tissue with sharp mouth hooks. Bacterial superinfection can also occur because of the NWS infestation.
Practitioners in states that border Mexico should be aware of this possibility, but given the possible impact on returning travelers, the advisory is relevant to all wound care practitioners. Healthcare providers who suspect a patient has an NWS infestation, report it immediately to your local or state health department – I was surprised to learn that the CDC can provide telemedicine assistance! Diagnostic confirmation for suspected human cases is available through CDC’s Diagnostic Parasitology Laboratory, DPDx, at dpdx@cdc.gov. DPDx may also be able to telediagnose cases if photographs capture distinctive features of NWS larvae. Consult this laboratory bench aid for instructions and photo guidelines.
(And for goodness’ sake, if you get a case, share PHI redacted photos with us. We are wound care practitioners and we can’t help ourselves…)
Wound inflicted by NWS larvae infestation in a patient’s lower leg. Image c/o Dr Isaac Bogoch
Symptoms can include painful, non-healing and foul-smelling wounds. High risk patients include:
- Those living in rural areas in regions or countries where NWS is endemic- or traveling back from countries currently experiencing an outbreak, and where livestock are raised.
- People who frequently work with livestock.
- Anyone with open sores or wounds (meaning, all of our patients)
- Vulnerable populations, including people who are immunocompromised, those at extremes of age, and people experiencing malnutrition (also meaning all of our patients)
Key Points:
- New World screwworm (NWS) myiasis is typically a disease of livestock but can also affect humans.
- NWS is endemic in South America, Cuba, Haiti, and the Dominican Republic.
- Recently, Mexico and countries in Central America where NWS was previously controlled have been experiencing an outbreak and are reporting cases in both livestock and humans.
- Currently, the New World screwworm (NWS) fly is not present in the United States
- In this current outbreak period, the only U.S. case (August 2025) was in a man returning from El Salvador.
- NWS occurs in people with open wounds; it can also occur in other body cavities with mucus membranes (e.g., nasal passages).
- Infestations by NWS maggots can cause painful and foul-smelling wounds. If not caught early and treated, it can lead to extensive tissue damage, and potentially death.
- There is no medication to treat NWS; prevention and quick removal are key.
- Healthcare providers should remove the NWS larvae from the affected site.
- Kill and preserve the larvae by placing it directly into concentrated (70%) ethyl or isopropyl alcohol.
- Send all suspected NWS specimens to CDC for identification and diagnosis.
- Report cases to health officials
- Get help from the CDC: Diagnostic confirmation for suspected human cases is available through CDC’s Diagnostic Parasitology Laboratory, DPDx, at dpdx@cdc.gov. Consult this laboratory bench aid for instructions and photo guidelines
Specimen Handling & Submission
- Submit larvae for clinical diagnosis and confirmation at CDC.
- Send at least 10 larvae. If you have fewer than 10, submit them all.
- If you have more than 10, send at least 10.
- If multiple stages of larvae are present in the lesion, try to include a representative sample from each stage. Email dpdx@cdc.gov for specimen submission instructions.
- Place larvae and eggs in a leak-proof container with 70% ethanol. The volume of liquid should be sufficient to fully submerge larvae and eggs.
- The ethanol will both kill and preserve them for identification.
- 70% (or greater) isopropanol or 5% – 10% formalin are acceptable alternatives.
- Do not dispose of any larvae or eggs in the trash or outside on the ground.
- Collect the remaining larvae and eggs in a separate leakproof container, submerge them in alcohol, place the container into a zip-top plastic bag, and seal it. Dispose of the sealed bag in the trash.
Resources:
CDC
- New World screwworm – What You Need to Know
- New World screwworm – Recommendations for Healthcare Providers
- Laboratory Diagnosis of New World Screwworm Bench Aid (English and Spanish)
- New World screwworm life cycle
USDA/APHIS
- New World Screwworm: What You Need to Know (English and Spanish)
- New World Screwworm
- APHIS: New World Screwworm Story Map
Country-specific information on NWS
- COPEG: Current Situation
- HHS and USDA Confirm Singular Traveler-Associated New World Screwworm Case; Precautionary and Proactive Surveillance Ongoing | HHS.gov
- First human case of New World screwworm parasite confirmed in US
- New World Screwworm: Outbreak Moves into Northern Mexico | HAN | CDC
- New World Screwworm Outbreak | New World Screwworm | CDC
- USDA Announces Sweeping Plans to Protect the United States from New World Screwworm
The post CDC Alert About New World Screwworm (NWS) Which Can Present as Painful, Malodorous Wounds appeared first on Caroline Fife M.D..