State of California—Health and Human Services Agency
California Department of Public Health

Health Advisory

TO: Healthcare Providers, Local Health Departments, Laboratories, and Travelers

New World Screwworm Outbreak in Mexico and Central America

9/4/2025

Key Messages

New World Screwworm (NWS) is a parasitic fly whose larvae feed on living tissue and can infest mammals and birds; it largely affects livestock but can also affect pets, wildlife, and humans.

There have been increased detections of NWS in southern Mexico and Central America, where it had been previously eradicated.

The US Department of Health and Human Services (HHS) and the U.S. Centers for Disease Control and Prevention (CDC) recently announced the identification of a travel-associated case of NWS infestation in a U.S. resident who traveled to El Salvador.

NWS has not been found in livestock, pets, wildlife or in the environment in the United States (U.S.). In addition, no locally acquired cases of NWS infestation in humans have been reported to date.

The risk to human health in the U.S. remains very low.

Individuals who have traveled to areas where NWS is present in the past 10 days should contact a healthcare provider if they experience signs of myiasis (parasitic infestation by fly larvae [maggots]).

Healthcare providers who identify myiasis in a patient should ask about recent travel to a country where NWS is present and notify their local health department (LHD).

If a LHD is notified that a patient with myiasis has recently been in a country where NWS is present or has any other risk factors concerning for NWS infestation, the LHD should notify the California Department of Public Health (CDPH).

Laboratories can submit images of larvae from patients to CDC for telediagnosis or physical specimens can be shipped to CDC for confirmation.

Suspected cases of NWS infestation in any non-human animal must be reported immediately to the California Department of Food and Agriculture (CDFA).

If NWS is suspected or confirmed, do not discard live larvae in the trash. Larvae should be placed in a sealed alcohol container and disposed of as biohazardous waste, not regular waste.

Summary

CDPH is closely monitoring the increased detections of New World Screwworm (NWS) in southern Mexico and Central America. NWS may be directly introduced to California via an infested traveler or animal, or by continued spread of the NWS fly across Mexico and eventually into California. Since NWS introduced into California could propagate and spread, rapid identification, investigation, and communication will be critical for an effective response. CDC recently confirmed NWS infestation in a U.S. resident who recently traveled to El Salvador, but NWS has not otherwise been detected locally in the United States in humans, livestock, other animals, or in the environment.

Background

NWS is a parasitic fly (Cochliomyia hominivorax) whose larvae feed on living tissue and can infest mammals and birds. NWS largely affects livestock but can affect pets, wildlife, and humans. The adult fly is attracted to wounds or mucous membrane openings, where it will lay eggs. The larvae feed on healthy tissue, leading to pain, bloody discharge, and a foul-smelling odor from the site of infestation, with the potential for extensive tissue destruction and secondary bacterial infections. NWS is not directly contagious between animals or humans, as animals and humans are infested by adult flies in the environment.

Historically, NWS was present in the Southwestern and Southeastern U.S., given subtropical climates that created a favorable environment for NWS to survive. NWS was eradicated from the United States in 1966 through coordinated fly-control programs, but is endemic to South America, Cuba, Haiti, and the Dominican Republic. Since 2023, its range has expanded northward from Panama, and it has been detected in all countries in Central America and in southern Mexico. As of September 3, 2025, NWS has been identified in animals in the following seven southern states in Mexico: Campeche, Chiapas, Oaxaca, Quintana Roo, Tabasco, Veracruz, and Yucatan. In Central America and southern Mexico, there have been more than 720 human cases, including 6 deaths, and more than 111,000 animal cases.

Recommendations

Recommendations for Healthcare Providers

If you identify myiasis (parasitic infection by fly larvae [maggots]) in a patient, ask about recent travel to a country where NWS is present. These areas include countries that are currently experiencing an outbreak and those where it is endemic. Recent travel is considered travel in the 10 days before onset of clinical signs and symptoms.

Treatment is primarily through removal of all larvae from the affected site. Medications to treat or prevent secondary bacterial infections may be needed. There is no medication that can treat NWS infestation in people.

Collect all suspected NWS larvae and place them in a leak-proof container with 70% ethanol, or other alcohol (5 to 10% formalin is an acceptable alternative if alcohol is not available). Alcohol will kill the larvae and preserve them for identification.

Never discard live larvae in the trash, as larvae that are not appropriately disposed of can develop into adult flies, establish a local fly population, and promote further spread within the U.S. Larvae should be placed in a sealed alcohol container and disposed of as biohazardous waste, not regular waste.

Notify your LHD about any suspected cases; the LHD can facilitate submission of images and specimens for diagnosis.

Refer to CDC’s COCA call on NWS for more information.

Recommendations for Local Health Departments

Notify CDPH if a healthcare provider suspects NWS infestation in a person. LHDs should contact the CDPH Infectious Diseases Branch (510-620-3434) during business hours, or the CDPH Duty Officer (916-328-3605) after hours or on weekends/holidays.
Complete the Case Report Form and case investigation guide for suspected cases. These documents will be available in the CalREDIE Document Repository under the CDPH tab in CalREDIE. The completed form and investigation guide should be returned to IDB via email (idb-sss@cdph.ca.gov) or fax (916-552-9725). There is currently no CalREDIE module for NWS.

Recommendations for Laboratories

Refer to CDC's Bench Aid for New World Screwworm (PDF) for guidance on identifying NWS larvae. CDC has offered to conduct one-on-one training with laboratories that want additional assistance.
Submit images of larvae from humans to CDC for telediagnosis. Laboratories can email dpdx@cdc.gov to request diagnostic assistance.
Ship physical specimens (larvae) from humans to CDC for confirmation. Specimens can be shipped from CDPH or a local public health laboratory. CDPH must coordinate the submission if specimens are shipped directly from a local laboratory.
The Association of Public Health Laboratories (APHL) held a webinar on NWS (PDF) with CDC subject matter experts on 7/30/2025. The webinar provides details on how to identify NWS and submit images and samples. A recording of the webinar is available for viewing through the APHL Learning Center.

Recommendations for Travelers to Areas Where NWS is Present

During travel, keep any open wounds clean and covered and take precautions to avoid insect bites, as even wounds as small as insect bites can attract the NWS fly. Insect bite prevention can include wearing a hat, loose-fitted long-sleeved shirts and pants, and socks, as well as using an EPA-registered insect repellant. Consider treating clothing with products containing 0.5% permethrin. In addition, consider sleeping indoors, and protecting sleeping areas with a screen or bed net.
Be mindful that flies are active during the day and most commonly affect livestock species (e.g., cattle), and avoid activities that could increase the likelihood of contact with the NWS fly. Individuals may be exposed to the fly in an affected area if they are sleeping outside, especially during the day, or if they work or live near livestock. Visitors and volunteers at or near areas where livestock are present may also be at risk.
People who are immunocompromised (from HIV, cancer treatment or medications that suppress their immune system) or have a medical condition with skin lesions or open wounds (e.g., recent surgery or cancer) may be at higher risk of infection if visiting an area where the NWS fly is present.
Monitor your health for 10 days after returning to the U.S. Seek medical attention if you observe larvae (maggots) on your body or have a wound that may be infested. Wounds may be painful or bleeding, have a foul-smelling odor or discharge, or there may be a sensation of movement within the wound.
If you see larvae, seek medical attention immediately and do not try to remove the larvae yourself.

Reporting Animal Cases

Anyone (e.g., veterinarian, laboratory, owner) who suspects NWS in an animal must report it to CDFA's Animal Health Branch. NWS is a foreign animal disease, requiring a prompt response, and must be reported by phone (916-900-5002) or email (ahbfeedback@cdfa.ca.gov) within 24 hours of discovery. Specimens from animals or the environment will be submitted to the U.S. Department of Agriculture (USDA) for confirmatory identification.