New World Screwworm: Outbreak Moves into Northern Mexico

New World screwworm (NWS), “gusano barrenador del Nuevo Mundo” in Spanish, is a devastating pest. NWS myiasis occurs when NWS flies lay eggs in wounds or in other body cavities with mucus membranes, such as the nose, ears, eyes, or mouth. The eggs develop into parasitic larvae (maggots) that feed on and burrow into living flesh. Although they primarily affect livestock, such as cattle and horses, the flies can also lay eggs on people and other warm-blooded animals, both domestic and wildlife. Cases of NWS in people can be fatal if left untreated.

An outbreak of NWS infestations in people and animals has developed across Central America and Mexico. During this outbreak, more than 1,190 cases and seven deaths in people have been reported in Central America and Mexico as of January 20, 2026. Mexico has reported 24 hospitalizations among people and 601 active cases among animals. (An animal case is defined as “active” at the time of diagnosis and becomes inactive if officials verify the absence of new wounds or larvae after 15 days.) The Mexican state of Tamaulipas, which borders the U.S. state of Texas, reported eight active animal cases. The NWS fly has not been detected in the United States and risk to people remains low. For more information about the more than 148,000 animals affected by the outbreak, please see Current Status of New World Screwworm | Screwworm.gov.

NWS was previously a significant economic pest of livestock in the United States. The United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) developed an eradication strategy based on releasing sterile male flies. Since the female NWS fly only mates once in her 21-day lifespan, mating with a sterile male fly means any resulting eggs are not fertilized and no larvae can hatch. This strategy was successful in eradicating NWS from the United States, Mexico, and southward through Central America. The United States successfully used the strategy again in 2017 following a 2016 re-introduction of NWS into Key deer in the Florida Keys.

Since 2006, animal health and agricultural officials had maintained a biological barrier at the Darien Gap along the Panama and Colombia border, where sterile flies were continuously released to prevent reintroduction into Central America. In 2023, Panama reported a spike in cases of NWS—more than 9,300 in one year. Unregulated cattle movement, increased movement through the Darien Gap, and new areas of farming contributed to rapid northward spread of NWS. In addition to this increased activity in Central America and Mexico, NWS has remained persistent in South America and the Caribbean, including in Cuba, Haiti, and the Dominican Republic.

The USDA’s APHIS current strategy to control and re-eradicate NWS includes

1. Releasing sterile male flies.
2. Developing and enforcing animal movement controls.
3. Increasing passive and active surveillance, outreach, and education in impacted areas.

An additional and important part of this strategy includes rapid identification and appropriate clinical management of infestations in people. Timely public health response allows for prompt investigations and control measures to prevent onward spread outside of the area where sterile flies are being released. CDC is collaborating with federal, state, and local agencies and international organizations to assist with efforts to control the spread of NWS through public health action.

People might be at increased risk for developing NWS if they spend time in geographic areas where NWS flies are present and

• Have an open wound, including scratches, cuts, insect bites, or surgical wounds.
• Have a medical condition that might affect how quickly a wound heals or that may cause bleeding or open sores (e.g., skin cancer, psoriasis, or eczema).
• Are very old or very young.
• Experience malnutrition.
• Sleep outdoors, especially during daytime hours when NWS flies are more active.
• Live, work, or spend an extended amount of time near livestock or other warm-blooded animals in these areas.

Clinical presentation and guidance

• Consider NWS in people who present with any of the following:
  • Visible larvae or egg masses in a wound, ears, eyes, nose, mouth, or other body orifice (e.g., genitals).
  • Destruction of healthy tissue.
  • Sensation of movement, foul odor, bloody discharge, swelling, and pain.
  • Recent travel to regions where NWS is present
• In areas where NWS is present, advise patients to:
  • Clean and cover all wounds, no matter how small or the location on the body.
  • Wear loose-fitting, long-sleeved shirts and pants, socks, and hats to limit exposed skin and use Environmental Protection Agency (EPA)-registered insect repellents.
  • If possible, avoid spending time where livestock are located or housed in rural areas.
  • Avoid sleeping outdoors, especially during daytime hours when NWS flies are most active.
  • Use bed nets to protect sleeping quarters from insects if windows are left open and not screened. In hospitals, screening windows and doors is essential.

Reporting to public health

• Immediately report all suspected human cases to your state, tribal, local, or territorial health department, and work with your health department to seek confirmatory diagnosis with CDC’s Diagnostic Parasitology Lab (DPDx).
• Direct clinical inquiries and patient management-related questions to your health department or to CDC’s Parasitic Diseases Hotline (404-718-4745 or parasites@cdc.gov) during business hours, or to CDC’s Emergency Operations Center (770-488-7100) after regular business hours.
• Non-clinical questions can be directed to newworldscrewworm@cdc.gov.

Patient management and specimen submission

• For patients with larval infestations in a lesion:
  • In consultation with your health department and CDC, collect and submit larvae from patients to CDC for clinical diagnosis and confirmation. Many morphologically similar species of flies can cause myiasis; submitting larvae is critical for species identification. Diagnostic confirmation for suspected human cases is available through CDC’s Diagnostic Parasitology Laboratory (DPDx).
  • Submit at least 10 larvae to CDC.
    • If you have fewer than 10 larvae, submit them all.
    • 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.
    • It may be possible to make a diagnosis more rapidly through telediagnosis, which is available by contacting dpdx@cdc.gov.
  • Promptly remove and kill all larvae and eggs from patients with suspected NWS. This usually requires physical removal. Failure to kill and properly dispose of all larvae or eggs could result in the new introduction and spread of NWS in the local environment.
    • Place larvae and eggs in a leak-proof container containing a volume of liquid sufficient to fully submerge larvae and eggs.
    • 70% ethanol (preferred) will both kill and preserve the larvae and eggs for identification. Alternative liquids include 70% (or greater) isopropanol or, if no alcohol is available, 5%–10% formalin is acceptable.
    • Collect any 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.
• The treatment of NWS in humans is removal of all eggs and larvae, which might require surgical extraction if the larvae are embedded deeply into tissues. There have not been any studies to prove that any specific medication is useful in treatment in humans. Patients with secondary infections or additional symptoms may need supportive care.
• Reexamine treated lesions after 24–48 hours to confirm no live larvae remain. Remove and safely dispose of any remaining larvae as described above.

• Emphasize to medical professionals and the community the importance of promptly removing and killing all larvae and eggs from suspected NWS cases. Dispose of larvae and eggs properly. Do not dispose of any larvae (maggots) or eggs in the trash or outside on the ground.
  • Place the larvae and eggs in a leak-proof container containing a volume of liquid sufficient to fully submerge larvae.
  • 70% ethanol (preferred) will both kill and preserve the larvae and eggs for identification. Alternative liquids include 70% (or greater) isopropanol or if no alcohol is available, 5%–10% formalin is acceptable.
  • Advise healthcare providers to collect at least 10 larvae to submit for confirmation at CDC.
    • If you have fewer than 10 larvae, submit them all.
    • If multiple stages (or species) of larvae are present in the lesion, try to include a representative sample from each stage (or species). Email dpdx@cdc.gov for specimen submission instructions.
    • It may be possible to make a diagnosis more rapidly through telediagnosis, which is also available by contacting dpdx@cdc.gov.
    • Collect any remaining larvae and eggs in a separate leakproof container, submerge them in 70% ethanol, place the container into a zip-top plastic bag, and seal it. Dispose of the sealed bag in the trash.
• Human infestations might indicate local circulation of flies and possible infestations in animals. Alert your state animal health official (SAHO) so they can monitor for NWS flies and take steps to stop further spread in coordination with agriculture and public health officials.
• Consider directed outreach to and surveillance of communities with people at higher risk of NWS, including individuals with unstable housing and agricultural workers.
• For any NWS cases identified in humans, complete the case reporting form available on the One CDC Data Platform (1CDP) and submit to CDC (newworldscrewworm@cdc.gov). If you need access to 1CDP or have trouble accessing the case report form in 1CDP, please email newworldscrewworm@cdc.gov.
• Report all suspected NWS cases in animals and pets to the state animal health official and APHIS office.
• Direct clinical inquiries to CDC’s Parasitic Diseases Branch (404-718-4745 or parasites@cdc.gov) during business hours or CDC’s Emergency Operations Center (770-488-7100) after regular business hours. Non-clinical questions can be directed to newworldscrewworm@cdc.gov.
• Diagnostic assistance for suspected human cases is available through CDC’s Diagnostic Parasitology Laboratory, DPDx, at dpdx@cdc.gov.

• Ensure all larvae are placed in a leak-proof container containing a volume of liquid sufficient to fully submerge larvae. 70% ethanol (preferred) will both kill and preserve the larvae and eggs for identification. Alternatives include 70% (or greater) isopropanol or 5% – 10% formalin. Do not dispose of any larvae (maggots) or eggs in the trash or outside on the ground.
• Immediately report suspect human NWS cases to your state, tribal, local, or territorial health department.
• Immediately report suspect animal NWS cases to state animal health official and APHIS office.
• For diagnostic questions about human NWS, contact CDC’s Diagnostic Parasitology Laboratory, DPDx, (dpdx@cdc.gov) or Parasitic Diseases Branch (404-718-4745 or parasites@cdc.gov) during business hours or CDC’s Emergency Operations Center (770-488-7100) after regular business hours. Non-clinical questions can be directed to newworldscrewworm@cdc.gov.

• Prevention is key to protecting yourself from NWS in areas where the NWS fly is present.
  • Keep open wounds clean and covered, no matter how small or location on the body.
  • Wear loose-fitting, long-sleeved shirts and pants, socks, and hats to limit areas where you could get bitten by insects or scratched.
  • Prevent insect bites, especially when visiting areas where NWS flies are present and spending time outdoors.
  • Use an EPA-registered insect repellent.
  • Treat clothing and gear with products containing 0.5% permethrin.
  • Sleep indoors; if the room has windows, the windows should be screened. If you are outside, sleep under a bed net or inside a screened tent.
• If you see or feel maggots (larvae) in or on a wound or other area of your body, seek medical care immediately.
• Do not try to remove any maggots or egg masses yourself. Do not throw any larvae (maggots) or eggs in the trash or outside on the ground as this could result in NWS spreading in your area. If maggots or eggs fall out of the wound, cover them with alcohol (70% ethanol) in a leak-proof container and bring them to your healthcare provider.
• Contact your state, tribal, local, or territorial health department if you have questions about NWS.

• Immediately report all suspected NWS cases in animals to your state animal health official and APHIS office.
• Larvae and eggs from NWS cases in animals can be submitted to USDA’s National Veterinary Services Laboratory for identification. See guidance from APHIS on specimen submission.
• The Food and Drug Administration (FDA) has issued an Emergency Use Authorization for the use of Credelio (lotilaner) chewable tablets to treat NWS infestations in dogs and cats.
• For animals with larval infestations in a lesion:
  • Promptly remove and kill all larvae and eggs in suspected NWS cases. This might require physical removal. Failure to kill and properly dispose of all larvae or eggs could result in the new introduction and spread of NWS in the local environment.
    • Place larvae and eggs in a leak-proof container containing a volume of liquid sufficient to fully submerge larvae and eggs.
    • 70% ethanol (preferred) will both kill and preserve the larvae and eggs for identification. Alternative liquids include 70% (or greater) isopropanol or if no alcohol is available, 5%–10% formalin is acceptable.
    • Collect any 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.
    • Do not dispose of any larvae or eggs in the trash or outside on the ground.
• See guidance from APHIS on disposal of larvae and eggs and cleaning and disinfesting affected premises.

About New World screwworm

• About New World Screwworm | CDC / In Spanish: Acerca de la miasis por el gusano barrenador del Nuevo Mundo | El gusano barrenador del Nuevo Mundo | CDC
• New World Screwworm Outbreak | CDC
• Clinical Overview of New World Screwworm | CDC / In Spanish: Información clínica sobre la miasis por el gusano barrenador del Nuevo Mundo | CDC
• Resurgence of New World Screwworm in the Americas: What Healthcare Providers Need to Know | COCA | CDC
• Laboratory Identification of New World Screwworm (bench aid) | CDC / In Spanish: Identificación en laboratorio del gusano barrenador del Nuevo Mundo (bench aid) | CDC
• Telediagnosis of New World Screwworm (bench aid) | CDC
• gov | Stop Screwworm: Unified Government Response to Protect the United States | USDA
• New World Screwworm | USDA
• New World Screwworm: What You Need to Know (brochure) | USDA / In Spanish: Gusano barrenador del Nuevo Mundo: Lo que necesita saber (folleto) | USDA
• New World Screwworm (story map) | USDA
• Screwworm: An International Threat to Human and Animal Health (poster) | USDA
• New World Screwworm Draft Response Playbook | APHIS | USDA

Where to report cases of New World screwworm

• Human cases: local and state health departments (EPI On Call)
• Animal cases: state animal health official and APHIS office

Preventing insect bites

• Preventing Mosquito Bites | CDC
• Preventing Mosquito Bites While Traveling | CDC
• About Permethrin-Treated Clothing and Gear | CDC
• How to Prevent Mosquito and Tick Bites | CDC
• Repellents: Protection against Mosquitoes, Ticks and Other Arthropods | EPA