New World Screwworm Insights and Action Steps

September 11, 2025 | Shalini Nair

CDC, in coordination with the Maryland Department of Health, recently confirmed a human case of New World screwworm (NWS) in the United States associated with the ongoing outbreak in Central America. Prior to late 2023, NWS was present in South America and certain countries of the Caribbean, and a biological barrier in eastern Panama prevented the pest from spreading through Central America. Although the United States has experienced travel-associated cases from countries where NWS is endemic in the past, this is the first travel-associated case from a country affected in this outbreak.

The parasite, most often seen in animals (especially livestock), was confirmed on Aug. 4 in a Maryland resident with recent travel to El Salvador — a country affected by the current outbreak. While the risk to public health in the United States remains low at this time, it is important for state and territorial health officials to stay informed and prepared for this evolving threat.

What Is New World Screwworm?

New World screwworm, or Cochliomyia hominivorax, is a parasitic fly whose larvae (maggots) feed on healthy tissue. NWS flies are attracted to and lay eggs on and inside of open wounds, which leads to myiasis, or a parasitic infestation in which fly larvae burrow into the flesh of the affected host. While NWS can affect various warm-blooded animals, most commonly livestock and wildlife, it does not spread between humans and animals. Further, the fly is not a carrier of vector-borne disease. Risk factors for contracting NWS myiasis include:

• An open wound (even wounds as small as a tick bite may attract the flies).
• A weakened immune system.
• A medical condition that contributes to bleeding or open sores.
• Spending extended amounts of time outdoors or near livestock in areas where NWS flies are present.

NWS infestation can be very painful. In humans, symptoms may include unexplained skin wounds or lesions that worsen over a few days, bleeding from an open sore, the presence or feeling of maggots around or in open wounds, and a foul-smelling odor at the infestation site. Larvae can also be present in the nose, eyes, mouth, or ears. In animals, signs may include irritated behavior, head shaking, presence of larvae in wounds, and the smell of decay; animals may also stop eating and self-isolate. There is currently no drug-only cure for NWS but effective treatment consists of quick and thorough removal of larvae. The USDA also maintains a list of EPA-registered pesticides to use against NWS on pastures, agricultural buildings, livestock, and other animals.

Framework for Outbreak Prevention

NWS was formerly eradicated from the United States in the 1960s, following a large-scale, coordinated effort and the creation of a “barrier” zone between NWS-endemic South America and NWS-free Central and North America. While subsequent outbreaks have occurred since then, the last confirmed (and later eradicated) U.S. outbreak of NWS was in the Florida Keys in 2016, affecting wild Key deer. However, NWS has slowly migrated northward towards the southern U.S. border since the emergence of a new outbreak in Panama in 2023.

In the 1950s, USDA developed a successful method for eradicating NWS, referred to as sterile insect technique (SIT). This method involves the mass-rearing and release of sterilized male flies into infested areas to mate with wild females, resulting in nonviable eggs and a vast reduction in the target fly population. It has been used in the United States and across North and Central Mexico and was successful in maintaining eradication until the outbreak in recent years.

Addressing the Current Outbreak

USDA and HHS have initiated proactive measures to address the current threat to the United States. This includes CDC collaborating with health care professionals, state and local health departments, and tribal organizations to prepare for the potential arrival of NWS in the United States and developing clinical guidance and other resources for health care and public health partners on how to identify and respond to NWS myiasis cases in humans.

Additionally, USDA is taking comprehensive action in coordination with U.S. Government partners to protect the United States and prevent the further spread of NWS. In May, USDA suspended imports of cattle, horses, and bison from Mexico, following recent detections of NWS as close as 370 miles from the border. In June, USDA announced the construction of an $8.5 million sterile NWS fly production facility in South Texas, aimed at bolstering their ability to control and eliminate this pest. Also, the new NWS Domestic Readiness and Response Policy Initiative outlines a five-pronged plan to mitigate the threat of NWS. In response to the travel-associated human case, USDA has initiated targeted surveillance for NWS in nearby areas of Washington, D.C., Maryland, and Virginia. As of September 5, no trap results have come back positive for NWS flies.

Need-to-Know Information for Health Officials

Veterinary Considerations

Currently, the potential threat of NWS looms largest for livestock such as cattle, horses, and pigs — although no infestations in animals have been identified in the United States since the outbreak in Key Deer was eradicated in 2017. Coordination between public health agencies and the agricultural and animal health sectors is strongly encouraged as part of a One Health approach to strengthening disease surveillance. Veterinarians should report any suspicious cases in any animal species immediately to their state animal health official and to USDA-APHIS. HHS recently authorized FDA to issue emergency use authorizations for drugs to treat infestations in animals, should they be necessary. While no FDA-approved drugs currently exist for treating NWS infestation in animals in the United States, this may expedite the use of drugs approved for other purposes or those available in other countries to treat NWS-infested animals.

Food Safety

NWS is not transmitted through consuming appropriately cooked meat or poultry products. In addition, all livestock used for food production in the United States must pass inspection both before and after slaughter, and the presence of infestations or treatment residues that deem meat unsafe for human consumption will prevent the affected product from entering the food supply.

Current Human Epidemiologic Situation

Over the years, the United States has experienced occasional travel-associated cases of NWS in people traveling from endemic countries; however, the confirmation of a travel-associated case of NWS in Maryland marks the first human case of this parasitic infestation in the United States from the outbreak area. The patient has since recovered, and officials confirmed that there is no indication that the infestation spread to other humans or animals.

At this time, there are no active human cases in the United States. The risk to public and traveler health in the United States remains low, and cases of NWS in humans remain much lower than animal cases in countries affected by the current outbreak in Mexico and Central America. Public health officials are encouraged to promote clinician awareness of NWS and consider developing coordinated public communications resources as necessary.

Diagnosing and Reporting Suspected Human NWS Infestations

Health care providers should remain aware of the risks in patients who have traveled to areas affected by the current outbreak, in addition to those where NWS is endemic. If a human case is suspected, providers should report it immediately to their local or state health department for further investigation. Any maggots found in suspected cases should be placed into a leak-proof container filled with 70% ethanol for proper disposal, and providers should contact CDC’s DPDx team for further specimen submission instructions. Proper disposal of suspected NWS larvae is critical to preventing the parasite from spreading to the environment. If a suspect case is identified, contact CDC (newworldscrewworm@cdc.gov) to obtain the case report form and case investigation guide.

ASTHO will continue to monitor developments on this emerging public health issue.

Supplemental Resources

Overview of NWS – CDC

• About New World Screwworm
• Clinical Overview of New World Screwworm
• Acerca de la miasis por el gusano barrenador del Nuevo Mundo
• Información clínica sobre la miasis por el gusano barrenador del Nuevo Mundo
• Lab Identification of New World Screwworm (PDF)

Local/State Public Health Departments

• Epi On Call by CSTE

Clinical or Diagnostic Assistance – CDC

• Clinical inquiries and patient management related questions: parasites@cdc.gov or 404-718-4745. Direct after-hours inquiries to CDC’s Emergency Operations Center at 770-488-7100.
• Diagnostic assistance for suspected human cases: dpdx@cdc.gov.

USDA

• New World Screwworm
• New World Screwworm: What You Need to Know PDF (English and Spanish)
• New World Screwworm Story Map
• Screwworm: An International Threat to Human and Animal Health (PDF)

Veterinarians

• Report any suspicious cases immediately to your state animal health official and APHIS office.

Insect Bite Prevention – CDC

• Preventing Mosquito Bites
• Preventing Mosquito Bites While Traveling
• About Permethrin-Treated Clothing and Gear
• How to Prevent Mosquito and Tick Bites

Insect Repellents

• Repellents: Protection Against Mosquitoes, Ticks and Other Arthropods

This product was reviewed by ASTHO Chief Medical Officer, Susan Kansagra.