Responding to New World Screwworm in the U.S.
The USGS National Wildlife Health Center (NWHC) is supporting the Department of the Interior’s (DOI) preparedness planning and response for New World screwworm (NWS), a foreign animal disease detected in the U.S. in June 2026 after its northward spread from Central America.
What is New World Screwworm?
New World screwworm is a fly (Cochliomyia hominivorax) that lays its eggs in the wounds of living warm-blooded animals. Within 12-24 hours the eggs hatch into larvae (maggots) which burrow into the wound to feed, creating a larger wound and leading to illness and death if left untreated. The flies primarily affect livestock but can also infect people, pets and wildlife.
New World screwworm was once native to the southern U.S. and Central America but was eradicated in the U.S. in 1966 by the targeted release of sterilized male flies (sterile insect technique). Females only mate once in their lifetime, so mating with sterilized male flies results in unfertilized eggs which reduces the population over time.
Starting in 2023, NWS began to move north from Panama through Central America. In June 2026, NWS was detected in southern Texas. Notably, isolated detections do not indicate that the parasite is established in a location because it may be possible to control isolated events through quarantine and sterile insect technique. However, NWS has the potential to significantly impact agriculture and vulnerable wildlife species if it re-establishes in the U.S. The U.S. Fish and Wildlife Service National Wildlife Refuge System is urging DOI personnel in southern states, particularly those that border Mexico, to be on high alert for NWS in wildlife.
What is NWHC doing for NWS?
Early detection and rapid response are critical to controlling the spread of the parasite. To help prepare for the possibility of sustained NWS re-emergence in the U.S., NWHC is developing products to inform decision-making, including an annotated bibliography and a wildlife-focused risk assessment.
The annotated bibliography, published in April 2026, summarizes scientific reports of NWS in wildlife and serves as a plain-language reference to help wildlife professionals, land managers, and policymakers quickly access key information. While this document does not replace the need to consult original studies, it is intended to provide a consolidated summary of existing information needed to inform NWS surveillance, management, and response decisions. The risk assessment, planned for publication in summer 2026, is a framework for understanding risks to wildlife and wildlife’s role in sustaining NWS given changes in wildlife populations and other factors since the disease was eradicated in the 1960s.
The NWHC is also coordinating closely with our partners in the US Department of Agriculture on data, modeling, and laboratory working groups. Through our diagnostic program, we are screening all morbidity and mortality submissions for suspect NWS larvae or wounds.
The NWHC is available for consultation to help wildlife partners navigate this emerging threat in wildlife. If you have a question regarding NWS in wildlife, please contact our Epidemiology Team at NWHC-epi@usgs.gov.
For additional information about NWS myiasis (the disease caused by NWS), see:
Animal disease: New World Screwworm (USDA)
Human disease: Clinical Overview of New World Screwworm | New World Screwworm | CDC
The USGS National Wildlife Health Center (NWHC) is supporting the Department of the Interior’s (DOI) preparedness planning and response for New World screwworm (NWS), a foreign animal disease detected in the U.S. in June 2026 after its northward spread from Central America.
What is New World Screwworm?
New World screwworm is a fly (Cochliomyia hominivorax) that lays its eggs in the wounds of living warm-blooded animals. Within 12-24 hours the eggs hatch into larvae (maggots) which burrow into the wound to feed, creating a larger wound and leading to illness and death if left untreated. The flies primarily affect livestock but can also infect people, pets and wildlife.
New World screwworm was once native to the southern U.S. and Central America but was eradicated in the U.S. in 1966 by the targeted release of sterilized male flies (sterile insect technique). Females only mate once in their lifetime, so mating with sterilized male flies results in unfertilized eggs which reduces the population over time.
Starting in 2023, NWS began to move north from Panama through Central America. In June 2026, NWS was detected in southern Texas. Notably, isolated detections do not indicate that the parasite is established in a location because it may be possible to control isolated events through quarantine and sterile insect technique. However, NWS has the potential to significantly impact agriculture and vulnerable wildlife species if it re-establishes in the U.S. The U.S. Fish and Wildlife Service National Wildlife Refuge System is urging DOI personnel in southern states, particularly those that border Mexico, to be on high alert for NWS in wildlife.
What is NWHC doing for NWS?
Early detection and rapid response are critical to controlling the spread of the parasite. To help prepare for the possibility of sustained NWS re-emergence in the U.S., NWHC is developing products to inform decision-making, including an annotated bibliography and a wildlife-focused risk assessment.
The annotated bibliography, published in April 2026, summarizes scientific reports of NWS in wildlife and serves as a plain-language reference to help wildlife professionals, land managers, and policymakers quickly access key information. While this document does not replace the need to consult original studies, it is intended to provide a consolidated summary of existing information needed to inform NWS surveillance, management, and response decisions. The risk assessment, planned for publication in summer 2026, is a framework for understanding risks to wildlife and wildlife’s role in sustaining NWS given changes in wildlife populations and other factors since the disease was eradicated in the 1960s.
The NWHC is also coordinating closely with our partners in the US Department of Agriculture on data, modeling, and laboratory working groups. Through our diagnostic program, we are screening all morbidity and mortality submissions for suspect NWS larvae or wounds.
The NWHC is available for consultation to help wildlife partners navigate this emerging threat in wildlife. If you have a question regarding NWS in wildlife, please contact our Epidemiology Team at NWHC-epi@usgs.gov.
For additional information about NWS myiasis (the disease caused by NWS), see:
Animal disease: New World Screwworm (USDA)
Human disease: Clinical Overview of New World Screwworm | New World Screwworm | CDC