New World screwworm (NWS) is a parasitic fly that poses a serious threat to animals and, in rare cases, humans. Unlike typical flies, screwworm larvae feed on living tissue, causing painful and potentially severe wounds. While NWS has not yet established a presence in the United States, outbreaks in certain regions of Central America and Mexico with northward spread highlight the importance of awareness and the ability to detect cases of NWS. Prompt identification and reporting are crucial to prevent the spread of this parasite and to protect public health, livestock, wildlife, and pets. This page provides information on symptoms, reporting guidelines, prevention measures, and updates on any current outbreaks.
Human Prevention Measures
To reduce the risk of New World screwworm infection, keep all open wounds clean, properly treated, and covered. Use EPA-registered insect repellents as directed, and wear long-sleeved shirts, long pants, and closed-toe shoes in areas where flies may be present. Avoid areas with known outbreaks whenever possible. If traveling to or living in high-risk areas, use window and door screens, bed nets, and other protective coverings to limit exposure to flies. Assist with the monitoring of any family members at risk who may be unable to monitor themselves (e.g., newborns/infants).
General Information
New World Screwworm Outbreak (CDC)
CDC update on the current outbreak of New World screwworm (NWS) in Mexico and Central America, including case counts in animals and people, and U.S. risk status.
New World Screwworm: What You Need to Know (CDC)
Overview of what NWS is, how it spreads, risk factors, key signs and symptoms in humans and animals, and why early detection and reporting are critical to preventing spread.
New World Screwworm: Life Cycle, Distribution, and Identification Poster (USDA)
Illustrated overview of NWS life cycle, transmission, distribution, and larval identification features to support early detection and prevention.
New World Screwworm: Guidance for Hunters (USDA)
USDA information on identifying NWS in wildlife, protecting yourself during hunting activities, and reporting suspected cases
New World Screwworm: International Travel with Pets (USDA)
USDA guidance on protecting dogs and cats, meeting U.S. entry requirements, and recognizing signs of NWS when traveling internationally
Healthcare Provider Resources
Note: Healthcare providers who suspect a human case of NWS should immediately contact the South Dakota Department of Health (SD-DOH) at 605-773-3737 for guidance on specimen handling, reporting, and coordination with CDC.
New World Screwworm: Recommendations for Healthcare Providers (CDC)
Clinical guidance for evaluating suspected cases, wound assessment considerations, reporting requirements, and coordination with public health authorities.
Webinar - Resurgence of New World Screwworm in the Americas: What Healthcare Providers Need to Know (CDC)
Telediagnosis of New World Screwworm (CDC)
Guidance for specimen collection, handling, submission, identification of larvae, and laboratory confirmation procedures.
Animal & Agricultural Resources
Note: Detection of New World screwworm (NWS) in domestic animals or livestock should be reported immediately to the South Dakota Animal Industry Board (AIB). Rapid reporting helps prevent the spread and protects South Dakota’s livestock, domestic animals, and wildlife.
New World Screwworm – Myth vs. Fact (USDA)
USDA factsheet clarifying common misconceptions about NWS, including transmission, food safety, livestock management, and U.S. preparedness efforts
APHIS: New World Screwworm Story Map (USDA)
USDA APHIS interactive resource outlining outbreak history, response efforts, geographic spread, and prevention strategies to protect livestock and wildlife.
Resource for Agriculture Producer: English | Spanish
Resource for Agriculture Employees: English | Spanish
Report Suspected Animal Cases:
South Dakota Animal Industry Board: 605-773-3321
https://aib.sd.gov/