Flesh-Eating Parasite Returns: First U.S. Human Case of New World Screwworm Detected

In a concerning development, the United States has confirmed its first human case of New World screwworm (NWS) myiasis since 2017. The infection was identified in a Maryland resident who had recently traveled to El Salvador. This marks a significant reemergence of the parasitic threat that primarily affects livestock but can also infest humans. 

The Threat of New World Screwworm

New World screwworm (Cochliomyia hominivorax) are parasitic flies whose larvae infest the living tissue of warm-blooded animals, including humans. They are attracted to open wounds and mucous membranes, where they lay their eggs. Upon hatching, the larvae burrow into the tissue, feeding and causing extensive damage. While primarily a concern for livestock, human cases, though rare, have occurred, particularly in travelers to endemic regions.

Federal Response and Surveillance Efforts

Following the confirmation of the human case, the U.S. Department of Health and Human Services (HHS) and the Centers for Disease Control and Prevention (CDC) initiated an epidemiological investigation in coordination with local health authorities. The risk to public health in the United States from this isolated case remains very low.

The U.S. Department of Agriculture (USDA) has implemented targeted surveillance within a 20-mile radius of the affected area, encompassing portions of Maryland, Virginia, and the District of Columbia. To date, all trap results have been negative for NWS. There have been no detections of NWS in U.S. livestock since the last outbreak in the Florida Keys was resolved in 2017.

Global Context and Ongoing Threat

Since 2023, NWS has been spreading northward from Central America, with significant outbreaks reported in Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and Mexico. In response, the USDA has implemented measures such as shutting down U.S. southern border ports to livestock trade and releasing sterile male flies to reduce the screwworm population.

Preventive Measures and Public Awareness

Health officials advise travelers to endemic areas to take precautions to prevent NWS infestations. These include avoiding exposure to flies in endemic areas and seeking prompt medical attention for suspected infestations. The CDC provides detailed information on New World screwworm infestations, emphasizing that the primary risk is to livestock. Human infestations are rare and typically associated with travel to endemic areas.

What This Means for the U.S.

While the recent human case of New World screwworm in the U.S. is concerning, it remains an isolated incident. Federal agencies continue to monitor the situation closely and have implemented measures to prevent the spread of the parasite. Public awareness and adherence to preventive measures remain crucial in mitigating the risk of further cases.