New World Screwworm Outbreak Intensifies Across Central America and Mexico, Prompting Urgent International Response

Widespread Infestation Raises Alarms

The New World Screwworm (NWS), scientifically known as Cochliomyia hominivorax, a parasitic fly notorious for infesting the living tissue of warm-blooded animals, is experiencing a rapid and concerning spread across Central America and Mexico. This outbreak has prompted urgent action from health agencies and agricultural authorities across the region and internationally.

Originating in Panama and Costa Rica in 2023, the infestation has since moved northward, impacting Nicaragua, Honduras, Guatemala, Belize, and El Salvador, before reaching Mexico. As of December 9, 2025, the Centers for Disease Control and Prevention (CDC) reported more than 138,000 animal cases and over 1,000 human cases across the affected countries.

Impact on Animals and Humans

The NWS larvae, unlike typical maggots that feed on dead tissue, burrow into and consume the living flesh of their hosts. This can lead to painful wounds, extensive tissue damage, and, if left untreated, can be fatal. The parasitic fly poses a severe threat to:

• Livestock: Causing significant economic losses to the agricultural sector.
• Wildlife: Endangering various species.
• Pets: Infesting domestic animals.
• Humans: Leading to painful myiasis, with Mexico reporting 92 human cases as of November 28, 2025, including five deaths in the states of Chiapas and Campeche. A travel-associated human case was also confirmed in the U.S. in August 2025, linked to travel in El Salvador.

The outbreak's northward progression is particularly alarming, with a case detected in Sabinas Hidalgo, Nuevo León, Mexico, in September 2025, less than 70 miles from the U.S.-Mexico border.

Coordinated Eradication Efforts Underway

In response to the escalating crisis, the U.S. Department of Agriculture (USDA) and its international partners, including Mexico and Central American nations, have launched a comprehensive 'One Health' strategy. A cornerstone of this strategy is the Sterile Insect Technique (SIT), a proven method involving the release of sterile male flies to disrupt the reproductive cycle of the NWS, leading to a decline in the population.

Key initiatives include:

• Facility Expansion: The USDA is investing $21 million to upgrade a facility in Metapa, Mexico, aiming to produce an additional 60-100 million sterile NWS flies weekly. A new sterile fly dispersal facility also commenced operations in Tampico, Mexico, in November 2025.
• Existing Production: The Panama-U.S. Commission for the Eradication & Prevention of Screwworm (COPEG) facility in Panama currently produces 100 million sterile flies weekly, which are dispersed across affected areas.
• Import Restrictions: To prevent further spread, the USDA suspended imports of live cattle, bison, and horses from Mexico in May 2025. While some equine import restrictions were later adjusted with strict mitigation protocols in January 2025, broader livestock import restrictions remain in effect.
• Enhanced Surveillance: Rigorous surveillance at borders, rapid diagnostic testing, and strict animal movement controls are being implemented to detect and contain new cases.

Historical Context and Future Outlook

The New World Screwworm was successfully eradicated from the United States in 1966 and from Mexico in 1991 through extensive SIT programs. A biological barrier zone has been maintained in eastern Panama since 2006 to prevent re-infestation from South America. The current outbreak represents a significant challenge to these long-standing eradication efforts.

The collaborative efforts between the USDA, Mexican authorities, and other Central American countries are crucial in pushing the NWS population southward and preventing its re-establishment in previously eradicated areas. The focus remains on early detection, rapid response, and sustained international cooperation to mitigate the severe animal, human, and economic impacts of this resurgent parasite.