Abstract

The New World screwworm (Cochliomyia hominivorax; NWS) — a devastating obligate myiasis-causing fly eradicated from the United States in the 1960s — re-emerged as a near-border threat in 2024–2025. The resurgence prompted emergency animal health responses across U.S. federal agencies, including stepped-up surveillance, temporary trade restrictions, investments to expand sterile-insect production, and regulatory action to enable rapid access to therapeutics. This paper summarizes the epidemiology of the current NWS threat, details USDA and HHS/FDA actions (including a planned U.S. sterile-fly production and dispersal facility), summarizes the FDA’s emergency use authority and initial EUAs/conditional approvals for drugs to treat animals (including a recent EUA directed at dogs), and reviews recommended clinical treatments for cattle, companion animals, wildlife and other at-risk species.

Introduction

The New World screwworm (NWS) is an obligate parasitic fly whose larvae invade and consume living tissue in wounds of warm-blooded animals and occasionally humans. Historically, a U.S. eradication campaign using the sterile insect technique (SIT) succeeded in eliminating reproducing NWS populations from the continental United States by the late 1970s. Beginning in the early 2020s and accelerating in 2024, NWS populations expanded northward from endemic regions in South and Central America, producing outbreaks in parts of Mexico and increasing risk of incursions into southern U.S. states (AP News, 2025).

Current Epidemiology and Risk

Since eradication, surveillance and international control (primarily SIT releases from Panama and Mexico) have prevented re-establishment. Recent detections in northern Mexico increased the probability of establishment in the southern U.S., affecting livestock, dogs, wildlife, and occasionally humans (USDA, 2025).

Federal Response: Surveillance, Trade Measures, and Sterile-Fly Capacity

USDA and state animal health authorities expanded surveillance and temporary import restrictions. A cornerstone of the response has been scaling up SIT capacity. In 2025 USDA announced major investments to construct a domestic sterile-fly facility in Texas with a production capacity of 300 million sterile males per week, reducing dependence on Panama and improving rapid response (USDA, 2025; AP News, 2025).

Regulatory Actions: HHS Declaration and FDA Emergency Authority

In August 2025, HHS issued a declaration allowing FDA to authorize Emergency Use Authorizations (EUAs) for animal drugs against NWS. FDA subsequently issued an EUA permitting use of the isoxazoline lotilaner (Credelio) for treating dogs and conditional approval for doramectin formulations for livestock (FDA, 2025).

Clinical Management and Treatments

Treatment involves (1) prompt removal of larvae, (2) topical and systemic therapy, and (3) prevention of reinfestation. Dogs: lotilaner (Credelio) under EUA; Livestock: macrocyclic lactones like ivermectin or doramectin; Topical larvicides such as coumaphos under veterinary oversight. Wildlife: managed via trapping and area-wide SIT (FDA, 2025; AVMA, 2025).

Implications and Outlook

Federal investments in SIT capacity and emergency regulatory tools are crucial to prevent NWS re-establishment. While construction of the new facility continues, collaboration with Mexico and Panama remains vital (USDA, 2025).

Conclusion

NWS poses a renewed threat to animal health and the U.S. livestock economy. Coordinated surveillance, SIT expansion, and therapeutic EUAs offer the best chance of maintaining NWS-free status (FDA, 2025; USDA, 2025).

References

AP News. (2025, June 18). The US plans to build a $750M fly factory in Texas to stop a flesh-eating cattle parasite. Associated Press. https://apnews.com

FDA. (2025, August 19). HHS allows FDA emergency use of animal drugs to combat New World screwworm, protect U.S. food supply. https://www.fda.gov/news-events/press-announcements/hhs-allows-fda-emergency-use-animal-drugs-combat-new-world-screwworm-protect-us-food-supply

FDA. (2025). New World Screwworm: Information for veterinarians. https://www.fda.gov/animal-veterinary/safety-health/new-world-screwworm-information-veterinarians

USDA. (2025, June 18). Secretary Rollins announces plan to combat New World screwworm’s northward spread. https://www.usda.gov/about-usda/news/press-releases

APHIS, USDA. (2025). Eradicating New World Screwworm with Sterile Insect Technique. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm

AVMA. (2025). First drug for screwworm treatment, prevention receives conditional approval. https://www.avma.org/news/first-drug-screwworm-treatment-prevention-receives-conditional-approval