Screwworm threat returns to U.S.

Dr. Jay K. Varma is a special contributor to Healthbeat. Public health, explained: Sign up to receive Healthbeat’s free national newsletter here. In 2025, the United States faced a series of reminders that the security of our food supply depends on public health systems that are largely invisible to the general population. Poultry and dairy production were disrupted by H5N1 avian influenza and infant formula by botulism contamination. Less visible was a parasite that once terrified American ranchers: the New World screwworm. The resurgence of screwworm highlights the fragility of infectious disease control and the importance of sustained government cooperation across different sectors and across international borders. What screwworm is and why it matters The New World screwworm is caused by the larvae of a blowfly called Cochliomyia hominivorax. The adult fly targets warm-blooded animals, most often cattle, and lays its eggs in open wounds. These wounds can be large, such as those caus