Borderline Outbreak

A flesh-eating parasite is spreading north from the Darién Gap, threatening U.S. livestock and decades of containment efforts

- JESUS MESA

THE DARIÉN GAP, A REMOTE AND BIOLOGI-cally rich corridor between Central and South America, is becoming an unexpected conduit for a growing public health concern: the northward spread of the New World screwworm.

Now challenging decades of containment efforts, the flesh-eating parasitic fly, whose larvae burrow into the wounds of living animals, has surged north through Central America since 2022, recently reaching the Mexican states of Oaxaca and Veracruz—roughly 700 miles from the Texas border. Scientists and federal officials now warn that the screwworm could cross into the U.S. as early as this summer if containment efforts falter.

The screwworm, scientifically known as Coch-liomyia hominivorax—roughly translated as “man-eater”—was eradicated from the U.S. by 1982 using the Sterile Insect Technique, or SIT, which involves releasing sterilized male flies to prevent reproduction and collapse the population. The parasite targets livestock, wildlife and pets—and humans—by burrowing into open wounds. Infected animals are once again being found in areas previously considered screwworm-free, triggering emergency responses from governments.

For decades, the United States and Panama maintained a binational barrier to block the parasite at the Darién, which has long been a perilous route for millions of migrants traveling north toward the U.S. They used a program that dropped millions of sterilized screwworm flies over the jungle to prevent breeding. That system began unraveling in 2022, amid pandemic-era supply-chain disruptions and record human and animal movement through the region.

Between 2021 and 2024, more than 1.2 million migrants crossed the Darién, according to data from Panama's National Migration Service and UNHCR. The crossings peaked at 520,085 in 2023 before falling to 302,203 in 2024. U.S. officials believe that mass movement contributed to operational breakdowns in the region's screwworm control infrastructure.