In the 1990s, whirling disease slammed into Montana’s trout streams like a wrecking ball, wiping out up to 95% of rainbow trout in the Madison River. Caused by the microscopic parasite Myxobolus cerebralis, the disease deforms young trout’s spines and brains, making them swim in frantic, uncontrolled circles—hence the name. The outbreak sparked scientific alarms, community outrage, and even inspired a fly-fishing murder mystery novel. Fast forward to 2025, and trout are back—sort of. While numbers have rebounded, researchers say today’s fish may have developed resistance at the cost of genetic diversity, raising new questions about the long-term health of one of Montana’s most iconic fisheries.
Want to dive deeper into the science, the mystery, and the comeback story? Check out the full Big Why feature at mtpr.org. this story is full of twists—both biological and literary.
