Is it mosquito season in Colorado?

Denver sits at 5,280 feet — the Mile High City — and still records West Nile Virus cases every summer. That's the Colorado paradox: a state most associated with altitude and outdoor recreation has a genuine, persistent WNV problem along its Front Range urban corridor. The reason is the South Platte River system, which threads irrigated agriculture through the Denver metro and provides Culex tarsalis with productive warm-water habitat within miles of 3.5 million people. East of the Front Range, the high plains amplify that risk considerably — the agricultural counties along the South Platte, Republican, and Arkansas rivers rank among the Mountain West's most active WNV zones most years.

The Front Range corridor: altitude doesn't protect you

Denver, Aurora, Lakewood, and Fort Collins all lie along a narrow band where the Great Plains meets the Rockies — and where Culex tarsalis finds the warm agricultural water it needs. The South Platte River corridor through Greeley and Brighton is the primary breeding engine for the metro, with irrigation return flows sustaining populations through the dry summer months when the river would otherwise run low. Arapahoe and Adams counties record the metro's highest WNV case totals most years. Fort Collins — home of Colorado State University's arthropod-borne disease research — runs detailed surveillance that makes its county data among the most carefully tracked in the region.

The eastern plains: Colorado's highest-risk zone

Drive two hours east of Denver and the landscape flattens into some of the most intensively irrigated farmland in the West. Weld County — one of the nation's top agricultural producers — sustains Culex tarsalis populations through the summer on a scale that dwarfs the metro. Morgan, Logan, and Yuma counties along the South Platte, and Otero and Prowers counties along the Arkansas River, routinely record WNV cases at rates that, adjusted for population, rival the worst of Kansas and Nebraska. The birds, the warm water, and the heat are all there.

Mountain communities: the genuine respite

Above roughly 7,000 feet, meaningful mosquito activity drops sharply. Summit County, Pitkin County (Aspen), and the mountain resort communities sit at elevations where cold nights prevent larvae from completing development reliably. Boulder — at 5,430 feet with significant creek and reservoir habitat — occupies an intermediate zone: more exposure than the mountains, but measurably less than the plains. The rule of thumb holds: every thousand feet of elevation gain shortens the season by roughly two weeks.