West Nile virus remains leading US mosquito-borne illness as cases surge this summer

Deaths from West Nile virus have surfaced again this summer across Illinois, Nebraska, and Colorado, a reminder that the disease remains the most consequential mosquito-borne illness in America. The virus spreads when infected mosquitoes bite people, and it has held that grim distinction for years. A report released Thursday by the Centers for Disease Control and Prevention examined what happened in 2021, the most recent year with complete data, and the picture was stark: across 49 states and Washington, D.C., there were more than 3,000 cases of arboviral disease—the medical term for illnesses spread by insects—and West Nile accounted for 2,911 of them. Nearly 2,100 people required hospitalization. Two hundred twenty-seven died.

The disease hit hardest in the West. Arizona saw an outbreak severe enough to drive the national rate of neuroinvasive West Nile—the form that attacks the nervous system—to its highest level since 2012. Three Arizona counties alone, Maricopa, Pima, and Pinal, reported more than half of all neuroinvasive cases in the entire country. Texas followed with 130 cases, Colorado with 101, California with 96, and Nebraska with 69. The virus works through a cycle: mosquitoes bite infected birds, then bite people. This is why public health officials monitor both bird populations and mosquito abundance as early warning systems.

La Crosse virus, a different arboviral disease, tells a different story. It rarely kills, but it strikes children with particular force. In 2021, forty cases were reported across eight states in the Midwest and South, and thirty-five of those were children. All forty patients were hospitalized, but none died. The contrast illustrates how arboviral diseases vary in their severity and in whom they target.

This year, conditions are aligning in ways that worry experts. A historically wet winter followed by an unusually hot summer has created ideal conditions for mosquito breeding across the Western states. Daniel Markowski, a technical adviser for the American Mosquito Control Association, described the mosquito hatch following spring snowmelt as "pretty tremendous" in Colorado, Utah, and California. When you combine the right temperature, the right number of mosquitoes, and the right time of year, localized outbreaks become likely. Many mosquito control districts are already detecting West Nile virus in their mosquito populations—a signal that transmission is beginning.

As of late August, 247 West Nile cases had been reported to the CDC for 2023, and that number continues to climb. The disease's unpredictability makes prevention difficult. Most people infected with West Nile never develop symptoms. About one in five develop fever and other mild signs. But roughly one in 150 infected people develop severe illness—encephalitis, which inflames the brain, or meningitis, which inflames the membranes surrounding the brain and spinal cord. These severe cases can be fatal.

The CDC researchers acknowledged the challenge in their report: weather patterns, the abundance of animal hosts and mosquitoes, and human behavior all influence where and when outbreaks occur. This complexity makes prediction nearly impossible. What remains clear is that surveillance matters. Detecting cases quickly allows public health officials to direct prevention efforts where they are needed most. For now, the advice is straightforward: use insect repellent, wear long sleeves and long pants, and avoid mosquito bites. The virus is circulating, the conditions favor its spread, and the season is far from over.