West Nile Virus in 2021: Arizona's Outbreak Drove the Worst Year Since 2012

In 2021, three Arizona counties — Maricopa, Pima and Pinal — accounted for more than half of all the severe West Nile virus cases in the United States. Arizona alone reported 1,140 cases of West Nile neuroinvasive disease, at a rate of 15.7 per 100,000 people, pushing the national rate to 0.61 per 100,000: above the 2010–2020 median of 0.39, and the highest since 2012.

Incidence of West Nile virus neuroinvasive disease by state, 2021. Credit: CDC, Morbidity and Mortality Weekly Report.

The year's arboviral diseases

Arboviruses — arthropod-borne viruses — reach people mainly through the bites of infected mosquitoes and ticks. Most infections cause no symptoms; some cause a fever, and a few cause neuroinvasive disease such as encephalitis or meningitis. In 2021, 49 states and the District of Columbia (every state but Hawaii) reported 3,035 cases of domestically acquired arboviral disease, 70 percent neuroinvasive, from 500 of the nation's 3,143 counties. (Chikungunya, dengue, yellow fever and Zika were excluded, since those cases were mostly caught while traveling.)

West Nile, up close

West Nile disease came from 432 counties; 71 percent of patients fell ill in July–September. Of the neuroinvasive cases, 64 percent were encephalitis, 30 percent meningitis, and 2 percent acute flaccid paralysis. After Arizona, the most cases were in Texas (130), Colorado (101), California (96) and Nebraska (69), with the highest rates in the Mountain and West North Central states.

Arizona's outbreak came later in the year than usual, raising the share of cases that began in October–December. Likely reasons: late-season rain, recent population growth and housing development, and little West Nile activity the year before.

Far more than reported

Only diagnosed and reported cases are counted. Studies estimate 30 to 70 milder, non-neuroinvasive cases for every neuroinvasive one reported — so 2021 probably saw 60,240 to 140,560 such West Nile cases, of which only 903 were reported. The COVID-19 pandemic likely made underreporting worse: 69 percent of reported West Nile cases were neuroinvasive, against 63 percent in 2010–2020.

Prevention

There are no specific treatments and no human vaccines for these domestic viruses. Prevention depends on communities and households reducing mosquitoes and ticks, on personal protection against bites, and on screening donated blood. Clinicians should consider arboviral infection in patients with aseptic meningitis or encephalitis, especially in summer, test for it and report cases promptly. New detections, such as Jamestown Canyon in Indiana and Powassan in Ohio, likely reflect more awareness and testing — but could also mean the viruses are spreading.

Sources

Based on "West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2021," by Anna C. Fagre and colleagues, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.