Today in Morbidity and Mortality Weekly Report, researchers using the Centers for Disease Control and Prevention (CDC) arthropod-borne disease surveillance data confirm that West Nile virus remains the most common mosquito-borne disease in the United States, with almost 3,000 cases reported in 2021.
The surveillance data shows 3,035 cases of domestic arthropod-borne diseases, including 2,911 of West Nile, 40 cases of La Crosse, 32 cases of Jamestown Canyon, 24 cases of Powassan virus, 17 cases of St. Louis encephalitis, 5 cases of eastern equine encephalitis, and 6 cases of an unspecified California serogroup virus.
All cases were reported from all states except Hawaii and the District of Columbia, with 16% of all US counties reporting at least one case of arboviral disease.
Of the West Nile virus cases noted, 2,008 (69%) were classified as neuroinvasive, which included cases reporting encephalitis (64%), meningitis (30%), and acute flaccid paralysis (2%). Cases were reported from 432 counties in 49 states and the District of Columbia, with most seasonally reported (71%) during July, August, and September.
Arizona outbreak increased case counts
"Most patients with neuroinvasive disease (1,907; 95%) were hospitalized and 225 (11%) died," the authors wrote. Arizona had the largest number of neuroinvasive cases (1,140), or 57% of all US cases. The activity in Arizona made 2021 the most active year for West Nile in the United States since 2012.
"Reasons for the outbreak likely included late-season rain, recent population growth and housing development, and low levels of WNV [West Nile virus] circulation during the preceding year,” the authors wrote.
They said continued surveillance is needed, as there are no treatments for West Nile virus.