West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2022

Arthropodborne viruses (arboviruses) primarily infect humans through the bite of an infected mosquito or tick. Infections are commonly asymptomatic; however, the clinical signs and symptoms can range from a mild febrile illness to severe neuroinvasive disease. This report summarizes data for six nationally notifiable arboviral diseases for 2022 reported to ArboNET, the national surveillance system for arboviral diseases, including eastern equine encephalitis, Jamestown Canyon, La Crosse, Powassan, St. Louis encephalitis, and West Nile viruses. In 2022, these viruses caused 1,247 human disease cases, 968 (78%) hospitalizations, and 103 (8%) deaths. Reported case counts decreased from 2021 for all viruses except Powassan and St. Louis encephalitis viruses. Despite a substantial decrease in reported cases from 2021, West Nile virus remained the leading cause of arboviral disease in the continental United States. Variations in annual arboviral disease incidence and distribution highlight the importance of high-quality surveillance. Health care providers should suspect arboviral infection in patients with a clinically compatible illness, consider testing, and report positive findings to their state or local health department. In areas with arboviral activity, community and household efforts to reduce vector populations (e.g., applying insecticides and reducing breeding sites) and personal protective measures to decrease mosquito and tick exposures (e.g., wearing repellents and protective clothing) can reduce arboviral disease morbidity and mortality.


Introduction
Arthropodborne viruses (arboviruses) are transmitted to humans primarily through the bite of an infected mosquito or tick.Rarely, transmission occurs through blood transfusion and organ transplantation.West Nile virus (WNV) is the leading cause of arboviral disease in the continental United States (1).Other domestic arboviruses cause sporadic cases and occasional outbreaks.Most arboviral infections are asymptomatic, with clinical signs and symptoms ranging from a mild febrile illness to severe neuroinvasive disease (2).This report summarizes nationally notifiable arboviral diseases reported to CDC for 2022.

Methods
Data for six nationally notifiable, domestic arboviruses (eastern equine encephalitis, Jamestown Canyon, La Crosse, Powassan, St. Louis encephalitis, and West Nile viruses) were analyzed and are included in this report.Chikungunya, dengue, yellow fever, and Zika virus disease cases are excluded because these infections are primarily travel-associated when they occur in U.S. states (3,4).Surveillance data are obtained from ArboNET, the national surveillance system for arboviral diseases.Disease cases are reported by state health departments to ArboNET using a standard case definition that includes clinical and laboratory criteria.*Cases reported as probable † and confirmed § are included in this report and are reported on the basis of state and county of residence.Cases are described by demographic characteristics including age and sex, quarter year of illness onset (January-March, April-June, July-September, and October-December), clinical syndrome (neuroinvasive [acute flaccid paralysis, encephalitis, meningitis, or other neurologic signs and symptoms] versus nonneuroinvasive [all other cases]), and outcome (hospitalization and death).Incidence was calculated using 2022 midpoint population estimates from the U.S. Census Bureau.¶ All statistical analyses were conducted using SAS software (version 9.

West Nile Virus Disease
The 1,132 WNV disease cases were reported from 358 counties in 42 states and DC; 966 (85%) patients had illness onset during July-September.Median patient age was 63 years, and 61% were male.A total of 862 (76%) patients were hospitalized, and 93 (8%) died.Three patients with nonfatal neuroinvasive disease were infected through solid organ transplants from a common donor.

St. Louis Encephalitis Virus Disease
Thirty-three cases of St. Louis encephalitis virus disease were reported from 12 counties in three states.Illness onset occurred most frequently during July-September (45%), although 39% of cases occurred during October-December (Table 1).All late-season cases were reported in the southwestern United States (Arizona, California, and Texas).Median patient age was 65 years, and 61% of patients were male.Twenty-seven (82%) patients had neuroinvasive disease, 29 (88%) were hospitalized, and three (9%) died.The highest incidences of neuroinvasive disease were reported from Arizona (0.16 per 100,000) and California (0.04) (Table 2).All patients who died were aged >65 years (median age = 83 years; range = 68-85 years).

La Crosse Virus Disease
Twenty-two cases of La Crosse virus disease were reported from 19 counties in five states.Twenty (91%) patients experienced illness onset during July-September (Table 1); the median patient age was 9 years, and 55% of patients were male.Nineteen (86%) patients had neuroinvasive disease.Twenty-one (95%) patients were hospitalized; none died.Ohio reported the highest number of neuroinvasive disease cases (12; 63%) (Table 2), and the highest incidences of neuroinvasive disease occurred in Ohio (0.10 per 100,000), West Virginia (0.06), and Minnesota (0.05).

Jamestown Canyon Virus Disease
Among 12 cases of Jamestown Canyon virus disease reported from 12 counties in five states, illness onset occurred most frequently during April-June (five cases) (Table 1).The median patient age was 60 years, and nine of the 12 patients were male.

Eastern Equine Encephalitis Virus Disease
One case of eastern equine encephalitis virus disease was reported.The patient was a woman aged >60 years with illness onset in August.The patient experienced neuroinvasive disease and was hospitalized.

Discussion
Overall, the number of arboviral disease cases reported in 2022 (1,247) decreased 59% compared with the 3,035 cases reported in 2021.This decrease was largely driven by a 61% decrease in reported WNV disease cases in 2022 (1,132) compared with the 2,911 cases reported in 2021, when a large WNV disease outbreak occurred in Arizona (1).WNV disease remained the most commonly reported domestic arboviral disease.La Crosse virus remained the most common cause of neuroinvasive arboviral disease in children.
In contrast to other arboviruses, historically high numbers of St. Louis encephalitis virus and Powassan virus disease cases were reported in 2022.The 33 St. Louis encephalitis virus disease cases represent the highest number of cases since 2003, when 49 cases were reported (5).The 47 Powassan virus disease cases represent the highest number ever reported in a single year; the previous high was 43 cases reported in 2019 (6).
Arboviral diseases remain an important cause of morbidity in the United States.Although most human infections occur through the bite of infected mosquitoes or ticks, organ transplant transmission continues to occur.Currently, no national policy exists requiring arboviral screening of deceased donors (7).The complex interaction among humans, animals, and environment that contributes to vectorborne transmission poses challenges to predicting and controlling disease.Timely and high-quality surveillance (e.g., accurate and complete case identification, investigation, and reporting) is important to detecting arboviral disease risk and implementing interventions to lower disease incidence such as distributing prevention messaging and performing vector control activities.

Limitations
The findings in this report are subject to at least two limitations.First, ArboNET is a passive surveillance system and, as such, likely underestimates disease prevalence.Identifying

TABLE 1 . Number and percentage of reported cases of nationally notifiable nonneuroinvasive and neuroinvasive arboviral diseases, by virus type and selected patient characteristics (N = 1,247)* -United States, 2022 Characteristic Virus type, no. (%) of cases
Abbreviation: AFP = acute flaccid paralysis.*One eastern equine encephalitis virus disease case was also reported.† Date of illness onset is unknown for one case of West Nile virus disease.§ Percentages of cases of encephalitis, meningitis, AFP, and unspecified neurologic signs or symptoms are percentages of neuroinvasive cases.¶ Among the 41 West Nile virus disease cases with AFP, 10 (24%) also had encephalitis or meningitis.** Among the four Powassan virus disease cases with AFP, three also had encephalitis or meningitis.