West Nile Virus and Other Domestic Nationally Notifiable Arboviral Diseases — United States, 2019

Arthropod-borne viruses (arboviruses) are transmitted to humans primarily through the bites of infected mosquitoes and ticks. West Nile virus (WNV) is the leading cause of domestically acquired arboviral disease in the United States (1). Other arboviruses, including La Crosse, Jamestown Canyon, Powassan, eastern equine encephalitis, and St. Louis encephalitis viruses, cause sporadic disease and occasional outbreaks. This report summarizes surveillance data for nationally notifiable domestic arboviruses reported to CDC for 2019. For 2019, 47 states and the District of Columbia (DC) reported 1,173 cases of domestic arboviral disease, including 971 (83%) WNV disease cases. Among the WNV disease cases, 633 (65%) were classified as neuroinvasive disease, for a national incidence of 0.19 cases per 100,000 population, 53% lower than the median annual incidence during 2009-2018. More Powassan and eastern equine encephalitis virus disease cases were reported in 2019 than in any previous year. Health care providers should consider arboviral infections in patients with aseptic meningitis or encephalitis, perform recommended diagnostic testing, and promptly report cases to public health authorities. Because arboviral diseases continue to cause serious illness, and annual incidence of individual viruses continues to vary with sporadic outbreaks, maintaining surveillance is important in directing prevention activities. Prevention depends on community and household efforts to reduce vector populations and personal protective measures to prevent mosquito and tick bites such as use of Environmental Protection Agency-registered insect repellent and wearing protective clothing.*,†.

by an infected mosquito or tick. Most human arboviral infections are asymptomatic; symptomatic infections commonly manifest as systemic febrile illness, similar to bacterial or parasitic diseases transmitted by ticks, and less commonly as neuroinvasive disease.
Most endemic arboviral diseases are nationally notifiable and reported by state health departments to CDC through ArboNET, the national arboviral surveillance system managed by CDC and state health departments, using standard surveillance case definitions that include clinical and laboratory criteria (3). Cases are reported by patient's state and county of residence. Confirmed and probable cases were included for 2019. Cases reported as meningitis, encephalitis, acute flaccid paralysis (AFP), or unspecified neurologic presentation were classified as neuroinvasive disease; the remainder were considered nonneuroinvasive disease. Incidence was calculated using U.S. Census 2019 midyear population estimates and reported neuroinvasive disease cases, which are more reliably diagnosed and reported than nonneuroinvasive disease cases because of the associated morbidity.
Fifty-five La Crosse virus disease cases were reported from 10 states, with the highest number of cases reported from Ohio, Tennessee, and North Carolina ( Table 2). The median patient age was 8 years (IQR = 5-12 years), and 51 (93%) were aged <18 years (Table 1). Thirty-three (60%) patients were male. Illness onset dates ranged from June to October,  with 41 (75%) occurring during July-September. Forty-eight (87%) cases were neuroinvasive, and 54 (98%) patients were hospitalized; none died. Forty-five Jamestown Canyon virus disease cases were reported from six states, with the highest number of cases reported from Minnesota and Wisconsin (Table 2). A disease case was reported for the first time from Illinois; however, the patient had traveled during the likely period of infection. The median patient age was 59 years (IQR = 31-70 years); 30 (67%) were male (Table 1). Illness onset ranged from April to November, with 29 (64%) cases occurring during July-September. Twenty-five (56%) cases were neuroinvasive, 26 (58%) patients were hospitalized, and two (4%) died, both aged 25-35 years and both with neuroinvasive disease.
Thirty-eight cases of eastern equine encephalitis virus disease were reported from 10 states. Twenty-two (58%) cases were reported from Massachusetts (12) and Michigan (10) ( Table 2); cases were reported for the first time from Indiana and Tennessee. The median patient age was 64 years (IQR = 54-72 years); 27 (71%) were male. Illness onset dates ranged from June to November, with 36 (95%) occurring during July-September. All cases were neuroinvasive, and all patients were hospitalized. Nineteen (50%) patients died, all of whom were aged >50 years.

Discussion
As in previous years, WNV was the most common cause of domestic arboviral neuroinvasive disease in 2019. However, WNV neuroinvasive disease incidence (0.19 per 100,000) was 53% lower than the median annual incidence during 2009-2018 (0.40; range = 0.13-0.92) (4). The decrease in incidence was most notable in Midwestern and South Central states, particularly Texas, which reported 24 neuroinvasive disease cases, 78% lower than its annual median of 111 (range = 20-844) during 2009-2018 (4). Despite overall low WNV disease incidence, multiple states reported more cases than their annual median during 2009-2018, mostly in the Mountain region (4).
La Crosse virus continued to be the most common cause of neuroinvasive arboviral disease in children (5) (8). Eastern equine encephalitis virus remained the deadliest arbovirus disease, with one half of patients dying. For viruses with higher than average case numbers in 2019, whether the increase reflects an actual increase in disease incidence or increased awareness, surveillance, and testing is unknown.

Summary
What is already known about this topic?
West Nile virus (WNV) is consistently the leading cause of domestically acquired arboviral disease, but other arboviruses cause sporadic cases and outbreaks of neuroinvasive disease, resulting in substantial morbidity and mortality.
What is added by this report?
In 2019, WNV neuroinvasive disease incidence was 53% lower than the median annual incidence during 2009-2018. More Powassan and eastern equine encephalitis virus disease cases were reported than in any previous year.
What are the implications for public health practice?
Health care providers should consider arboviral infections in patients with aseptic meningitis or encephalitis, perform recommended diagnostic testing, and promptly report cases to public health authorities. Surveillance is important to identify outbreaks and guide prevention strategies, which include wearing insect repellent, long pants, and long-sleeved shirts when outdoors.