LAUSR

Arthropodborne 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 continental United States.1 Other arboviruses, including eastern equine encephalitis, Jamestown Canyon, La Crosse, Powassan, and St. Louis encephalitis viruses, cause sporadic cases of disease and occasional outbreaks. This report summarizes surveillance data reported to CDC for 2018 on nationally notifiable arboviruses. It excludes dengue, chikungunya, and Zika viruses because they are primarily nondomestic viruses typically acquired through travel. In 2018, 48 states and the District of Columbia (DC) reported 2813 cases of domestic arboviral disease, including 2647 (94%) WNV disease cases. Of the WNV disease cases, 1658 (63%) were classified as neuroinvasive disease (e.g., meningitis, encephalitis, and acute flaccid paralysis), for a national incidence of 0.51 cases of WNV neuroinvasive disease per 100 000 population. Because arboviral diseases continue to cause serious illness and have no definitive treatment, maintaining surveillance is important to direct and promote prevention activities. Health care providers should consider arboviral infections in patients with aseptic meningitis or encephalitis, perform appropriate diagnostic testing, and report cases to public health authorities. Arboviruses are maintained in a transmission cycle between arthropods and vertebrate hosts, including humans and other animals.2 Humans primarily become infected when bitten by an infected mosquito (West Nile, La Crosse, Jamestown Canyon, St Louis encephalitis, and eastern equine encephalitis viruses) or tick (Powassan virus). Most human infections are asymptomatic; symptomatic infections commonly manifest as a systemic febrile illness and less commonly as neuroinvasive disease. Most endemic arboviral diseases are nationally notifiable and are reported by state health departments to CDC through ArboNET, the national arbovirus surveillance system, using standard surveillance case definitions that include clinical and laboratory criteria.3 Cases are reported by the patient's state of residence. Confirmed and probable cases were included in this analysis. Cases reported as acute flaccid paralysis, encephalitis, meningitis, or an unspecified neurologic presentation were classified as neuroinvasive disease; cases with more than one neuroinvasive presentation were counted once according to the order specified above. Other clinical presentations were considered nonneuroinvasive disease. Incidence rates were calculated using neuroinvasive disease cases and the U.S. Census 2018 midyear population estimates. A total of 2813 cases of domestic arboviral disease were reported to CDC for 2018. Cases were caused by WNV (2647 cases, 94%), La Crosse virus (86), Jamestown Canyon virus (41), Powassan virus (21), St. Louis encephalitis virus (eight), eastern equine encephalitis virus (six), and unspecified California serogroup virus (four). Cases were reported from all states except Hawaii and New Hampshire. Of the 3142 U.S. counties, 858 (27%) reported one or more arboviral disease cases. Overall, 2647 WNV disease cases were reported from 787 counties in 48 states and DC. Of these, 1658 (63%) cases were neuroinvasive and 2435 (92%) patients had illness onset during July– September (Table 1). In 2018, WNV disease was reported for the first time from a resident of Alaska; however, the patient's likely location of infection was reported as a state with previously documented transmission. Two WNV disease cases were reported in solid organ transplant recipients with a common donor, and subsequent investigation demonstrated transmission via organ transplantation. The median age of patients with WNV disease was 59 years (interquartile range [IQR] = 44– 70); 1638 (62%) were male. A total of 1774 (67%)