West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2014

Nicole P. Lindsey, MS1; Jennifer A. Lehman1; J. Erin Staples, MD, PhD1; Marc Fischer, MD1

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). However, several other arboviruses also cause sporadic cases and seasonal outbreaks. This report summarizes surveillance data reported to CDC in 2014 for WNV and other nationally notifiable arboviruses, excluding dengue. Forty-two states and the District of Columbia (DC) reported 2,205 cases of WNV disease. Of these, 1,347 (61%) were classified as WNV neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis), for a national incidence of 0.42 cases per 100,000 population. After WNV, the next most commonly reported cause of arboviral disease was La Crosse virus (80 cases), followed by Jamestown Canyon virus (11), St. Louis encephalitis virus (10), Powassan virus (8), and Eastern equine encephalitis virus (8). WNV and other arboviruses cause serious illness in substantial numbers of persons each year. Maintaining surveillance programs is important to help direct prevention activities.

In the United States, most arboviruses are maintained in transmission cycles between arthropods and vertebrate hosts (typically birds or small mammals). Humans usually become infected when bitten by infected mosquitoes or ticks. Person-to-person transmission also occurs rarely through blood transfusion and organ transplantation. The majority of human arboviral infections are asymptomatic. Symptomatic infections most often manifest as a systemic febrile illness and, less commonly, as neuroinvasive disease. Most endemic arboviral diseases are nationally notifiable and are reported to CDC through ArboNET, a national arboviral surveillance system managed by CDC and state health departments (2,3). Using standard definitions, human cases with laboratory evidence of recent arboviral infection are classified as neuroinvasive disease or nonneuroinvasive disease (2). Cases reported as encephalitis, meningitis, or acute flaccid paralysis are collectively referred to as neuroinvasive disease; others are considered nonneuroinvasive disease. Acute flaccid paralysis can occur with or without encephalitis or meningitis. In this report, any case reported as acute flaccid paralysis (with or without another clinical syndrome) was classified as acute flaccid paralysis and not included in the other categories. Because of the substantial associated morbidity, detection and reporting of neuroinvasive disease cases is assumed to be more consistent and complete than that of nonneuroinvasive disease cases. Therefore, incidence rates were calculated for neuroinvasive disease cases using U.S. Census 2014 mid-year population estimates.

In 2014, CDC received reports of 2,327 cases of nationally notifiable arboviral disease, among which 1,453 (62%) were classified as neuroinvasive disease. Cases were caused by WNV (2,205 cases, 95%), La Crosse virus (80), Jamestown Canyon virus (11), St. Louis encephalitis virus (10), Powassan virus (8), Eastern equine encephalitis virus (8), and unspecified California serogroup virus (5). Cases were reported from 568 (18%) of the 3,141 U.S. counties; no cases were reported from Alaska, Delaware, Rhode Island, or Vermont.

A total of 2,205 WNV disease cases, including 1,347 (61%) neuroinvasive cases, were reported from 503 counties in 42 states and the District of Columbia. WNV disease cases peaked in late August; 90% of cases had illness onset during July–September (Table 1). The median age of patients was 57 years (interquartile range [IQR] = 44–67 years); 1,403 (64%) were male. Overall, 1,589 (72%) patients were hospitalized, and 97 (4%) died. The median age of patients who died was 75 years (IQR = 65–83 years).

Of the 1,347 WNV neuroinvasive disease cases, 620 (46%) were reported as encephalitis, 565 (42%) as meningitis, 132 (10%) as acute flaccid paralysis, and 30 (2%) as other neurologic presentation. Among the 132 patients reported to have acute flaccid paralysis, 102 (77%) also had encephalitis or meningitis. Among all patients with WNV neuroinvasive disease, 1,294 (96%) were hospitalized, and 87 (6%) died.

The national incidence of WNV neuroinvasive disease was 0.42 per 100,000 population (Table 2). States with the highest incidence rates included Nebraska (2.2 per 100,000), North Dakota (1.6), California (1.4), South Dakota (1.4), Louisiana (1.3) and Arizona (1.2) (Table 2) (Figure). Three states reported two thirds (66%) of the neuroinvasive disease cases: California (561 cases), Texas (253), and Arizona (80). WNV neuroinvasive disease incidence increased with increasing age, ranging from 0.03 per 100,000 among persons aged <10 years to 1.15 per 100,000 among those aged ≥70 years, and was higher among males (0.57 per 100,000) than among females (0.29).

Eighty La Crosse virus disease cases were reported from nine states; 76 (95%) were neuroinvasive (Table 1). Dates of illness onset for La Crosse virus disease cases ranged from March to October; 73 (91%) had onset during July–September. Forty-two (53%) patients were female. The median age of patients was 8 years (IQR = 6–11 years); 72 (90%) were aged <18 years. A total of 79 (99%) patients were hospitalized; three (4%) died. La Crosse virus neuroinvasive disease incidence was highest in Ohio (0.26 per 100,000), North Carolina (0.23), and Tennessee (0.17) (Table 2).

Eleven Jamestown Canyon virus disease cases were reported from four states (Massachusetts, Minnesota, Tennessee, and Wisconsin); six were neuroinvasive (Table 1). Tennessee reported its first Jamestown Canyon virus disease cases in 2014. Dates of illness onset ranged from May to September, with eight occurring during July–September. The age distribution of patients was bimodal, with four patients aged <18 years and six aged >60 years. Six patients were female. Seven patients were hospitalized; none died. In addition to the La Crosse virus and Jamestown Canyon virus cases, five other cases of California serogroup virus disease were reported for which the specific infecting virus was unknown.

Ten St. Louis encephalitis virus disease cases were reported from five states (Alabama, Arizona, Florida, Mississippi, and Texas); six were neuroinvasive (Table 1). Dates of illness onset ranged from January–October; six had onset during July–September. The median age of patients was 55 years (IQR: 47–60 years); six were female. All patients were hospitalized; none died.

Eight Powassan virus disease cases were reported from four states (Massachusetts, New Jersey, New York, and Wisconsin); seven were neuroinvasive (Table 1). Three patients (38%) had onset in May, and 5 (62%) had onset during July–September. The median age of patients was 65 years (IQR = 51–70 years); six were male. All patients were hospitalized; none died.

Eight Eastern equine encephalitis virus neuroinvasive disease cases were reported from five states (Alabama, Maine, Michigan, New Hampshire, and New York (Table 1). All eight patients had illness onset during July–September. The median age of patients was 60 years (IQR = 52–69 years); four were male. All eight patients were hospitalized; two died.

Discussion

In 2014, WNV was the most common cause of neuroinvasive arboviral disease in the United States, accounting for 93% of all neuroinvasive cases. Nationally, WNV neuroinvasive disease incidence was similar to the median incidence during 2002–2013 (median = 0.40; range = 0.13–1.02) (3,4). However, California reported a record 561 neuroinvasive disease cases, 83% more than the next highest year (2005). Within California, 70% of all neuroinvasive disease cases were reported from just two counties (Los Angeles and Orange). These findings highlight the focal nature of WNV outbreaks.

As has been reported in previous years, La Crosse virus was the most common cause of neuroinvasive arboviral disease among children (1); it is not known why the incidence of La Crosse virus disease is highest among children (5). Jamestown Canyon virus disease cases continue to be reported from new locations (e.g., Tennessee) following the implementation of routine Jamestown Canyon virus antibody testing at CDC in 2013 (6). Eastern equine encephalitis virus disease, although rare, remained the most severe domestic arboviral disease, with two deaths among eight patients. Over 90% of arboviral disease cases occurred during April–September, emphasizing the importance of focusing public health interventions during this period.

The findings in this report are subject to at least three limitations. First, ArboNET is a passive surveillance system that relies on clinicians to consider the diagnosis of an arboviral disease and obtain appropriate diagnostic tests, and on health care providers and laboratories to report laboratory-confirmed cases to public health authorities. Second, testing and reporting are incomplete, leading to a substantial underestimate of the actual number of cases (7). For example, data from previous studies suggest there are an estimated 30–70 nonneuroinvasive disease cases for every reported case of WNV neuroinvasive disease (8–10). Extrapolating from the 1,347 WNV neuroinvasive disease cases reported, an estimated 40,000–94,000 nonneuroinvasive disease cases might have occurred in 2014. However, only 858 (1%–2%) were diagnosed and reported. Finally, this report underestimates the overall disease burden for arboviral diseases in the United States during 2014, because it does not include dengue or arboviral diseases that were not nationally notifiable such as Colorado tick fever and chikungunya. Chikungunya became a nationally notifiable condition in 2015.

Arboviruses continue to cause substantial morbidity in the United States, although reported numbers of cases vary annually. Cases occur sporadically, and the epidemiology varies by virus and geographic area. The weather, zoonotic host and vector abundance, and human behavior are all factors that can influence when and where outbreaks occur. Because of this complex ecology, it is difficult to predict how many cases of disease might occur in the future and in what areas; therefore, surveillance is essential to identify outbreaks and guide prevention efforts. Health care providers should consider arboviral infections in the differential diagnosis of cases of aseptic meningitis and encephalitis, obtain appropriate specimens for laboratory testing, and promptly report cases to public health authorities (2). Because human vaccines against domestic arboviruses are not available, prevention depends on community and household efforts to reduce vector populations (e.g., applying insecticides and reducing breeding sites), personal protective measures to decrease exposure to mosquitoes and ticks (e.g., use of repellents and wearing protective clothing), and screening of blood donors.

Acknowledgments

ArboNET surveillance coordinators in state and local health departments.

1Division of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC.

Corresponding author: Nicole Lindsey, nplindsey@cdc.gov, 970-221-6400.

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