West Nile Virus Disease and Other Arboviral Diseases — United States, 2011

Arthropodborne viruses (arboviruses) are transmitted to humans primarily through the bites of infected mosquitoes and ticks. Symptomatic infections most often manifest as a systemic febrile illness and, less commonly, as neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis). West Nile virus (WNV) is the leading cause of domestically acquired arboviral disease in the United States (1). However, several other arboviruses also cause seasonal outbreaks and sporadic cases (1). In 2011, CDC received reports of 871 cases of nationally notifiable arboviral diseases (excluding dengue); etiological agents included WNV (712 cases), La Crosse virus (LACV) (130), Powassan virus (POWV) (16), St. Louis encephalitis virus (SLEV) (six), Eastern equine encephalitis virus (EEEV) (four), and Jamestown Canyon virus (JCV) (three). Of these, 624 (72%) were classified as neuroinvasive disease, for a national incidence of 0.20 per 100,000 population. WNV and other arboviruses continue to cause focal outbreaks and severe illness in substantial numbers of persons in the United States.

In the United States, most arboviruses are maintained in transmission cycles between arthropods and vertebrate hosts (typically birds or small mammals). Humans can become infected when bitten by mosquitoes and ticks that carry blood from those hosts. Person-to-person transmission can occur 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 arboviruses are nationally notifiable and are reported to CDC through ArboNET (2,3). In addition to human disease cases, ArboNET collects data on viremic blood donors, veterinary disease cases, and infections in mosquitoes, dead birds, and sentinel chickens.* Using standard definitions, human cases with laboratory evidence of recent arboviral infection are classified as neuroinvasive disease or nonneuroinvasive disease (2). Because of the considerable morbidity associated with neuroinvasive disease cases, detection and reporting is assumed to be more consistent and complete than for nonneuroinvasive disease cases. Therefore, for this report, incidence rates were calculated only for neuroinvasive disease cases using U.S. Census Bureau 2011 mid-year population estimates.

In 2011, CDC received reports of 871 cases of nationally notifiable arboviral diseases (excluding dengue), including those caused by WNV (712 cases), LACV (130), POWV (16), SLEV (six), EEEV (four), and JCV (three) (Table 1). Arboviral disease cases caused by these viruses were reported from 331 (11%) of the 3,141 U.S. counties. No cases were reported from Alaska, Hawaii, Maine, New Hampshire, Oregon, or Washington. Of the 871 total cases, 624 (72%) were reported as neuroinvasive disease, for a national incidence of 0.20 per 100,000 population.

A total of 712 WNV disease cases were reported from 238 counties in 43 states and the District of Columbia (Figure), including 486 (68%) neuroinvasive and 226 (32%) nonneuroinvasive cases (Table 1). Presumptive WNV infections were identified in 137 blood donors through routine screening of the blood supply. Of these, one (1%) subsequently developed neuroinvasive disease, and 32 (23%) developed nonneuroinvasive disease and are included in the case totals. WNV disease cases peaked in late August with 663 (93%) cases having illness onset during July–September. The median age of patients with WNV disease was 57 years (range: 7–96 years); 424 (60%) were male. Overall, 547 (77%) persons were hospitalized with WNV disease, and 43 (6%) died. The median age of patients who died was 74 years (range: 32–96 years).

Of the 486 WNV neuroinvasive disease patients, 273 (56%) had encephalitis, 183 (38%) had meningitis, and 30 (6%) had acute flaccid paralysis; 28 (93%) of the 30 patients with acute flaccid paralysis also had encephalitis or meningitis. The national incidence of neuroinvasive WNV disease was 0.16 per 100,000 population (Table 2). The highest reported rates were in the District of Columbia (1.62), Mississippi (1.04), Nebraska (0.76), and Arizona (0.76). Five states reported 51% of WNV neuroinvasive disease cases: California (110 cases), Arizona (49), Michigan (32), Mississippi (31), and New York (28). Neuroinvasive WNV disease incidence increased with age, with the highest incidence among persons aged ≥70 years. Among patients with neuroinvasive disease, 42 (9%) died.

The 130 LACV disease cases were reported from 81 counties in 14 states (Figure); 116 (89%) were considered neuroinvasive (Table 1). Dates of illness onset for LACV disease cases ranged from May through October; 110 (85%) had illness onset during July–September. Eighty-two (63%) patients were male. Among patients, median age was 8 years (range: 3 months–84 years), and 123 (95%) patients were aged <18 years. LACV neuroinvasive disease incidence was highest in West Virginia (1.19 per 100,000), Ohio (0.38), and North Carolina (0.27) (Table 2). Those three states reported 102 (78%) LACV disease cases. A total of 118 (91%) patients were hospitalized; one fatal case (1%) was reported.

Of the 16 POWV disease cases reported, 12 (75%) were neuroinvasive (Table 1). Cases were reported from 13 counties in three states: Minnesota (11 cases), Wisconsin (four), and Pennsylvania (one). Dates of illness onset ranged from May through November, with 13 (81%) occurring during May–July. The median age of patients was 59 years (range: 3 months–70 years); 13 (81%) were male. Twelve (75%) patients were hospitalized; one died.

Four states (Alabama, Arkansas, Maryland, and Missouri) reported six SLEV disease cases overall; four were neuroinvasive (Table 1). Dates of illness onset ranged from July through October. All cases occurred in adults (median age: 69 years, range: 56–81 years); three were male. Four of the six SLEV patients were hospitalized; none died.

One EEEV neuroinvasive disease case was reported from each of four states: Massachusetts, Missouri, New York, and Wisconsin. The Missouri patient acquired the infection in Massachusetts. Dates of illness onset ranged from August through October. Cases occurred in one child (aged 4 years) and three adults (aged ≥60 years); two cases occurred in males. All four patients were hospitalized; three died (Table 1).

Two neuroinvasive and one nonneuroinvasive JCV disease cases were reported from Wisconsin and Mississippi (Table 1). Dates of illness onset ranged from April through September. All three cases occurred in adults aged >50 years; two patients were men. One patient was hospitalized; none died.

Reported by

Nicole P. Lindsey, MS, Jennifer A. Lehman, Grant L. Campbell, MD, J. Erin Staples, MD, Marc Fischer, MD, Div of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases; Stephanie J. Yendell, DVM, EIS Officer, CDC. Corresponding contributor: Stephanie J. Yendell, s[email protected], 970-221-6400.

Editorial Note

In 2011, WNV was the most common cause of neuroinvasive arboviral disease in the United States; however, LACV was the most common cause of arboviral disease among children, a finding consistent with previous reports (1,4). EEEV disease, although rare, remained the most severe arboviral disease, resulting in three deaths among four patients. In 2011, 91% of mosquitoborne disease cases (i.e., those caused by WNV, LACV, SLEV, EEEV, and JCV) occurred during July–September, and 81% of tickborne disease cases (POWV) occurred during May–July, emphasizing the importance of targeting public health interventions for these periods.

Reported numbers of arboviral disease cases vary from year to year. The national incidence of WNV neuroinvasive disease in 2011 was 0.16 per 100,000 population, which is consistent with incidence rates during 2008–2010 (median: 0.20; range: 0.13–0.23) (3–5). The number of LACV neuroinvasive disease cases reported increased by 73% from 2010 to 2011. More POWV disease cases were reported in 2011 than in any previous year, and included the first case ever reported from Pennsylvania. Wisconsin reported its first EEEV case since 1984. In addition to nationally notifiable arboviral diseases, two other domestic arboviral diseases were reported to CDC: Colorado tick fever (two cases) and Cache Valley virus disease (one case).

The findings in this report are subject to at least two 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 providers and laboratories to report confirmed cases to public health authorities. Second, testing and reporting are incomplete, leading to a substantial underestimate of the actual number of cases (6). Based on previous studies, for every reported case of WNV neuroinvasive disease, approximately 140–350 human WNV infections occur, with approximately 80% of infected persons remaining asymptomatic and 20% developing nonneuroinvasive febrile disease (7–9). Extrapolating from the 486 WNV neuroinvasive disease cases reported, an estimated 13,600–34,000 cases of nonneuroinvasive febrile disease might have occurred in 2011; however, only 226 (1%–2%) nonneuroinvasive disease cases were reported.

WNV and other arboviruses continue to cause severe illness in substantial numbers of persons in the United States. However, cases are focal and sporadic, and the epidemiology varies by virus and area. Surveillance is important to identify outbreaks and guide prevention efforts (10). Health-care providers should consider arboviral infections in the differential diagnosis of aseptic meningitis and encephalitis, obtain appropriate specimens for laboratory testing, and promptly report cases to state health departments to allow for appropriate control measures (2). Human vaccines against domestic arboviruses are not available commercially in the United States. Therefore, prevention of arboviral disease depends on community and household efforts to reduce vector densities (e.g., applying insecticides and reducing numbers of mosquito breeding sites), personal protective measures to decrease exposure to mosquitoes and ticks (e.g., use of repellents and long-sleeved shirts and long pants), and screening blood donors.

Acknowledgment

ArboNET surveillance coordinators in local and state health departments.

References

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