Arthropodborne viruses (arboviruses) are transmitted to humans primarily through the
bites of infected mosquitoes or ticks. West Nile virus (WNV) is the leading cause
of domestically acquired arboviral disease in the continental United States (
1
). Other arboviruses, including Jamestown Canyon, La Crosse, Powassan, St. Louis encephalitis,
and eastern equine encephalitis viruses, cause sporadic cases of disease and occasional
outbreaks. This report summarizes surveillance data reported to CDC from U.S. states
in 2017 for nationally notifiable arboviruses. It excludes dengue, chikungunya, and
Zika viruses because, in the continental United States, these viruses are acquired
primarily through travel. In 2017, 48 states and the District of Columbia (DC) reported
2,291 cases of domestic arboviral disease, including 2,097 (92%) WNV disease cases.
Among the WNV disease cases, 1,425 (68%) were classified as neuroinvasive disease
(e.g., meningitis, encephalitis, or acute flaccid paralysis), for a national rate
of 0.44 cases per 100,000 population. More Jamestown Canyon and Powassan virus disease
cases were reported in 2017 than in any previous year. Because arboviral diseases
continue to cause serious illness, maintaining surveillance is important to direct
and promote prevention activities.
Arboviruses are maintained in transmission cycles between arthropods and vertebrate
hosts, including humans and other animals (
2
). Humans primarily become infected when bitten by an infected tick or mosquito. 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 using standard surveillance case definitions
that include clinical and laboratory criteria (
3
). Confirmed and probable cases are included in this analysis. Cases reported as meningitis,
encephalitis, acute flaccid paralysis, or other neurologic illnesses are classified
as neuroinvasive disease; the remainder are considered non-neuroinvasive disease.
Incidence was calculated using neuroinvasive disease cases and the 2017 U.S. Census
mid-year population estimates.
In 2017, 2,291 cases of domestic arboviral disease were reported to CDC; 1,596 (70%)
were neuroinvasive. Cases were caused by the following viruses: WNV (2,097 cases;
92%), Jamestown Canyon (75), La Crosse (63), Powassan (34), St. Louis encephalitis
(11), unspecified California serogroup (six), and eastern equine encephalitis (five).
No cases were reported from Alaska or Hawaii. Among the 3,007 counties in the contiguous
United States, 722 (24%) reported one or more arboviral disease cases.
Overall, 2,097 WNV disease cases were reported from 641 counties in 47 states and
DC. Among these cases, 1,425 (68%) were neuroinvasive, and 1,814 (87%) patients had
illness onset during July–September (Table 1). The median patient age was 59 years
(interquartile range [IQR] = 46–69); 1,301 (62%) were male. A total of 1,545 (74%)
patients were hospitalized, and 146 (7%) died. The median age of patients who died
was 77 years (IQR = 68–84).
TABLE 1
Number and percentage of reported cases of West Nile virus and other arboviral diseases,
by virus type and selected patient characteristics — United States, 2017
Characteristic
Virus
no. (%)
West Nile*
(N = 2,097)
Jamestown Canyon
(N = 75)
La Crosse
(N = 63)
Powassan
(N = 34)
St. Louis encephalitis*
(N = 11)
Eastern equine encephalitis
(N = 5)
Age group (yrs)
<18
50 (2)
4 (5)
54 (86)
5 (15)
0 (0)
0 (0)
18–59
1,020 (49)
36 (48)
6 (10)
10 (29)
5 (45)
5 (100)
≥60
1,027 (49)
35 (47)
3 (5)
19 (56)
6 (55)
0 (0)
Sex
Male
1,301 (62)
46 (61)
34 (54)
28 (82)
8 (73)
2 (40)
Female
796 (38)
29 (39)
29 (46)
6 (18)
3 (27)
3 (60)
Period of illness onset
January–March
7 (<1)
1 (1)
1 (2)
0 (0)
1 (9)
0 (0)
April–June
87 (4)
25 (33)
4 (6)
21 (62)
0 (0)
0 (0)
July–September
1,814 (87)
45 (60)
53 (84)
7 (21)
8 (73)
1 (20)
October–December
185 (9)
4 (5)
5 (8)
6 (18)
1 (9)
4 (80)
Clinical syndrome
Nonneuroinvasive
672 (32)
17 (23)
0 (0)
1 (3)
5 (45)
0 (0)
Neuroinvasive
Encephalitis
714 (34)
29 (39)
53 (84)
22 (65)
2 (18)
3 (60)
Meningitis
530 (25)
5 (7)
10 (16)
7 (21)
3 (27)
0 (0)
AFP
89 (4)
4 (5)
0 (0)
2 (6)
1 (9)
0 (0)
Other
92 (4)
20 (27)
0 (0)
2 (6)
0 (0)
2 (40)
Outcome
Hospitalization
1,545 (74)
46 (61)
63 (100)
33 (97)
6 (55)
5 (100)
Death
146 (7)
2 (3)
0 (0)
2 (6)
0 (0)
2 (40)
Abbreviation: AFP = acute flaccid paralysis.
*Date of illness onset missing for four cases of West Nile virus and one case of St.
Louis encephalitis virus.
Among the 1,425 WNV neuroinvasive disease cases, 714 (50%) were reported as encephalitis,
530 (37%) as meningitis, 89 (6%) as acute flaccid paralysis, and 92 (6%) as other
neurologic illness. Among the 89 patients with acute flaccid paralysis, 34 (38%) also
had encephalitis or meningitis. Among patients with neuroinvasive disease, 1,346 (94%)
were hospitalized, and 146 (10%) died. The rate of neuroinvasive disease in the United
States was 0.44 per 100,000 population (Table 2). The highest rates were in South
Dakota (3.10 per 100,000), North Dakota (2.65), Mississippi (1.54), Arizona (1.40),
and Utah (1.26) (Figure). The largest numbers of neuroinvasive disease cases were
reported from California (401), Arizona (98), Texas (87), and Illinois (72), and together
accounted for 46% of neuroinvasive disease cases. The rate of WNV neuroinvasive disease
increased with patient age, from 0.02 per 100,000 in children aged <10 years to 1.28
in adults aged ≥70 years. The rate was higher among males (0.57 per 100,000) than
among females (0.31).
TABLE 2
Number and rate* of reported cases of arboviral neuroinvasive disease, by virus type,
U.S. Census division, and state — United States, 2017
U.S. Census Division/State
Virus
West Nile
Jamestown Canyon
La Crosse
Powassan
St. Louis encephalitis
Eastern equine encephalitis
No.
Rate
No.
Rate
No.
Rate
No.
Rate
No.
Rate
No.
Rate
United States
1,425
0.44
58
0.02
63
0.02
33
0.01
6
<0.01
5
<0.01
New England
10
0.07
5
0.03
—†
—
9
0.06
—
—
—
—
Connecticut
2
0.06
—
—
—
—
—
—
—
—
—
—
Maine
—
—
1
0.07
—
—
3
0.22
—
—
—
—
Massachusetts
5
0.07
2
0.03
—
—
3
0.04
—
—
—
—
New Hampshire
—
—
2
0.15
—
—
1
0.07
—
—
—
—
Rhode Island
1
0.09
—
—
—
—
2
0.19
—
—
—
—
Vermont
2
0.32
—
—
—
—
—
—
—
—
—
—
Middle Atlantic
66
0.16
—
—
—
—
13
0.03
—
—
—
—
New Jersey
6
0.07
—
—
—
—
4
0.04
—
—
—
—
New York
45
0.23
—
—
—
—
5
0.03
—
—
—
—
Pennsylvania
15
0.12
—
—
—
—
4
0.03
—
—
—
—
East North Central
192
0.41
37
0.08
16
0.03
3
0.01
—
—
1
<0.01
Illinois
72
0.56
—
—
1
0.01
—
—
—
—
—
—
Indiana
18
0.27
—
—
—
—
—
—
—
—
—
—
Michigan
32
0.32
—
—
—
—
—
—
—
—
—
—
Ohio
23
0.20
1
0.01
13
0.11
—
—
—
—
—
—
Wisconsin
47
0.81
36
0.62
2
0.03
3
0.05
—
—
1
0.02
West North Central
118
0.55
14
0.07
2
0.01
8
0.04
—
—
—
—
Iowa
10
0.32
—
—
1
0.03
—
—
—
—
—
—
Kansas
12
0.41
—
—
—
—
—
—
—
—
—
—
Minnesota
13
0.23
14
0.25
1
0.02
7
0.13
—
—
—
—
Missouri
17
0.28
—
—
—
—
—
—
—
—
—
—
Nebraska
19
0.99
—
—
—
—
—
—
—
—
—
—
North Dakota
20
2.65
—
—
—
—
1§
0.13
—
—
—
—
South Dakota
27
3.10
—
—
—
—
—
—
—
—
—
—
South Atlantic
91
0.14
1
<0.01
28
0.04
—
—
—
—
4
0.01
Delaware
—
—
—
—
—
—
—
—
—
—
—
—
District of Columbia
1
0.14
—
—
—
—
—
—
—
—
—
—
Florida
4
0.02
—
—
—
—
—
—
—
—
1
<0.01
Georgia
44
0.42
—
—
2
0.02
—
—
—
—
2
0.02
Maryland
5
0.08
—
—
1
0.02
—
—
—
—
1
0.02
North Carolina
8
0.08
1
0.01
21
0.20
—
—
—
—
—
—
South Carolina
16
0.32
—
—
—
—
—
—
—
—
—
—
Virginia
12
0.14
—
—
—
—
—
—
—
—
—
—
West Virginia
1
0.06
—
—
4
0.22
—
—
—
—
—
—
East South Central
117
0.61
—
—
17
0.09
—
—
1
0.01
—
—
Alabama
40
0.82
—
—
—
—
—
—
1
0.02
—
—
Kentucky
9
0.20
—
—
—
—
—
—
—
—
—
—
Mississippi
46
1.54
—
—
—
—
—
—
—
—
—
—
Tennessee
22
0.33
—
—
17
0.25
—
—
—
—
—
—
West South Central
174
0.44
1
<0.01
—
—
—
—
—
—
—
—
Arkansas
15
0.50
—
—
—
—
—
—
—
—
—
—
Louisiana
38
0.81
1
0.02
—
—
—
—
—
—
—
—
Oklahoma
34
0.86
—
—
—
—
—
—
—
—
—
—
Texas
87
0.31
—
—
—
—
—
—
—
—
—
—
Mountain
243
1.01
—
—
—
—
—
—
3
0.01
—
—
Arizona
98
1.40
—
—
—
—
—
—
3
0.04
—
—
Colorado
29
0.52
—
—
—
—
—
—
—
—
—
—
Idaho
16
0.93
—
—
—
—
—
—
—
—
—
—
Montana
3
0.29
—
—
—
—
—
—
—
—
—
—
Nevada
31
1.03
—
—
—
—
—
—
—
—
—
—
New Mexico
23
1.10
—
—
—
—
—
—
—
—
—
—
Utah
39
1.26
—
—
—
—
—
—
—
—
—
—
Wyoming
4
0.69
—
—
—
—
—
—
—
—
—
—
Pacific
414
0.78
—
—
—
—
—
—
2
<0.01
—
—
Alaska
—
—
—
—
—
—
—
—
—
—
—
—
California
401
1.01
—
—
—
—
—
—
2
0.01
—
—
Hawaii
—
—
—
—
—
—
—
—
—
—
—
—
Oregon
3
0.07
—
—
—
—
—
—
—
—
—
—
Washington
10
0.14
—
—
—
—
—
—
—
—
—
—
* Per 100,000 population, based on July 1, 2017, U.S. Census population estimates.
† Dashes indicate none reported.
§ Patient reported travel to a state with a history of the virus.
FIGURE
Rate* of reported cases of West Nile virus neuroinvasive disease — United States,
2017
* Per 100,000 population.
The figure is a map of the United States showing the rate of cases of West Nile virus
neuroinvasive disease reported in 2017.
Seventy-five Jamestown Canyon virus disease cases were reported from eight states,
primarily in the Northeast and upper Midwest (Table 2). In 2017, Jamestown Canyon
virus disease was reported for the first time from Louisiana, Maine, and North Carolina.
The median patient age was 58 years (IQR = 41–68), and 46 (61%) were male (Table 1).
Illness onset ranged from January to November, with 45 (60%) patients reporting onset
during July–September. Fifty-eight (77%) cases were neuroinvasive, 46 (61%) patients
were hospitalized, and two (3%) died; both deaths were among patients aged ≥58 years
with neuroinvasive disease. The rate of Jamestown Canyon virus neuroinvasive disease
was highest in Wisconsin (0.62 per 100,000).
Sixty-three La Crosse virus disease cases were reported from 10 states, all in the
Southeast and Midwest (Table 2). The median age of patients was 8 years (IQR = 5–12),
and 54 (86%) were aged <18 years (Table 1). Illness onset dates ranged from March
to October, with 53 (84%) reporting onset during July–September. All 63 cases were
neuroinvasive and the patients were hospitalized; none died.
Thirty-four Powassan virus disease cases were reported from 10 states, primarily in
the Northeast and Midwest (Table 2). The median patient age was 63 years (IQR = 48–74)
and 28 (82%) were male (Table 1). Illness onset dates ranged from April to December,
with 21 (62%) reporting onset during April–June. Powassan virus disease was reported
for the first time from North Dakota in 2017, but the patient had history of travel
to a state with previously documented transmission. Thirty-three (97%) cases were
neuroinvasive, 33 (97%) patients were hospitalized (32 with neuroinvasive disease
and one with non-neuroinvasive disease), and two (6%) patients died.
Eleven cases of St. Louis encephalitis virus disease were reported from three states
(Alabama, Arizona, and California) (Table 2). The median patient age was 60 years
(IQR = 48–63), and eight were male (Table 1). Illness onset dates ranged from January
to October, with eight patients reporting onset during July–September. Six cases were
neuroinvasive; all patients with neuroinvasive disease were hospitalized. No patients
died.
Five cases of eastern equine encephalitis virus disease were reported from four states
(Florida, Georgia, Maryland, and Wisconsin) (Table 2); however, infection occurred
in three of the cases through organ transplantation. The median patient age was 42
years (IQR = 27–42), and two were male. All cases occurred in September and October.
All cases were neuroinvasive, and the patients were hospitalized; two (40%) patients
died.
Discussion
As in previous years, WNV was the most common cause of neuroinvasive arboviral disease
in the United States, accounting for 89% of reported neuroinvasive disease cases.
The rate of WNV neuroinvasive disease in 2017 (0.44 per 100,000) was comparable to
the median rate during 2007–2016 (0.41) (
4
). La Crosse virus continued to be the most common cause of neuroinvasive arboviral
disease in children (
5
).
In 2017, eastern equine encephalitis virus transmission via organ transplantation
was reported for the first time (
6
). More cases of Jamestown Canyon and Powassan virus disease were reported in 2017
than in any previous year: 75 Jamestown Canyon virus disease cases were reported in
2017 compared with a previous high of 16 cases in 2013, and 34 Powassan virus disease
cases were reported in 2017 compared with a previous high of 22 cases in 2016 (
7
,
8
). These recent increases are likely caused by an increase in awareness and testing,
but increased activity of these viruses cannot be ruled out. Deaths possibly associated
with Jamestown Canyon virus infection are rare; however, two deaths were reported
in 2017.
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. Consistent with previous years, in 2017, approximately
90% of arboviral disease cases occurred during April–September. Weather, zoonotic
host and vector abundance, and human behavior are all factors that can influence when
and where outbreaks occur. These factors make it difficult to predict future locations
and timing of cases and emphasize the importance of surveillance to identify outbreaks
and inform public health prevention efforts.
The findings in this report are subject to at least two limitations. First, because
ArboNET does not require information about clinical signs and symptoms or laboratory
findings, cases might be misclassified. Second, ArboNET is a passive surveillance
system that only collects cases that are diagnosed and reported, resulting in underestimation
of the actual incidence of disease. Detection and reporting of neuroinvasive disease
are thought to be more consistent and more complete than they are for non-neuroinvasive
disease. Previous studies have estimated that between 30 and 70 non-neuroinvasive
disease cases occur for every reported case of WNV neuroinvasive disease (
9
). Based on the number of neuroinvasive disease cases reported in 2017, between 42,750
and 99,750 non-neuroinvasive disease cases of WNV would have been expected to occur;
however, only 672 (1%–2%) were reported.
Health care providers need to consider arboviral infections in the differential diagnosis
of aseptic meningitis and encephalitis, obtain appropriate specimens for laboratory
testing, and promptly report cases to public health authorities (
2
,
3
). Understanding the epidemiology, seasonality, and geographic distribution of these
viruses will assist with clinical recognition and differentiation from other neurologic
infections. 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 blood donor screening.
Summary
What is already known about this topic?
West Nile virus (WNV) is the leading cause of arboviral disease in the continental
United States, but several other arboviruses cause sporadic cases and outbreaks of
neuroinvasive disease.
What is added by this report?
In 2017, eastern equine encephalitis virus transmission via organ transplantation
was reported for the first time. More cases of Jamestown Canyon and Powassan virus
neuroinvasive disease were reported in 2017 than in any previous year.
What are the implications for public health practice?
Health care providers need to consider arboviral infections in the differential diagnosis
of aseptic meningitis and encephalitis, obtain appropriate specimens for laboratory
testing, and promptly report cases to public health authorities.