West Nile Virus in Maricopa County

Culex larvae found collecting in standing water

Image by CDC/James Gathany - License: Public Domain.

Maricopa County Department of Public Health
Office of Epidemiology
June 2011

January 1, 2010—December 31, 2010

Commentary
West Nile virus (WNV) is a mosquito-borne virus that causes a non-specific, selflimited, febrile illness. Mosquitoes become infected when they feed on infected birds that
have migrated into an area. The mosquitoes then bite people who may or may not become
infected. The cycle of WNV occurs at an unusually high intensity when there is both a large
number of infected birds and a high concentration of infected mosquitoes in a relatively
small geographic area. Mosquitoes are the known carriers (vectors) of the virus from the
host birds to humans. Humans and animals (i.e., horses) are incidental hosts in this birdmosquito cycle, and thus cannot pass the virus to others. Because WNV causes death in
birds, we expect dead birds to be the first warning of WNV activity in an area.
WNV is widespread in Africa, North America, Europe, the Middle East, India,
Southeast Asia, Australia, the Caribbean and Central and South America. Although it is
now widespread in the United States, WNV was not present in Arizona until 2003. WNV is
now endemic in Maricopa County and is expected to be a public health concern
indefinitely. WNV surveillance season begins April 1st and ends November 30th, however,
in Arizona the majority of cases occur between the months of June and October, with cases
as early as January and as late as November. All residents and visitors are urged to
continue to take precautions against WNV infection every year.
The Maricopa County Department of Public Health (MCDPH) and the Maricopa
County Department of Environmental Services (MCDES) work closely to educate and
protect the residents of Maricopa County against WNV. MCDES has an extensive
mosquito-trapping program that enables staff to identify areas where there is a lot of
mosquito activity and to detect mosquito-borne diseases. Various types of mosquito traps
are set up throughout the county to collect mosquitoes for testing. These mosquitoes are
pooled together, by species, into groups of 1-50 mosquitoes. This collection of mosquitoes
is called a mosquito pool. The mosquitoes are then ground up and tested for WNV and
other mosquito-borne diseases. MCDES responds to complaints regarding green
(unattended) swimming pools, stagnant water, dead birds, and mosquitoes.
MCDPH is responsible for the interaction of the virus with humans. Working with
hospitals and medical providers throughout the county, Public Health conducts disease
surveillance to find “hot spots” in the Valley where there are a particularly high number of
people with WNV. MCDPH nurses and epidemiologists monitor patients with potential
WNV symptoms who seek medical care, monitor disease patterns in order to stop

-2-

transmission of the virus, and assist the public by giving recommendations for controlling
the spread of or exposure to WNV in many different settings.
The majority (~80%) of people infected with WNV will show no symptoms at all.
For those that are symptomatic (~20%), symptoms will appear 2-14 days after receiving
the mosquito bite. Symptomatic cases are characterized by the acute onset of fever,
headache, arthralgias, myalgias, and sometimes accompanied by a maculopapular rash or
lymphadenopathy. Rarely do symptoms get more severe; however, 1-3% of symptomatic
infections will develop into a neuroinvasive form of the disease. In neuroinvasive West Nile
virus, the central nervous system (CNS) is involved and clinical syndromes ranging from
meningitis (inflammation of the lining of the brain and spinal cord) to encephalitis
(inflammation of the brain), or acute paralysis can occur. There is no treatment for WNV;
only supportive care can be given.
Between January 1, 2010 and December 31, 2010, the MCDPH received reports of
126 residents infected with WNV, 115 of whom developed symptoms, and 12 of whom died
from the disease. This was the second highest year for West Nile cases in Maricopa County
ever recorded, but was still substantially lower than the 355 cases reported in 2004.
This represents a substantial increase in cases from the 19 cases reported in 2009. It
should be noted that the 2009 season coincided with the increase in summer influenza
cases during the H1N1 influenza pandemic, and therefore fewer WNV cases may have been
detected and/or reported.
The 2010 WNV season was unusual as there were a high number of cases and
mosquito positives reported early in the summer (mostly in July) from residents and
mosquito pools in the East Valley. This intense, localized transmission of WNV was unlike
transmission patterns seen from 2005 to 2009, which typically showed the peak number of
cases and positives being reported in late August to September throughout Maricopa
County (Figure 1). This focus of transmission occurred while there was minimal WNV
activity throughout the rest of the United States. MCDPH and ADHS requested assistance
from the Centers for Disease Control & Prevention (CDC) to define the size and scope of
the outbreak, identify modifiable risk factors for WNV infection, and determine prevention
techniques and messages to help curtail the outbreak. Reports on the findings from the
CDC’s investigation are forthcoming.
The following pages summarize the incidence of WNV cases in humans by age,
gender, case classification and by city of residence. In addition, WNV surveillance
indicators

and

data

on

mosquito

-3-

pools

are

also

summarized.

Figure 1. WNV Cases in Humans in Maricopa County by Month of Onset, 2005 - 2010
45

2005 (n=70)

2006 (n=75)

40

2007 (n=68)
2008 (n=91)

35

2009 (n=19)
2010 (n=115)

Human Cases

30
25
20

15
10

5
0

Jan

Feb

Mar

Apr

May

Jun

-4-

Jul
Aug
Month

Sep

Oct

Nov

Dec

Figure 2. WNV Cases in Humans and WNV Positive Mosquito Pools in Maricopa County by Month, 2010
45
70
Human Cases¹ (n=115)

Mosquito Pools² (n=163)

40

60

50
Human Cases

30
25

40

20

30

15
20

WNV Positive Mosquito Pools

35

10
10

5
0

0

Jan

Feb

Mar

Apr

May

Jun

Jul
Aug
3
Month

1A

Sep

Oct

Nov

Dec

human case is an individual infected with WNV and with a clinically compatible illness.
pools are a collection of mosquitoes that have been collected from a county trap that are pooled together, by species, into groups of 1-50
mosquitoes and then ground up and tested for WNV.
3 Human data are based on the date of symptom onset. Mosquito data are based on date specimen was collected. Since WNV has an incubation
period ranging from 2-14 days, it is expected that there will be a lag between mosquito positives and human case onsets.
2 Mosquito

-5-

Figure 3. West Nile Virus Surveillance Indicators: Dates of First Positive Findings in a Mosquito Pool and
Onset in a Human in Maricopa County, 2010 *
1

5/25/10
Mosquito

January

February

March

April

May

June

July

August

September

October

6/1/10
Human

*Reports from previous years have included the first positive equine. In 2010, no horses tested positive for WNV in Maricopa County.

-6-

November

December

Table 1. West Nile Virus Cases in Maricopa County by Gender and Case
Classification, 2010
Case

WNV Cases
Male Female

Classification1

% of Total
Cases

Meningitis

17

18

35

30%

Encephalitis

20

10

30

26%

3

1

4

3%

Neuroinvasive Disease Total

40

29

69

60%

Fever

19

8

27

23%

4

2

6

5%

2

4

6

5%

2

5

7

6%

Non-Neuroinvasive Total

27

19

46

40%

Total Number of Cases

67

48

115

100%

5

6

11

-

Paralysis Syndrome

Fever in a viremic

donor2

Unspecified3
Unspecified in a viremic

Viremic Donors
1

Total

donor3,4

Asymptomatic5

Case Classification may differ from the numbers reported by Arizona Department of Health Services.

2Fever

in a viremic donor cases are included in fever cases throughout the rest of this report.

3In

2010, MCDPH expanded the interpretation of the WNV case definition to include mild cases of illness
without fever, captured in the “Unspecified” category.
4Unspecified

in a viremic donor cases are included in fever cases throughout the rest of this report.

5Fever,

fever in a viremic donor, unspecified, unspecified in a viremic donor are included in nonneuroinvasive cases throughout the rest of this report.

Case Classifications:
The Maricopa County Department of Public Health uses the Centers for Disease Control
and Prevention’s (CDC) case definition to confirm a case of West Nile virus. CDC case
definitions are reviewed and modified as more information is learned about the disease.
Since WNV is a relatively new disease, changes to the WNV case definition are common. A
case of WNV is a clinically compatible illness with supporting laboratory evidence. Cases of
WNV are classified either as neuroinvasive or non-neuroinvasive, which are explained
below. WNV infection can also be asymptomatic or result in a febrile illness of variable
severity sometimes associated with CNS involvement.
Non-Neuroinvasive West Nile Virus: Requires, at minimum, the presence of
documented fever, as measured by the patient or clinician, the absence of
neuroinvasive disease, and the absence of a more likely clinical explanation for the
illness.
-7-

o Fever: A non-localized, self-limited febrile illness characterized by the acute
onset of fever, headache, arthralgias, myalgias, and sometimes accompanied
by a maculopapular rash or lymphadenopathy.
o Unspecified: A non-localized, self-limited illness with the same symptoms as
listed (above) for fever, but in the absence of a fever. (This was a new
classification category for the 2010 season).
Neuroinvasive West Nile Virus: Requires the presence of fever, the absence of
a more likely clinical explanation for the illness, and at least one of the following
documented by a physician:
1. Acutely altered mental status (e.g., disorientation, obtundation, stupor, or
coma)
2. Acute signs of central or peripheral neurologic dysfunction (e.g., paresis or
paralysis, nerve palsies, sensory deficits, abnormal reflexes, generalized
convulsions, or abnormal movements)
3. Pleocytosis (increased white blood cell concentration in cerebrospinal fluid
[CSF]) associated with illness clinically compatible with meningitis (e.g.,
headache or stiff neck)
Neuroinvasive cases are classified using the following categories:
o Meningitis: WNV meningitis is characterized by fever, headache, stiff neck,
and pleocytosis.
o Encephalitis: WNV encephalitis is characterized by fever, headache, and
altered mental status ranging from confusion to coma with or without
additional signs of brain dysfunction (e.g., paresis or paralysis, cranial nerve
palsies, sensory deficits, abnormal reflexes, generalized convulsions, and
abnormal movements).
o Paralysis syndrome: No CDC case definition for this syndrome exists at the
present time. The criteria applied to Maricopa County cases are:
Laboratory confirmed acute WNV-infection; AND
Physician documented acute paralysis.
Viremic Donor: The CDC defines a WNV positive viremic donor as a person who
donated blood and had a positive test when screened for the presence of West Nile
-8-

virus. Viremic donors are followed up by the blood agency to verify their infection
with additional tests. Some viremic donors will remain asymptomatic, but others
will go on to develop symptoms.
o Asymptomatic Viremic Donor: Viremic donors who do not become
symptomatic are not included in case numbers. The criteria applied to
Maricopa County cases are:
Laboratory confirmed acute WNV-infection; AND
No report of symptoms.
o Symptomatic Viremic Donor: Once the symptoms are reviewed, the case will
be classified as a confirmed case of viremic donor encephalitis, Viremic donor
meningitis, Viremic donor paralysis syndrome, Viremic donor fever, or
Viremic donor unspecified. The criteria applied to Maricopa County cases
are:
Laboratory confirmed acute WNV-infection; AND
A report of symptoms compatible with West Nile Fever,
meningitis, encephalitis, paralysis syndrome, or unspecified
(see above definitions).
For more information visit:
http://www.cdc.gov/ncidod/dvbid/westnile/clinicians/surveillance.htm#casedef
http://www.azdhs.gov/phs/oids/pdf/casedefinitions.pdf

-9-

Figure 4. West Nile Virus Human Cases in Maricopa County by Gender and Case Classification, 2010 (n=115)

40
Female

35

35
33

30

30

Number of Cases

Male

25

20

15

13

10

4

5

0

Meningitis

Encephalitis

Paralysis Syndrome
Case Classification

- 10 -

Fever

Unspecified

Figure 5. West Nile Virus Cases in Maricopa County by Case Classification, 2006-2010

80
2006 (n=75)

2007 (n=68)

2008 (n=91)

2009 (n=19)

69

70

Number of Cases

60

55

50

40

30

46

40

38

36

35
30

20
12
10

2010 (n=115)

7

0
Non-neuroinvasive

Neuroinvasive
Case Classification

- 11 -

Table 2. Mean, Median, and Range of Ages of West Nile Virus Deaths and
Survivors in Maricopa County, 2010 (n=115)
Mean Age

Median Age

Age Range

Died (n=12)

66.3

67.5

32-89

Survived (n=103)

51.0

50.0

10-85

Total

52.6

51.0

10-89

Figure 6. West Nile Virus Deaths in Maricopa County by Gender, 2006-2010
(n=28)
14
Male (n=20)

Female (n=8)

12

12

Number of Deaths

10

8

6

6

6

4

4

2
0

0

2006

2007

2008
Year

- 12 -

2009

2010

Table 3. Mean, Median, and Range of Ages of West Nile Virus Cases in
Maricopa County, 2010 (n=115)
Mean Age

Median Age

Age Range

Non-neuroinvasive

49.9

49.0

10-80

Neuroinvasive

54.41

54.0

14-89

Total

52.6

51.0

10-89

Mean age of West Nile virus non-neuroinvasive cases vs. neuroinvasive cases was not significantly
different, p=<.05
1

Figure 7. West Nile Virus Cases in Maricopa County by Age and Case
Classification, 2010 (n=115)
30
28

Non-neuroinvasive (n=46)

Neuroinvasive (n=69)

25

25

Number of Cases

20
16

16

15
11

10

10

6
5
3

0
0-17

18-29

30-39

40-49

50-59

Age (Years)

- 13 -

60-69

70-79

80+

Figure 8. Mean Ages in Years of West Nile Virus Cases in Maricopa County by Case Classification, 20062010
70
Non-neuroinvasive

58.6

60

Mean Age (Years)

50

58.5
56.4

50.1

48.0

Neuroinvasive

59.3
54.4

53.8
49.9

48.8

40

30

20

10

0
2006

2007

2008
Year

- 14 -

2009

2010

Figure 9. Rates of West Nile Virus in Maricopa County by City, 2010

0.0

2.0

4.0

Chandler

6.0

8.0

10.0

12.0

1.32

City1

6.15

Peoria

3.25

0.76

Queen Creek

17.12

Scottsdale
Surprise
Tempe

5.98
3.40
3.71

Rate per 100,000 Population2
1 Population
2 Rate

18.0

14.87

Mesa

Phoenix

16.0

4.66

Gilbert
Glendale

14.0

statistics for each city were obtained from United States Census Bureau 2010 Census.
per 100,000 population = (N/population) * 100,000.

- 15 -

Figure 10. Rates of West Nile Virus in Maricopa County by City1, 2006-2010
16.0
2006
2007

14.0

2008

Rate per 100,000 Population1

2009
2010

12.0

10.0

8.0

6.0

4.0

2.0

0.0
Chandler

Glendale

Gilbert

Mesa

Peoria
City2

1 Rates

Phoenix

Scottsdale

Tempe

Total³

shown for the top 8 cities by population in Maricopa County.
per 100,000 population = (N/population) * 100,000.
3Cities population statistics for each city were obtained from United States Census Bureau 2006, 2007, 2008, and 2009 estimates and from the
2010 Census.
4Total includes cities displayed in figure, as well as other cities and unincorporated areas with <5 cases.
2Rate

- 16 -