West Nile Virus & St. Louis Encephalitis End of Year Report, 2020
West Nile virus (WNV) arrived in Arizona in 2003 and is now endemic in Maricopa County. St. Louis encephalitis virus (SLE) is a rare mosquito-borne disease in Maricopa County. Maricopa County Department of Public
Health Office of Epidemiology collaborates with the Maricopa County Department of Environmental Services Office of Vector Control to identify and monitor areas of WNV and SLE activity. This report describes trends in
human infections during 2020. Maricopa County saw the least WNV cases this year since the virus first arrived in Arizona. Some data has been suppressed to protect confidentiality.

Year
Total

Non-neuroinvasive

Neuroinvasive

Hospitalized

Deaths

Gender
M/F

Median Age

Race/Ethnicity

WNV
3

<5

<5

<5

<5†

<5

69

<5

SLE
6

<5

<5

<5

<5‡

<5

71

6 White, Non-Hispanic

†The difference in the mean age of WNV survivors vs deaths was not statistically significant (α=0.05).

No viremic blood donors who did not develop symptoms were identified for either WNV or SLE

‡ The difference in the mean age of SLE survivors vs deaths was not statistically significant (α=0.05).

30

Human Infections of WNV by Month Onset*

WNV Deaths 2016–2020*

20
15

20

10
10
0

5

*
Jan

4

*
Feb

Mar

Apr

May

Jun

*

0
Jul

Aug

Sep

Oct

Nov

2016

Dec

Human Infections of SLE by Month Onset*

2017

2018

2019

2020

SLE Deaths 2016–2020*

3

3

2
2
1

*
Feb

Mar

Apr

May

*

Jan

*

0

*

1

Jun

Jul

Aug

*

0
Sep

Oct

Nov

Dec

2016

*Data are suppressed to prevent identification of cases due to small number (< 5) of cases in each figure

2017

2018

2019

2020

West Nile Virus & St. Louis Encephalitis End of Year Report, 2020
Human Infections of WNV 1,3,* and WNV Positive Mosquito Pools by MMWR Week

3
2
1

Number of Mosquito Pools2

4

0

Rates of Human Illness per
100,000 Population
Maricopa County Overall Rate
and Cities with Highest Rates
WNV**
MARICOPA COUNTY
OVERALL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
MMWR Week3

0.07

0

Human Infections of SLE 1,3,* and SLE Positive Mosquito Pools by MMWR Week

0.02 0.04 0.06 0.08 0.1

25
20
15
10
5
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
1 A human case is an individual infected with WNV or SLE and with a clinically compatible illness
2 Mosquito pools are a collection of mosquitoes collected by MCDES from a county trap and tested positive for WNV and SLE
3 Human data are based on date of symptom onset, mosquito data are based on date of mosquito collection

*Data are suppressed to prevent identification of cases due to small number (< 5) of cases in each figure

Number of Mosquito Pools2

30

SLE**
MARICOPA COUNTY
OVERALL

0.1

0

0.2

**Due to the low number of cases in 2020,

rates of human WNV and SLE illness by
cities is not shown.