West Nile Virus & St. Louis Encephalitis End of Year Report, 2023 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 Division of Epidemiology and Informatics collaborates with the Maricopa County Environmental Services Department Vector Control Division to identify and monitor areas of WNV and SLE activity. This report describes trends in human infections during 2023. Year Total* WNV 72** Clinical Syndrome Nonneuroinvasive Neuroinvasive Encephalitis Meningitis or Not specified Deaths Sex at Birth M/F Median Age (IQR) 11 (all encephalitis) 40/32 65 (58-75) Hospitalized 14 58 48 Nonneuroinvasive Neuroinvasive <5 56 Median Age† (IQR) 80 (66-84) 10 Sex at Birth M/F * / <5 Race White Black/African American Unknown * <5 1 Ethnicity Not Hispanic or Latino Unknown 9 2 Race & Ethnicity Race White Black/African American Two or more races Some Other Race alone Unknown 53 <5 <5 <5 13 Ethnicity Not Hispanic or Latino Hispanic or Latino Unknown 46 9 17 There were no SLE reports in 2023. *Cases are counted in the year in which they are reported to Public Health. **WNV viremic donors who did not become symptomatic (n=5) and those who did not have fever or neurological illness (n=<5) are not included in year total. 120 Human Infections of WNV by Month Onset 35 30 25 20 15 10 5 0 5 Year Average 200 150 100 50 Jan 4 * * * * Feb Mar Apr May Jun Jul †The difference in the mean age of WNV survivors (65 yrs) vs deaths (73 yrs) was not statistically significant (α=0.05). * * Aug Sep 0 Oct Nov Human Infections of SLE by Month Onset Dec WNV Deaths 2019–2023 100 80 60 40 20 * 0 2019 2020 * 2021 2023 SLE Deaths 2019–2023 4 5 Year Average 2022 2 2 0 * 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2019 *Data are suppressed to prevent identification of cases due to small number (< 5) of cases 2020 * 2021 2022 2023 West Nile Virus & St. Louis Encephalitis End of Year Report, 2023 Human Infections of WNV1,3,* and WNV Positive Mosquito Pools2 by MMWR Week 35 Number of Human Cases1 30 25 10 5 Human 20 Mosquito 15 10 5 0 Number of Mosquito Pools2 15 Rates of Human Illness per 100,000 Population Maricopa County Overall Rate and Cities§ with Highest Rates 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 WNV MMWR Week3 *Data are suppressed to prevent identification of cases due to small number (< 5) of cases Human Infections of SLE1,3,* and SLE Positive Mosquito Pools2 by MMWR Week PHOENIX MESA 9 8 4 7 6 3 2 Human 5 Mosquito 4 3 2 1 1 0 Number of Mosquito Pools2 Number of Human Cases1 5 1.1 1.4 SCOTTSDALE 2.9 MARICOPA COUNTY OVERALL 1.5 0 1 2 3 SLE Maricopa County Overall 0.0 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 MMWR Week3 1 A human case is an individual infected with WNV or SLE and with a clinically compatible illness. §Rates for cities with ≥ 100,000 populations and case counts 2 Mosquito pools are a collection of mosquitoes collected by MCESD from a county trap that tested positive for WNV or SLE. >4 shown. 3 Human data are based on date of symptom onset; mosquito data are based on date of mosquito collection.