West Nile Virus & St. Louis Encephalitis End of Year Report, 2021 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 and Data Services 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 2021. Maricopa County saw more WNV cases in 2021 than any year since the virus first arrived in Arizona. Year Total* Hospitalized Deaths Sex at Birth M/F Median Age (Min, Max) 615 Encephalitis 319 Meningitis 20 Other Neuro 920 Neuroinvasive 91 Non-neuroinvasive 101 842/638 66 (2, 97) 1084 White, Non-Hispanic 77 Hispanic or Latino 23 Other, Non-Hispanic 16 Black, Non-Hispanic 13 Asian/Pacific Island 267 Unknown * * Neuroinvasive <5 Non-neuroinvasive * /<5 73 (52, 88) 5 White, Non-Hispanic <5 Asian/Pacific Island <5 Black, Non-Hispanic <5 Hispanic or Latino <5 Unknown Non-neuroinvasive WNV 1480** 526 Fever SLE 11 <5 Neuroinvasive Neuroinvasive 101 Median Age† (Min, Max) 79 (32, 95) Sex at Birth M/F 70/31 Race/Ethnicity 84 White, Non-Hispanic 5 Hispanic or Latino 12 Other/Unknown 0 Race/Ethnicity *Cases are counted in the year in which they are reported to Public Health. One case in the year total had illness onset in 2020; 10 cases and one death were reported in 2022 and are not in the year total. †The difference in the mean age of WNV survivors (62 yrs) vs deaths (77 yrs) was statistically significant (α=0.05). **WNV viremic donors who did not become symptomatic (n=79) and those who did not have fever or neurological illness (n=79) are not included in year total. 1000 Human Infections of WNV by Month Onset 5 Year Average WNV Deaths 2017–2021 120 100 80 500 60 * * * 20 * 0 0 Jan 10 40 Feb Mar Apr May Jun Jul Aug Sep Oct Human Infections of SLE by Month Onset Nov 2017 Dec 5 Year Average 8 2018 2019 2020 2021 SLE Deaths 2017–2021 10 8 6 6 4 4 2 0 Jan Feb Mar Apr May Jun Jul Aug * * * Sep Oct Nov 2 * 0 Dec 2017 *Data are suppressed to prevent identification of cases due to small number (< 5) of cases 2018 2019 2020 2021 West Nile Virus & St. Louis Encephalitis End of Year Report, 2021 240 220 200 180 160 140 120 100 80 60 40 20 0 160 140 120 100 80 Human 60 Mosquito 40 20 Number of Mosquito Pools2 Number of Human Cases1 Human Infections of WNV 1,3,* and WNV Positive Mosquito Pools by MMWR Week Rates of Human Illness per 100,000 Population Maricopa County Overall Rate and Cities§ with Highest Rates WNV 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 One case had illness onset in 2020 and is not included in the graph. MMWR Week3 Human Infections of SLE 1,3,* and SLE Positive Mosquito Pools by MMWR Week 4 3 2 1 0 10 9 8 7 6 5 4 Human 3 Mosquito 2 1 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 34 MESA 38 SCOTTSDALE Number of Mosquito Pools2 Number of Human Cases1 5 PEORIA 152 MARICOPA COUNTY OVERALL 33 0 50 100 SLE Maricopa County Overall 0.3‡ ‡Due to the low number of cases in 2021, rates of human SLE illness by cities was not done. 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 150 § Rates for cities with ≥ 100,000 populations shown