Unprecedented Outbreak of West Nile Virus — Maricopa County, Arizona, 2021

Investigation and Results

WNV, an arthropod-borne arbovirus, is primarily transmitted through bites of infected Culex mosquitoes and is the leading cause of domestically acquired arbovirus infections in the United States (1). Transmission is also possible through blood transfusions; since 2005, the Food and Drug Administration has recommended WNV nucleic acid testing of minipools consisting of combined individual blood donation samples, with an automatic switch to individual donation testing upon detection of a positive result (5). Persons with a WNV-positive reverse transcription–polymerase chain reaction (RT-PCR) or immunoglobulin M (IgM) blood or cerebrospinal fluid (CSF) test result are reported to public health. Health care providers are required to report patients within 5 working days of detection, diagnosis, or treatment of a suspected or confirmed WNV infection; laboratories are required to report positive WNV test results within the same period. WNV case reports are stored within the Arizona Medical Electronic Disease Surveillance Intelligence System.*^,† MCDPH investigates reports of positive WNV laboratory test results, classifies them according to national case definitions (6), and regularly communicates with health care providers via a mass notification system (SurvAlert) regarding community health threats. MCDPH responds to WNV outbreaks in partnership with MCESD-VCD, with support from ADHS.

No vaccine or specific therapy exists for WNV; thus, treatment is supportive. The case fatality rate in persons with neuroinvasive disease is 10% (2,3). The frequency and location of outbreaks vary annually and are challenging to predict (1). In Arizona, WNV was first detected in 2003 (12 cases); the majority of cases occurred among Maricopa County residents (2). The largest outbreak previously recorded in Maricopa County occurred in 2004 (355 cases).

MCESD-VCD conducts vector surveillance and abatement^§ based on resident complaints of mosquito abundance and routine mosquito trap deployments in specific locations throughout the county.^¶ When mosquitoes are found in traps, MCESD-VCD organizes them into groups (pools) of up to 50 female Culex spp. mosquitoes to be tested as one sample. Each pool is then tested for WNV using RT-PCR; a positive mosquito pool is one in which the sample is WNV-positive. From this testing, MCESD-VCD calculates a VI (the estimated proportion of infected mosquitoes of a particular species in a specific area collected during weekly mosquito surveillance). The highest VI previously recorded in Maricopa County was 19.4 in 2019 (7). When the VI exceeds 3.0 (based on analysis of data from previous seasons), MCESD-VCD notifies MCDPH that an increase in human WNV cases is anticipated within 2–3 weeks. Laboratory processing and notification of VI to MCDPH lags throughout the season (approximately 1–2 weeks). ADHS coordinates confirmatory human WNV testing with the Arizona State Public Health Laboratory and CDC, monitors WNV surveillance data statewide, provides resources, and issues health alert notifications (HANs).

On May 4, 2021, MCESD-VCD notified MCDPH of the first 2021 WNV-positive mosquito pool. MCESD-VCD continued mosquito surveillance and commenced application of adulticides based on WNV-positive pools. On June 11, MCESD-VCD notified MCDPH that the VI had exceeded 3.0 (Figure). MCDPH enhanced routine surveillance by forwarding WNV IgM-positive serum and CSF specimens collected from persons with suspected WNV cases to the Arizona State Public Health Laboratory (ASPHL) for confirmatory testing. WNV-positive RT-PCR samples are considered confirmatory tests and were not forwarded to ASPHL. On August 12, the VI had increased by approximately 127% from the previous week (from 5.11 to 11.57). By September 2, the WNV VI was 46.72, peaking the week of September 11 at 53.61; the highest level ever recorded in the county. A VI peak this late in the season (i.e., in September) has occurred twice before in Maricopa County, in 2014 (VI = 9.6) and 2018 (VI = 7.9).

During 2021, MCDPH identified 1,487 confirmed or probable human WNV cases and an additional 78 asymptomatic viremic blood donors. The majority (95%) of persons with WNV had illness onset during a 12-week period during August 15–November 6, 2021. On September 25, the outbreak peaked at 236 cases reported in a single week. The last adulticide application occurred November 9. The last positive mosquito trap was identified the week of November 14; adulticide was not applied because the temperatures had decreased to <50°F (<10°C); according to manufacturer instructions, the material cannot be applied at these temperatures.**^,††

Among the 1,487 WNV cases, 956 (64.3%) were classified as neuroinvasive disease, and 101 (6.8%) patients died; all deaths occurred among patients with neuroinvasive disease (Table). In addition to the 78 asymptomatic WNV reports identified through routine blood donation screening, 25 of the 1,487 WNV cases were identified as blood donors with symptomatic WNV; one of these symptomatic patients was diagnosed with neuroinvasive disease. The median age among all patients was 66 years (IQR = 53–75 years), and among those who died, the median age was 79 years (IQR = 71–83 years). Most cases occurred in persons who were White (78%), non-Hispanic or Latino (76%), and male (57%). In total, 1,014 (68.2%) patients were hospitalized, with 91% of hospitalizations occurring among persons with neuroinvasive disease. The median length of hospitalization for persons with neuroinvasive disease was 7 days (IQR = 4–11 days), compared with 4 days (IQR = 2–6 days) for those with nonneuroinvasive disease. During the investigation, cross-reactivity with mumps IgM testing was reported for 11 cases. MCDPH clinical staff members reviewed patient clinical courses, including symptoms, comorbidities, and potential exposures to determine compatibility with WNV and mumps; all patients’ clinical illnesses were considered to be more consistent with WNV than with mumps.