Unprecedented West Nile Virus Outbreak - Maricopa County, Arizona, 2021

ABSTRACT

Background. West Nile virus (WNV) causes a mosquito-borne disease that can lead to permanent paralysis or death. WNV became endemic in Maricopa County in 2003. The Maricopa County Department of Public Health (MCDPH) and Environmental Services Department Vector Control (MCESD VC) and Arizona Department of Health Services (ADHS) collaborated to monitor and respond to mosquito surveillance and human WNV cases. We describe an unprecedented WNV outbreak in 2021. Methods. MCESD VC monitored vector complaints, mosquito vector positive traps, and vector index (VI) data. Reported human cases (using the CSTE/CDC case definition) were interviewed by MCDPH. MCDPH and ADHS sent surveillance alerts to healthcare providers regarding WNV symptomology and clinical testing guidance. MCDPH consulted with providers and/or Infection Preventionists during case investigations. Results. The first WNV-positive mosquito trap occurred the week of May 1 (VI=0.235);VI peaked the week of September 11 at 53.608;the last positive trap was the week of November 20. MCESD VC fogged >400,000 acres (~2x the 10-year average) with adulticide, applied larvicide to ~25,000 sites, and received ~9,500 mosquito complaints (40% more than prior season). During the 2021 WNV season, 1,487 cases were reported, of which 960 (64.6%) had neuroinvasive illnesses (WNND) and 101 (6.8%) died (all were WNND cases). Median case age was 66 (IQR=53-75) years;for deaths, it was 79 (IQR=71-83) years. In total, 1,017 (68.4%) cases were hospitalized with a median length of stay (LOS) of 7 (IQR=4-10) days. WNND cases accounted for 91.2% of hospitalizations. Median LOS for hospitalized WNND cases was 7 (IQR=4-11) days, compared to 4 (IQR=2-6) days for nonneuroinvasive cases. Despite 3 surveillance alerts sent to providers and media coverage, providers were not consistently aware of the WNV outbreak or that serum can be tested for WNV. Conclusion. This was the largestWNVoutbreak in Maricopa County. It taxed an already stressed healthcare system during the COVID-19 pandemic with over 1,000 hospitalized cases, including nearly 100 in people without WNND. During WNV season, clinicians should consider WNV testing of both serum and CSF in severely ill patients with and without evidence of neuroinvasive disease.