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

What is already known about this topic? West Nile virus (WNV) is endemic in Maricopa County, Arizona. Since WNV was first detected in 2003, four outbreaks have occurred. What is added by this report? In 2021, Maricopa County experienced its fifth, and largest, WNV outbreak reported in the county: 1,487 cases, 1,014 (68%) hospi-talizations, and 101 (7%) deaths, taxing a stressed health care system during the COVID-19 pandemic. What are the implications for public health practice? Clinicians should consider WNV testing in serum and cerebrospinal fluid in patients with a clinically compatible illness. Public health agencies should continually review messaging to improve aware-ness. Human and mosquito surveillance is essential to mounting a rapid, coordinated response and limiting further spread.