What was the West Nile virus outbreak in Arizona during 2021?

The 2021 West Nile virus outbreak in Arizona was the largest county-level outbreak recorded in the United States. It occurred in Maricopa County and involved a significant number of infections and deaths. This event stands out because the virus vector index reached a record high, leading to a tenfold increase in human cases within two weeks.

What the research says

Maricopa County, Arizona, reported a total of 1,487 human West Nile virus cases during 2021, with 101 deaths ⁸⁹. Of those infected, 956 patients, or 64.3%, developed neuroinvasive disease affecting the brain or spinal cord ⁹. The outbreak was so severe that the local mosquito vector index reached its highest ever recorded level of 53.61 ⁹.

The virus is primarily spread by bites from infected Culex species mosquitoes ⁸⁹. Almost all infections occur this way, though rare transmission can happen through blood transfusions or organ transplants ⁸. Approximately 80% of human infections are asymptomatic, meaning people feel no symptoms ⁸¹⁰. About 20% cause a febrile illness known as West Nile fever, and less than 1% result in severe neuroinvasive disease like meningitis or encephalitis ⁸.

Mortality rates vary by health status. Overall mortality for patients with neuroinvasive disease is about 10% ⁸⁹. However, the risk is higher for older adults, those with hematologic malignancies, and patients receiving specific immunotherapies ⁸. In the 2021 Arizona outbreak, 6.8% of the total cases resulted in death ⁹.

What to ask your doctor

• What are my specific risk factors for severe West Nile virus if I live in an area with high mosquito activity?
• How does my age or existing health conditions affect my risk of neuroinvasive disease?
• What symptoms should I watch for if I have been bitten by mosquitoes in a high-risk area?
• Are there specific mosquito control measures I should take in my home or yard?
• What are the signs of neuroinvasive disease that require immediate medical attention?