Maricopa County had the worst outbreak ever for West Nile in 2021

It was a historically bad year for West Nile in Maricopa County.
It was a historically bad year for West Nile in Maricopa County.(Franco Patrizia)
Published: Apr. 27, 2023 at 9:46 PM MST
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PHOENIX (3TV/CBS 5) — A new report shows Maricopa County had a historically bad West Nile virus season in 2021. According to the report from the Centers for Disease Control and Prevention, Arizona’s largest county saw 1,487 cases of West Nile virus, with 956 patients having the disease, or 64%. Of those, 101 people died. It’s the largest recorded outbreak of West Nile virus in a U.S. county, with more than four times the number of cases reported than the previous largest outbreak in Maricopa County in 2004, the CDC said.

On August 12, health officials saw that the West Nile index, which is a measure of infected mosquitos, had increased 127% from the previous week. By early September 2021, they noticed the index was substantially elevated, and they told the Maricopa County Department of Public Health and the Arizona Department of Health Services. By then, the CDC said, there were already 100 West Nile cases in the Valley. Within two weeks, the index had reached its highest ever recorded level, 53.61, with human cases increasing tenfold, the CDC said. The CDC report said the MCDPH tried to get the word out about how quickly the West Nile virus was spreading, but there was still a lack of awareness of the outbreak.

The county’s Vector Control Division deploys around 800 routine traps each week.

In the report, federal officials couldn’t pinpoint an exact cause but said many factors played into the outbreak. A wet monsoon, recent population growth, housing development and changes in how people get health care during the COVID-19 pandemic impacted the outbreak. Most of the cases were in adults 60 and older, with more than 1,000 patients needing to be hospitalized.

The Maricopa County Environmental Services Department sets traps and fogs highly-populated mosquito areas throughout the year. Crews also set traps and bring back mosquitos to test them. Female adult mosquitoes are the biggest concern because they can bite and transmit diseases like West Nile.