West Nile Virus 2021 Case Count Passes 1,000 In Maricopa County

PHOENIX, AZ — Maricopa County broke another record for West Nile virus cases reported in humans this week, with 1,053 cases so far in 2021. There have been 78 deaths from the disease this year, also a record, according to health department data.

The previous annual record of 355 cases was set in 2004.

Contributing to this astronomical number of cases, compared to years past, was the particularly wet monsoon this year after an incredibly dry 2020, according to Maricopa County Health Department Epizoologist, Melissa Kretschmer. There were only three human cases of West Nile virus and one death in the county last year.

The wet monsoon this year contributed to more standing water in the Valley and, "standing water is the perfect breeding ground for the mosquitos that spread the virus,” she said.

That contributed to a larger mosquito population this year than is typical in the Valley and more of those mosquitos are testing positive for West Nile virus than in a usual year, Kretschmer said.

One of the reasons that more mosquitos are testing positive is likely because more birds are carrying that disease. Mosquitos catch the disease from birds.

In 2020, Mosquitos were testing positive for West Nile virus in low numbers. The county doesn't have data on infection or carrier rates for birds, but Kretschmer believes that the birds this year likely have little immunity to West Nile virus because they weren't exposed to it last year.

"The fact that they were not exposed the previous year led them to be more susceptible this year, helping to amplify the virus in the Valley,” she said. “So it’s a trifecta: The weather, the mosquitos and the birds."

West Nile virus was first detected in Arizona in 2003, according to the Arizona Department of Public Health. People contract the disease when an infected mosquito bites them. West Nile is not transmitted through human-to-human contact.

Around 80 percent of people bitten by infected mosquitos never develop symptoms or even know they’ve infected. Slightly fewer than 20 percent might develop a flu-like illness, Kretschmer said. Slightly fewer than 1 percent might develop a more severe illness that affects their brain and spinal cord.

Previous annual West Nile virus human case counts, according to Maricopa County Department of Public Health records:

• 2011: 50 cases and two deaths
• 2012: 98 cases and four deaths
• 2013: 52 cases six deaths
• 2014: 93 cases and 12 deaths
• 2015: 62 cases and two deaths
• 2016: 63 cases and five deaths
• 2017: 93 cases and six deaths
• 2018: 24 cases and six deaths
• 2019: 155 cases and 17 deaths
• 2020: Three cases and one death

It's possible that it will get cold enough in the Valley this winter to stop the West Nile virus season, but although numbers are decreasing, mosquitos are still testing positive in the county.

To avoid contracting West Nile virus, Kretschmer recommends:

• Wear mosquito repellant when spending time outside.
• Avoid being outdoors at dawn and dusk, when the mosquitos that carry the virus are most active.
• Get rid of standing water around your home, including in the saucers below potted plants, which can catch enough water for mosquitos to reproduce in. She recommends scrubbing them to remove any eggs left behind.
• Make sure your pools and water features are in good working condition and clean them regularly.
• Ensure your door and window screens are in good condition so mosquitoes can’t get inside your home.