In July, shortly after the first human case of 2025 was detected, the South Dakota Department of Health released a report projecting more than 130 cases for the year. That would have been the most in a single year for the state since 2018, when 169 cases were detected.

However, Joshua Clayton, South Dakota’s state epidemiologist, told the Argus Leader that a combination of factors helps explain why there were fewer cases than expected.

“Weather patterns likely played a significant role, as stretches of hot, dry conditions can reduce mosquito survival and limit the opportunity for transmission,” Clayton said. “City and county mosquito control programs also contributed through consistent management efforts throughout the season.”

The willingness of residents to protect themselves by using bug spray and limiting time outside during peak mosquito hours likely helped limit infections too, Clayton said.

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The majority of this year’s cases happened in East River counties, which Clayton said matches a “long-standing transmission pattern” in the state. The region also gets more rain and has more wetlands and other low-lying places that create “ideal breeding areas for mosquitoes,” he added. 

“Human factors add to this pattern, since eastern South Dakota has more residents, which increases the chance of exposure and increases the likelihood that cases will be detected and reported,” Clayton said.

South Dakota reported four deaths from West Nile virus in 2025. That matches the total from 2023, when 96 human cases were detected in the state, a health department report says. Nationally, 46 states had, as of Nov. 18, reported a total of 1,941 human cases of West Nile virus this year, according to the CDC.

What are the symptoms of West Nile virus?

Most people infected with West Nile virus don’t develop any symptoms, according to the Centers for Disease Control and Prevention. However, about one in five people develop a fever and other symptoms, and about one out of 150 people develop a serious, sometimes fatal, illness.

When it becomes a serious illness, symptoms can include: high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness or paralysis.

What are the best ways to prevent West Nile virus?

The health department says to take the following precautions to prevent mosquito bites and West Nile virus:

• Apply mosquito repellents to clothes and exposed skin, and limit exposure by wearing pants and long sleeves in the evening.
• Limit time outside between dusk and midnight, when mosquitoes are most active.
• Remove standing water, which provides mosquitoes with a breeding ground. Routinely change the water in birdbaths and pet dishes, and drain water from flowerpots and other.