A South Dakota county with the second-highest national rate of lethal West Nile virus in humans shares a border with a county that has never reported a single case.
Between 1999 and 2023, Dewey County recorded 13.74 neuroinvasive cases per 100,000 people, just behind King County in Texas, according to the U.S. Centers for Disease Control and Prevention. Neuroinvasive refers to those more serious cases that can affect the brain and can be deadly.
Ziebach County, which borders the western boundary of Dewey County in north-central South Dakota, shows zero neuroinvasive West Nile virus cases, according to the CDC. The Cheyenne River Indian Reservation covers most of both counties.
The data isn’t wrong but needs explaining.
“The medical facility for both counties is in Eagle Butte, Dewey County, and they are the ones who report to the health department,” said Michael Claymore, director of the Cheyenne River Sioux Tribe Prairie Management Program.
Population numbers also impact incidence rates, Tia Kafka, marketing and outreach director at the South Dakota Department of Health, said in an email. The fewer residents live in a county, the more impact a single West Nile virus case will have, she said.
“Historically, Brown County has been the hotspot for West Nile virus activity in South Dakota, with the most cumulative West Nile virus cases reported,” Kafka said.
Brown County, whose county seat is Aberdeen, recorded more than 340 West Nile virus cases between 1999 and 2023, which ranks it 24th among other counties in the United States, the CDC said. Within South Dakota, Brown County is followed by Pennington County, with more than 220 West Nile virus cases during that time, and Minnehaha County, with almost 200 incidents.
The U.S. Department of Health and Human Services defines a high incidence rate as one that is greater than 1.1 per 100,000 people. Analysis of the CDC’s historic data within this framework showed that 61 out of 66 South Dakota counties which amounts to 92% have a high incidence of West Nile virus cases in humans.
When bitten by a West Nile virus-carrying mosquito, most people will not show any symptoms, said Dr. Erin Staples, a medical epidemiologist with CDCs Division of Vector-Borne Diseases. Some will develop West Nile fever, but even fewer will get a neuroinvasive West Nile virus disease, which affects the brain or the space around it. Non-neuroinvasive cases are generally less serious.
Mosquitoes are ‘the deadliest animal in the world’
Since the virus reached South Dakota in 2002, the state Department of Health has recorded more than 2,800 human cases, 53 of which were fatal.
Staples, of the CDC, said West Nile virus is usually transmitted through a mosquito bite.
“Mosquitoes are often referred to as the deadliest animal in the world, as they cause more death and disease than any other animal on the planet,” Staples said.
Humans can also spread the virus to one another through blood transfusion, Staples said.
The South Dakota Department of Health has already detected the first West Nile virus case of 2024.
The department’s West Nile Virus Surveillance tool shows that this non-neuroinvasive case of West Nile virus disease was found in a resident of Beadle County who is at least 64 years old.
As of July 17, the CDC’s surveillance tool shows that neighboring North Dakota and Nebraska have already recorded one and four cases, respectively.
Local efforts in mosquito control
This year, the state Department of Health allocated $500,000 in grant funds to distribute among cities, counties and tribes for mosquito control in amounts ranging from $500 to $20,000.
The Cheyenne River Sioux Tribe Prairie Management Program received $9,325 as part of the grant.
Claymore said the Prairie Management Program tries to eradicate mosquitoes in several different ways. The team conducts larva sightings, which involve putting chemicals in standing water to kill mosquitoes before they get a chance to develop into adults, sprays the ground, fogs the trees and disperses microscopic droplets of a chemical compound mixed with oil through aerial fogging.
The city of Sioux Falls received $20,000 through the grant.
Dominic Miller, environmental health manager for the Sioux Falls Department of Health, said the number of mosquitoes in 2024 is “as bad as (the city has) ever seen it.”
The city of Sioux Falls is divided into 18 spraying zones, and each zone has at least one mosquito trap. This summer, one of the traps collected 6,300 mosquitoes overnight, Miller said.
Ways to prevent infection
Kafka said homeowners can minimize the risk of being bitten by installing or repairing window and door screens to keep mosquitoes outside of homes.
The state Department of Health also encourages people to reduce the risk of being bitten by mosquitoes and contracting West Nile virus by joining “the S.W.A.T. team:”
- Spray: When outdoors, apply mosquito repellents that contain compounds like DEET, picaridin, 2-undecanone, param-menthane-diol, IR3535 or oil of lemon eucalyptus.
- Water: Remove standing water from old tires, buckets and other containers that can give mosquitoes a place to breed; regularly change the water in bird baths and pet dishes, and stay away from areas near standing water.
- Arms: Wear pants and long-sleeved shirts to cover exposed skin.
- Time: Limit time outdoors at dawn and from dusk until midnight when mosquitoes are most active
Dr. Susan Hoover, an infectious disease physician at Sanford Health, said that since there is no treatment for West Nile virus, it’s paramount to take those precautions.
If a person is bitten and exhibits such symptoms as fever, vomiting, diarrhea, headache or body aches and suspects they may have West Nile virus infection, they should contact their health care provider, she said.
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