A South Dakota county with the second-highest rate of fatal West Nile virus in humans in the nation borders a county that has never reported a single case.
Between 1999 and 2023, Dewey County recorded 13.74 neuroinvasive cases per 100,000 residents, 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 affect the brain and can be fatal.
Ziebach County, which borders Dewey County to the west in northern South Dakota, has no cases of the neuroinvasive West Nile virus, according to the CDC. The Cheyenne River Indian Reservation covers most of both counties.
“This is crazy,” said Randolph Runs After, environmental health specialist with the Cheyenne River Sioux Tribe, after learning of Dewey County’s ranking. “We actually haven’t had this many cases in recent years.”
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The data is not wrong, but requires an explanation.
“The medical facility for both counties is located in Eagle Butte, Dewey County, and they are under the health department,” said Michael Claymore, director of the Cheyenne River Sioux Tribe Prairie Management Program.
Population numbers also affect incidence rates, Tia Kafka, marketing and outreach director for the South Dakota Department of Health, said in an email. The fewer residents in a county, the greater the impact of a single case of West Nile virus, she said.
“Historically, Brown County has been the hotspot of West Nile virus activity in South Dakota, and it has also reported the most cases of West Nile virus,” Kafka said.
Brown County, whose county seat is Aberdeen, recorded more than 340 cases of West Nile virus between 1999 and 2023, ranking 24th among other counties in the United States, according to the CDC. Within South Dakota, Brown County is followed by Pennington County, with more than 220 West Nile virus cases during that time period, and Minnehaha County, with nearly 200 incidents.
The U.S. Department of Health and Human Services defines a high incidence rate as a rate greater than 1.1 per 100,000 people. An analysis of CDC historical data in this context found that 61 of 66 counties in South Dakota—or 92%—have a high incidence of West Nile virus cases in humans.
When people are bitten by a mosquito carrying West Nile virus, most show no symptoms, says Dr. Erin Staples, a medical epidemiologist with the CDC’s Division of Vector-Borne Diseases. Some develop West Nile fever, but even fewer develop neuroinvasive West Nile virus disease, which affects the brain or the area around the brain. Nonneuroinvasive cases are generally less severe.
The virus reached its status in 2002 and has been declining since then
Runs After remembers the early years of the West Nile virus in South Dakota, when “you could find dead birds here and there.”
The first reported case occurred in 2002 in a crow in Aberdeen, according to a July 2002 article in the Rapid City Journal.
The West Nile virus first appeared in New York City in the United States in 1999 and reached the West Coast in 2004, Staples said. She added that many people were infected at that time because “nobody was immune.”
Since 2002, the South Dakota Department of Health has recorded more than 2,800 human cases, 53 of which were fatal.
In 2003, Virgil Christensen, an 89-year-old man from Wakonda in Clay County, became the first South Dakota resident to die from West Nile virus, the Vermillion Plain Talk reported.
Both South Dakota and the United States saw a peak in West Nile cases this year, with the state Department of Health recording 1,039 cases.
This year, the South Dakota Department of Health has already registered the first case of West Nile virus in 2024.
The department’s West Nile Virus Surveillance Tool indicates that this non-neuroinvasive case of West Nile Virus disease was identified in a Beadle County resident at least 64 years of age.
The CDC’s surveillance tool shows that one and four cases have already been registered in the neighboring states of North Dakota and Nebraska, respectively.
Weather and human activities lead to an increase in the mosquito population
As of July 17, the South Dakota Department of Health projected 48 cases of West Nile virus in the state by 2024.
Kafka said the state Department of Health uses a weekly updated model that takes into account factors such as historical data on human West Nile virus cases, environmental data and current mosquito testing data.
Historically, West Nile virus cases in the United States are most common in late July and August.
South Dakota is seeing the same trend due to “several interconnected factors,” Kafka said.
Mosquitoes that can transmit West Nile virus reach their highest population density in the warm summer. In addition, heavy rain creates puddles and stagnant water, creating favorable conditions for mosquitoes.
“Higher temperatures not only promote mosquito reproduction, but also accelerate the development of the virus in the mosquitoes, increasing the risk of transmission to humans,” Kafka said.
People also like to spend more time outdoors in the summer, and activities such as camping, hiking and attending evening gatherings increase the likelihood of encountering mosquitoes, she said.
Mosquitoes are “the main culprit”
Staples of the CDC said West Nile virus is usually transmitted through a mosquito bite.
Some people show no symptoms at all after being bitten by a mosquito that carries West Nile virus, she said. However, one in five people who are bitten develop a febrile illness that may include fever, rash, headache, muscle aches or joint pain. This is also known as West Nile fever.
Some people go on to develop neuroinvasive West Nile disease, which can lead to encephalitis, an inflammation of the brain, or meningitis, an inflammation of the tissue surrounding the brain and spinal cord, Staples said.
“Mosquitoes are often referred to as the deadliest animals in the world because they cause more death and disease than any other animal on the planet,” Staples said.
Birds also contribute to the spread of West Nile virus. Once infected, they can develop enough virus to transmit it to an uninfected mosquito, which is why they are sometimes called “amplifying hosts,” she said.
Humans do not have the same capabilities, but can transmit the virus to each other through blood transfusions, Staples said.
Several vaccines have progressed through the early stages of clinical development but are currently stuck in the efficacy trial phase, she said.
“You have to be able to say, ‘Where is West Nile? I’m going to start vaccinating people and make sure the vaccine is very effective and can prevent infection,'” Staples said. “Unfortunately, we couldn’t necessarily predict where West Nile would appear. That’s because of the sporadic and unpredictable nature of West Nile virus.”
CDC, government funds mosquito control
The South Dakota Department of Health received nearly $1.5 million in 2023 through the CDC’s Epidemiology and Laboratory Capacity Program for the department’s West Nile virus media campaign and laboratory testing of mosquito pools.
This year, the state Department of Health has allocated $500,000 in mosquito control grants, ranging from $500 to $20,000, to be distributed to cities, counties and tribes. Kafka said 166 applicants have received funds based on population size and history of human cases.
The Cheyenne River Sioux Tribe Prairie Management Program received $9,325 as part of the grant.
Claymore said the Prairie Management Program is trying to eradicate the mosquitoes in several ways. He said the team is monitoring traps and testing captured mosquitoes to see if some of them carry viruses.
The program also conducts larval monitoring, which involves adding chemicals to standing water to kill the mosquitoes before they can develop into adults.
The workers also spray the ground, mist the trees and disperse microscopic droplets of a chemical compound mixed with oil through the air.
Claymore said the program uses all of the Department of Health funding to buy just enough chemicals for one season.
“I tell people to just watch the news and see where the mosquitoes are. They’re coming our way,” Claymore said. “Now is really important to be more vigilant about personal protective equipment.”
Mosquito plague in Sioux Falls “as bad as ever”
The city of Sioux Falls, Aberdeen and Pennington County each received $20,000.
Dominic Miller, environmental health manager for the Sioux Falls Health Department, said in an email that the city’s mosquito control program was launched the same year, 2003, that the city recorded its first case of West Nile virus. Now, more than 20 years later, mosquito numbers in 2024 are “as bad as they’ve ever been (in the city),” he said.
The City of Sioux Falls is divided into 18 spray zones, and each zone has at least one mosquito trap. On weekdays, the mosquito control team checks all traps each morning to count and test the mosquitoes caught.
This summer, 6,300 mosquitoes were caught overnight in one of the traps, Miller said.
The mosquito control team determines which zones need to be sprayed based on the number of mosquitoes caught and aims to eradicate all types of mosquitoes, not just those that could transmit West Nile virus, he said.
Options for infection prevention
According to Kafka, homeowners can minimize the risk of mosquito bites by installing or repairing screens on windows and doors to keep mosquitoes out of the home.
The state Department of Health also recommends that people reduce their risk of mosquito bites and West Nile virus infection by joining a “SWAT team”:
- Spray: When outdoors, apply mosquito repellent containing compounds such as DEET, picaridin, 2-undecanone, param-menthane diol, IR3535, or lemon eucalyptus oil.
- Water: Remove standing water from old tires, buckets and other containers that can serve as breeding grounds for mosquitoes. Change the water in birdbaths and pet bowls regularly and avoid areas near standing water.
- Arms: Wear pants and long-sleeved shirts to cover exposed skin.
- Time: Limit time outdoors at dawn and from dusk to midnight when mosquitoes are most active
Susan Hoover, MD, an infectious disease specialist at Sanford Health, said since there is no treatment for West Nile virus, it is extremely important to take these precautions.
If a person has been bitten and is experiencing symptoms such as fever, vomiting, diarrhea, headache or body aches and suspects they may be infected with West Nile virus, they should contact their doctor, she said.
