South Carolina One of Top States for West Nile Virus

With the dog days of summer soon winding to a close, fears of disease-carrying mosquitoes may be dwindling with them.

But new research from the University of South Carolina shows that West Nile virus in the Palmetto State occurs at a rate up to 10 times the national average.

And what’s more, September is a prime month for the mosquitoes that carry the illness, according to the lead researcher.

West Nile virus is the most common mosquito-borne disease threat in the state, according to the South Carolina Department of Public Health.

Others include Eastern equine encephalitis, La Crosse encephalitis, Saint Louis encephalitis, and heartworm in dogs and cats, DPH reports.

While eight in 10 West Nile infections are asymptomatic, some people can suffer serious illness that can lead to long-term health complications, and even death, said Dr. Melissa Nolan, an epidemiologist and director of USC’s Institute for Infectious Disease Translational Research.

“Around 2002, West Nile virus was first identified in our state. It was considered invasive at the time. Now, it’s become endemic,” she said.

“Our biggest finding was that the true burden of disease is significantly higher than previously thought,” she added. “We think studies like this help identify gaps in knowledge … that have important personal, medical, and public health implications.”

Nolan’s research was prompted by the 2022 death of a Richland County resident from West Nile. It found that while about 2 percent of people are infected with the virus nationally, up to 22 percent of the population in South Carolina has been exposed to it, she said.

For this study, Nolan’s team tested samples of 2,000 residents statewide. The infection rate was 9 percent for the state overall, she said, but Spartanburg County was a hot spot, and the highest rate – 22 percent – was recorded in Richland County.

“The hypothesis is that Columbia is on a fall line and you tend to see higher bird cases along that fall line,” she said. “Downtown Columbia is an island. So could it be related to less diversity in our ecology?”

Previously, it was thought that less than 1 percent of people were infected statewide, Nolan said, adding that about 5 percent of residents are infected every summer.

“It was thought that the Atlantic coastal states were a hotbed for West Nile, but we tend to see more infections in the Midwest,” said Nolan, who is also associate professor at the Arnold School of Public Health.

“The theory is it’s more associated with the fact that migratory birds are a primary host,” she said. “And migratory birds are likely contributing to our own local transmission.”

By comparison, she said, Maine has a zero percent West Nile infection rate, while South Dakota’s rate is 25 percent.

Conditions thought to affect these rates include ambient temperature and the amount of time people spend outdoors, for example, golfing, she said. Mosquito populations can also be affected by drought, she said, noting that the insects can develop from eggs to adults in two days during the summer.

The researchers found that there was “a higher concentration in older urban, tree-dense neighborhoods with numerous domestic water features, such as fish ponds and bird baths.”

While South Carolina is home to 62 species of mosquito, it’s the southern house mosquito that transmits West Nile virus, said Nolan, who moved to South Carolina in 2018 to join a growing collaboration of state partners studying mosquito- and tick-borne diseases.

“This mosquito likes to feed on birds, and birds harbor West Nile in nature,” she said. “In a lot of years, the birds die off from West Nile, but the mosquitoes are still hungry, so they go to people. That’s when we see outbreaks.”

About 20 percent of people who are infected develop mild illness, according to the U.S. Centers for Disease Control and Prevention. Symptoms typically begin two to six days after being bitten by an infected mosquito and can range from fever and body aches to vomiting, diarrhea, and a rash, CDC reports.

And while most people recover completely, fatigue and weakness can last for weeks or months, the agency reports.

Severe illness can cause central nervous system involvement, including encephalitis and meningitis, and is marked by high fever, headache, coma, and convulsions among other symptoms, according to the CDC.

Fewer than 1 percent of those infected suffer severe illness and about 10 percent of those people die, the agency reports.

People with diabetes are at greater risk, Nolan said, and older women are more likely to develop chronic fatigue syndrome after having West Nile, though it’s not known why.

The researchers also uncovered an interesting shift in transmission that is a marker of climate change, she said.

“Historically, August was peak mosquito season and transmission time,” she said. “Now it’s coming in around June or later in September.”

That’s because mosquitoes have an upper range where it’s too hot for them, she said. They need warm, humid conditions, but not too hot.

“So now August is too hot for them,” she said. “We have more mosquitoes on days when we have consistent weather a few days in a row. Extreme changes of temperature see less mosquitoes.”

The state Department of Public Health says it works with the CDC to monitor mosquito populations for diseases that can be spread to humans, for example by trapping and testing the insects around the state and monitoring mosquito-borne disease activity in birds and domestic animals.   

DPH also conducts surveillance for human cases of mosquito-borne diseases and provides local mosquito control programs with recommendations for mosquito control activities when diseases are detected in birds, mosquitoes, people, and animals.

The state maintains a website with the most current case counts of mosquito-borne diseases in birds, mosquitoes, and certain animals.

And residents can also help in surveillance and control by submitting certain species of dead birds for testing from March through November. These include crows, blue jays, house finches, sparrows, and other songbirds.

Meanwhile, researchers are also exploring wastewater surveillance as an early detection tool, similar to Covid-era monitoring, according to Nolan.

Mosquito control programs are managed at the local level, DPH reports.

And while the CDC provides some funding for mosquito-borne disease monitoring and control, the bulk of those funds go to states in the Midwest, Nolan said.

South Carolina sees other human cases of mosquito-borne diseases from time to time, she said. Last year, for example, the state reported the first fatal case of Eastern equine encephalitis in a person in more than 20 years, she said.

La Crosse virus, named for the Wisconsin city where it was first identified, is also reported now and again, almost exclusively in the Greenville area, Nolan said.

It’s also found in an area in Asheville, North Carolina, where cases have been reported for more than 30 years.

“We had a case of that last year in South Carolina that was almost fatal,” she said. “It specifically attacks children for some reason. The average age is 8 years old. Adults rarely have symptoms.”

The symptoms of La Crosse are similar to those of West Nile, she said, and include meningitis, a swelling of the brain that can impair walking and speaking.

La Crosse is transmitted by the tree hole mosquito, which lays eggs in the knots of trees that fill up with water, she said.

Malaria outbreaks also occur occasionally in South Carolina, she said, noting almost all cases happen in homeless people.

All mosquitoes are a little different, Nolan said, adding that the one that carries West Nile doesn’t like clean water. It prefers slimy, green water, she said, so residents can reduce their numbers by emptying the bird bath every day.

Because mosquitoes can develop in anything that collects standing water such as flower pots, bird baths and even old tires, the DPH recommends that residents take steps to reduce their populations.

“When we each take small actions to eliminate breeding areas and prevent bites,” Dr. Chris Evans, state public health entomologist, said in a release, “we strengthen our defense against mosquito‑borne illnesses and contribute to a safer, healthier summer for everyone in South Carolina.”