Mosquito testing can help experts determine where West Nile cases are most likely to occur. Jon Cherry/Getty Images
  • West Nile virus is the leading cause of mosquito-borne disease in the continental United States.
  • Cases tend to be diagnosed in the summer and then continue into the fall months.
  • A small percent of those with West Nile virus can experience severe neurological disease, including brain inflammation and paralysis.

While we’re all concerned about the COVID-19 pandemic and preparing for the coming flu season, there’s another disease that returns every year and could have severe consequences.

It’s called the West Nile virus, and one of the most common warm weather pests carries it: mosquitoes.

According to the Centers for Disease Control and Prevention (CDC), the West Nile virus (WNV) is the leading cause of mosquito-borne disease in the continental United States. Infections occur during the typical mosquito season, which starts in summer and continues through fall.

West Nile virus history

This illness was first identified in 1937 in Uganda and followed by outbreaks in Egypt between 1951 and 1954, which helped researchers understand the clinical characteristics of the disease.

A few years later, an outbreak in Israel caused severe neurological issues in nursing home residents — the first time these symptoms were ever observed. The disease spread to the rest of Africa and even into Europe over the next 20 years.

It remained there until 1999, when the first case emerged in New York, after which the disease firmly established itself in every state except Hawaii and Alaska.

What is West Nile virus?

“West Nile virus is a Flavivirus and is in the same family as Zika, Dengue, Yellow Fever, and Japanese encephalitis,” Eric Cioe-Peña MD, director of Global Health at Northwell Health in New Hyde Park, New York, told Healthline.

According to Peña, WNV causes severe disease in only 1 percent of patients, and the most common severe symptoms include encephalitis or meningitis. He added that about 20 percent could have moderate symptoms “similar to a viral meningitis,” which are headache, fever, and flu-like symptoms.

He also confirmed that the virus can’t be passed from person to person.

“The virus is mosquito-borne and cannot be passed directly between people,” he said. “There is no known treatment or vaccine.”

West Nile infection can cause severe symptoms

“Most people who are infected with West Nile virus are asymptomatic,” said Marta Feldmesser, MD, chief of the Division of Infectious Diseases at Lenox Hill Hospital in New York.

However, she added that a small percentage might experience a range of symptoms, some serious.

“Twenty percent [of those] get West Nile fever,” she said. “Less than 1 percent get neuroinvasive West Nile virus.”

Feldmesser explained that neuroinvasive disease could take one of three forms:

The first is encephalitis, “which is infection and inflammation in the brain that causes varying degrees of change in level of consciousness and ability to think. About two-thirds get that.”

The next is meningitis, which she said “is infection/inflammation of the lining of the brain and the spinal cord, where people get very severe headaches, can have alterations in levels of consciousness, and can basically end up with very severe headaches and fever.”

The third version of the neuroinvasive disease involves infection in the spinal cord, where people experience paralysis syndrome. “It can be fairly similar to the paralysis you see with polio,” Feldmesser explained.

Where outbreaks are occurring

“Thirty-five states are reporting human WNV activity as of 9/21 (CDC data),” said Charles Bailey, MD, medical director for infection prevention and treatment at Providence St Joseph Hospital and Providence Mission Hospital in Orange County, California.

According to Bailey, the areas most heavily impacted with WNV, based on incidence per 100,000 of the population, include the upper Midwest and Four-Corners states. 

“Ten states are reporting WNV in animal hosts only, and five states have no evidence of WNV,” he continued. “Historically, states of the Intermountain West and Mississippi delta have also been among those with the highest WNV incidence.”

Total U.S. cases show no appreciable increase, expert says

As of Sept. 21, a total of 479 cases of WNV disease in people have been reported, according to the most recent CDC data.

Asked if the outbreak was unusually intense, Bailey said no.

“WNV cases reported to CDC for 2021 are running on a pace comparable to last year’s total of 664,” he noted.

Bailey noted that this was “lower than the 2019 total of nearly 1,000” and was the lowest annual total over the past decade from 2011-2020, during which yearly cases ranged from 700 to over 5,000.

Preventing West Nile virus infection

“The best way to prevent West Nile is to wear DEET or similar mosquito repellant and avoid mosquitoes,” said Peña.

According to the CDC, you can also use insect repellents that contain these active ingredients:

  • Picaridin
  • IR3535
  • Oil of lemon eucalyptus (OLE)
  • Para-menthane-diol (PMD)
  • 2-undecanone

But the CDC cautioned that PMD and OLE shouldn’t be used to protect children younger than 3 years old.

The bottom line

Summer and fall are when we’re at the most significant risk of being acquiring the West Nile virus, a mosquito-borne illness that can cause severe symptoms.

Experts say that a small percentage of people with the virus can experience severe neurological disease, including brain inflammation and paralysis.

They also say that the number of cases this year is comparable to previous years, and insect repellent is the best way to prevent infection.

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