Colorado experiencing worst West Nile virus outbreak in U.S. this year. So far, 11 people have died.

This mosquito-borne illness can strike people of all ages, but is preventable. If you’re spending time outdoors, it's vital to protect yourself. Learn what steps you can take.
September 1, 2023
West Nile virus has been deadly in Colorado in 2023. So far, 11 people have died. Colorado is experiencing the worst West Nile virus outbreak in the U.S. this year. Photo: Getty Images.
The West Nile virus outbreak has been deadly in Colorado in 2023. As of Sept. 1, 2023, 11 people have died. Colorado is experiencing the worst West Nile virus outbreak in the U.S. this year. Photo: Getty Images.

Colorado is experiencing the worst West Nile virus outbreak in the U.S. this summer so far. And it’s been one of the deadliest West Nile virus seasons in Colorado since the mosquito-borne illness arrived in the U.S. back in 1999.

As of Sept. 1, 2023, 11 people have died in Colorado this year, and scores of others have needed to be hospitalized.

West Nile virus can strike healthy people of all ages. And it can cause scary neurological impairments and swelling in the brain in those who get a severe case. Read about a young woman who is still recovering after a bad West Nile virus infection last year.

Dr. Michelle Barron, senior medical director of infection prevention and control for UCHealth, has two key messages for people who are spending time outdoors now and for those who may be feeling sick.

Protect yourself to avoid getting mosquito bites. And, if you are feeling sick and suspect you might have West Nile virus, seek help from a doctor immediately.

“If your infection progresses to the point where you are having persistent headaches, confusion, any kind of muscle weakness, numbness or tingling, you need to be seen by a doctor as quickly as possible,” Barron said.

To understand more about West Nile virus and to prevent people from getting sick, we asked Barron some of your top questions about West Nile virus.

Along with her work for UCHealth, Barron is a professor of medicine and infectious diseases at the University of Colorado School of Medicine on the Anschutz Medical Campus and is one of the top infectious disease experts in the Rocky Mountain region.

Dr. Michelle Barron is one of the top infectious disease experts in the Rocky Mountain Region. Photo by Sonya Doctorian.
Dr. Michelle Barron is one of the top infectious disease experts in the Rocky Mountain Region. She’s urging people to be on guard against West Nile virus, which can be deadly. Photo by Sonya Doctorian, UCHealth.

What is West Nile virus?

West Nile virus is a mosquito-borne illness that’s known as a “flavivirus.” It was first detected in 1937 in Uganda. There were several large outbreaks in Egypt and the Nile River Delta in the 1950s, which is how the illness got its name. The first case in the U.S. was detected in 1999. It has since spread throughout the U.S. But Coloradans get a disproportionately high number of West Nile virus infections compared to people in other larger, more populated states. So, people living in Colorado and other states where West Nile virus infections are more common need to be especially vigilant about preventing mosquito bites.

Mosquitos can pick the virus up from birds and other animals and then can spread it to people when they bite us.

Why is West Nile virus so bad this year in Colorado?

Colorado has had a wet spring and summer.

“The increase in rain has helped the mosquito populations to increase and spread more than usual this summer,” Barron said.

In addition, Coloradans love spending time outdoors. The combination of many people spending time in nature while mosquitos are spreading widely may contribute to a bad year for West Nile virus in Colorado in 2023.

What is the typical season for West Nile virus cases in Colorado?

The season usually lasts from June through September, but if we have a mild, wet fall, mosquitos may continue to circulate later in the fall, and thus, they could continue to infect people.

Is West Nile virus also bad elsewhere in the U.S. this summer?

Other states from California to Texas to Wyoming and the Dakotas are also experiencing West Nile virus outbreaks this summer, but data from the U.S. Centers for Disease Control and Prevention (CDC) show that Colorado so far is experiencing the worst West Nile virus outbreak in the U.S. this year.

Is this also one of the worst years on record for West Nile virus in Colorado?

Colorado has had bad years for West Nile virus over the last two years. Last year, there were 20 deaths and in 2021, there were 11 deaths, according to the Colorado Department of Public Health and Environment.

The worst year on record in Colorado was soon after West Nile virus arrived in the state. In 2003, 66 people died after becoming infected with the virus.

What are the common symptoms of West Nile virus?

The key symptoms of West Nile virus include:

  • Headache
  • Fever
  • Diarrhea
  • Muscle weakness
  • Tingling or numbness
  • Neck stiffness
  • Flu-like symptoms

“Most people will have minor symptoms and may not ever know that they had West Nile virus,” Barron said.

Do some people get rashes when they get West Nile virus?

Rashes are not common with West Nile virus, but it’s possible to get one. If you are not feeling well and have questions about symptoms, reach out to your doctor.

People with severe cases of West Nile virus can develop neurological symptoms. How would you know that you are having these kinds of issues?

The neurologic symptoms related to West Nile virus can include the following:

  • Clumsiness
  • Problems walking
  • Brain fog or cognitive problems
  • Memory issues

When a case of West Nile virus becomes severe, Barron said patients can suffer from critical problems like encephalitis, which is inflammation of the brain, and meningitis, which is swelling in the lining of the brain and the spinal cord.

“People sometimes will start acting strange. They’ll have cognitive issues. They might have trouble with word finding. They might be a little slower or will have some weaknesses develop. Walking can be a challenge,” Barron said.

“They can have a polio-like syndrome where they can’t walk and have paralysis,” Barron said.

Her advice for anyone having these types of neurologic symptoms is utterly clear: Seek help immediately.

Is there a test for West Nile virus?

Yes. If you are sick and suspect that you might have West Nile virus, see your medical provider so they can arrange for you to be tested for West Nile virus.

Are there any cures for West Nile virus?

No. Despite decades of research and efforts to come up with a cure, there’s not a cure yet for West Nile virus. In the past, Barron said, doctors experimented with using antibodies from previously infected people in patients with new infections. Unfortunately, clinical trials showed that those therapies were not effective.

While there are no cures for West Nile virus, doctors can help with other symptoms, like reducing a patient’s fever or providing supportive care if they have swelling in the brain.

“If you’re having neurological symptoms, you want to make sure you see a doctor because the symptoms can get worse,” Barron said.

There’s no way to reverse the virus, but doctors can provide help.

“If you’re having trouble breathing, we can monitor that. If you’re having fevers, we can control that. If you’re having weakness, we can work on rehabilitation,” she said.

Is there a vaccine to prevent West Nile virus?

No. As with cures, researchers have been working for years to create a vaccine for humans to prevent West Nile virus. Unfortunately, there is not a vaccine yet. But there is a vaccine to prevent West Nile virus in horses, so it’s possible that vaccines for humans could come soon.

What if I had West Nile virus in the past? Could I get it again, or would I still have immunities to fight another infection?

Researchers and doctors are still learning about West Nile virus, but they think that once a person has West Nile virus, they should have lifelong immunity to the virus.

What can I do to avoid getting West Nile virus?

West Nile virus is highly preventable, but if you like being outside, it’s not easy to avoid mosquitos.

Here are some ideas for staying safe:

  • If you’re spending time outdoors, consider wearing long sleeves and long pants, so it’s not easy for mosquitos to bite you.
  • Use highly effective insect repellant, preferably a type containing DEET.

“If you’re out hiking or even if you’re out sitting on your deck or your patio, it’s a good idea to protect yourself. You can use insect repellant or citronella candles or anything that helps keep the mosquitos down,” Barron said. “And make sure there’s no standing water where mosquitos can breed.”

Are there certain times of day when you should be more cautious?

Yes. Mosquitos are especially active at dawn and dusk.

“Avoiding being outside at those times may help you avoid West Nile virus,” Barron said.

Overall, her advice is to prevent bites. Be prepared and pay attention.

About the author

Katie Kerwin McCrimmon is a proud Colorado native. She attended Colorado College, thanks to a merit scholarship from the Boettcher Foundation, and worked as a park ranger in Rocky Mountain National Park during summer breaks from college. She is also a storyteller. She loves getting to know UCHealth patients and providers and sharing their inspiring stories.

Katie spent years working as a journalist at the Rocky Mountain News and was a finalist with a team of reporters for the Pulitzer Prize for their coverage of a deadly wildfire in Glenwood Springs in 1994. Katie was the first reporter in the U.S. to track down and interview survivors of the tragic blaze, which left 14 firefighters dead.

She covered an array of beats over the years, including the environment, politics, education and criminal justice. She also loved covering stories in Congress and at the U.S. Supreme Court during a stint as the Rocky’s reporter in Washington, D.C.

Katie then worked as a reporter for an online health news site before joining the UCHealth team in 2017.

Katie and her husband Cyrus, a Pulitzer Prize-winning photographer, have three children. The family loves traveling together anywhere from Glacier National Park to Cuba.