Colorado had worst West Nile virus outbreak in U.S. in 2023 with 50 deaths

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.
April 18, 2024
West Nile virus was deadly in Colorado in 2023 with 50 affected people dying. Colorado had the worst West Nile virus outbreak in the U.S. in 2023. Photo: Getty Images.
West Nile virus was deadly in Colorado in 2023 with 50 affected people dying. Colorado had the worst West Nile virus outbreak in the U.S. in 2023. Photo: Getty Images.

Read about a renowned producer and a young woman, both of whom dealt with recent tough cases of West Nile virus

How to avoid getting West Nile virus

  • Wear long sleeves and long pants when you’re outdoors.
  • Be especially wary of mosquitos in the mornings and evenings when they’re most active.
  • Avoid areas near standing water where mosquitoes breed.
  • Parents can get mosquito netting to cover babies in strollers.
  • Use highly effective insect repellents that contain one of the following ingredients: DEET; Picaridin (known as KBR 3023 and icaridin outside the U.S.); IR3535; Oil of lemon eucalyptus (OLE); Para-menthane-diol (PMD); 2-undecanone. Find the right insect repellent for you by using EPA’s search tool. (Some repellents are not appropriate for babies and children.)
  • If you are also using sunscreen, apply sunscreen first and insect repellent second.

Colorado experienced the worst West Nile virus outbreak in the U.S. in 2023 and one of the deadliest years for West Nile since the mosquito-borne illness arrived in the U.S. back in 1999.

Throughout 2023, 50 people who contracted West Nile virus died in Colorado, and scores of others 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 an award-winning photographer and producer who contracted West Nile virus last year and a young woman who endured a tough recovery after a bad West Nile infection in 2022.)

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 had a wet spring and summer in 2023 and that may be the case again this year.

“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 this year.

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) showed that Colorado experienced the worst West Nile virus outbreak in the U.S. in 2023.

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. In 2022, 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 repellent, 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 repellent 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 Coloradan. 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 summers in college.

Katie is a dedicated storyteller who loves getting to know UCHealth patients and providers and sharing their inspiring stories.

Katie spent years working as an award-winning journalist at the Rocky Mountain News and at an online health policy news site before joining UCHealth in 2017.

Katie and her husband, Cyrus — a Pulitzer Prize-winning photographer — have three adult children and love spending time in the Colorado mountains and traveling around the world.