It’s West Nile Virus Season Again. How Worried Should We Be?

In Colorado, 2023 was a bad year for West Nile virus. And while the state’s annual summer-and-fall outbreak of the mosquito-borne infection so far seems less severe this year, West Nile continues to spark concern and questions.

Last year, out of 634 people in Colorado who tested positive for West Nile virus infection, 386 people were hospitalized, and 51 died, according to state Department of Public Health & Environment data. It was the highest West Nile death toll in the state since 2003, which was a year after cases were first reported in the state.

This year, as of Aug. 14, 13 cases had been reported in Colorado, with seven hospitalizations and no deaths up to that point. On Aug. 16, Denver public-health officials confirmed the year’s first case in the city, and Boulder County announced its first case Aug. 20. It’s believed that thousands of Coloradans infected with West Nile virus never get tested because they experience no symptoms or only mild ones.

Even the nation’s former top government infectious disease expert has been impacted by West Nile virus. Anthony Fauci, MD, was hospitalized recently after being infected and was recovering at home, news media reported Aug. 24.

To learn more about West Nile virus in Colorado, the threat it poses, and what we can do to protect ourselves, we turned to Lakshmi Chauhan, MD, assistant professor in the University of Colorado Department of Medicine’s CU Division of Infectious Diseases. Chauhan practices in the infectious disease clinic at UCHealth University of Colorado Hospital, and is among 21 faculty members in the inaugural class of the department’s Clinical Excellence Society.

Photo at top courtesy U.S. Centers for Disease Control and Prevention.

What is West Nile virus?

It’s a viral infection primarily transmitted by mosquitoes. In rare instances, it can spread through blood transfusions, organ transplantation, and mother to child during pregnancy. It’s similar to other mosquito-borne infections in other parts of the world, such as dengue or chikungunya, or St. Louis encephalitis virus in the United States. In the continental U.S., West Nile virus is the leading cause for mosquito-borne disease.

What is the season for West Nile virus infection?

Infection occurs during summer and early fall when the mosquitoes come out. Mosquito season ends when there is constant frosting, usually October to November in Colorado.

When did it first appear?

West Nile virus was first isolated in 1937 from a woman in Uganda’s West Nile district. In the United States, it emerged in the late 1990s.

What are the symptoms?

About 70% to 80% of infected people are asymptomatic and don’t even know they have an infection. About one in five have symptoms, usually mild ones – fever, rash, not feeling great, diarrhea, fatigue, headache, and some abdominal symptoms. The symptoms could seem like any other viral infection, which is why people may not recognize it as West Nile and may not even get tested.

A very small proportion of people – about one in 150 – develop a more severe form, a neuroinvasive infection, including meningitis, which is inflammation of membranes surrounding the brain, or encephalitis, which is inflammation of the brain tissue itself. Some people can have spinal cord involvement, which is like polio in the old days. Those are the people who need hospital care. People with neuroinvasive West Nile infection could have long term consequences, because the infection can damage parts of the brain that are crucial for movement or various other functions.

Of the very small percentage of infected people who get neuroinvasive West Nile, the infection may be fatal to about 10%.

Are pre-existing medical conditions a factor?

The people who get more severe manifestations of West Nile generally tend to have some underlying conditions which make them more predisposed to serious infections. They may be older, or have immune suppressed conditions like cancers or a transplant. Most people with regular immune systems tend to fight off these viral infections.

What treatments are available?

Unfortunately, at this time, there is no treatment, there is no cure, there is no vaccine. But we can manage symptoms. In case of mild symptoms like most other viral infections, it’s rest, fluids, and management of symptoms like fever. It is also important to watch for symptoms which may not be improving (like fever) or any neurological symptoms, like headache, muscle weakness, and confusion.

For people who develop serious syndromes like meningitis or encephalitis, sometimes they can get seizures or breathing issues, so they may have to go to the ICU. We do a lot of supportive management in that situation to avoid complications that can arise from inflammation of the brain.

Why do we have some bad West Nile years in Colorado and other years where the numbers are lower?

West Nile comes in outbreaks or spurts. This has been happening since West Nile was recognized. Last year was a very bad year, and that may have been related to the heavy rains we had in May and June, increasing conditions for mosquitoes to proliferate. Climate variations can influence West Nile cases.

What can we do to protect ourselves from West Nile virus?

Since there is no treatment, prevention is the best way to avoid infection. Because it is a mosquito-borne illness, we counsel people to not be outside when there are a lot of mosquitoes. Obviously, people here are very interested in outdoor activities, but you can take precautions to avoid mosquito bites: wear a long-sleeve shirt, use mosquito repellant, and avoid times when mosquitos are heaviest, like dusk to dawn. And draining standing water in your back yard is a good precaution.

If someone gets a lot of mosquito bites, should they get tested?

If you get mosquito bites and some symptoms, report that to your primary care physician. But not all mosquitoes carry West Nile.

Overall, the serious manifestations of West Nile affect only a very small percentage of people. However, just as it is advisable to use sunscreen when outside, taking steps to prevent mosquito bites is the best way to prevent these infections.