West Nile Virus
Disclosure: I wrote this article and used AI as a “professional editor” to refine and shorten overly long or complicated sentences.
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Mosquitoes are known to transmit viruses that can cause major outbreaks, affecting millions of people. Well-known examples include dengue, Zika, chikungunya, and yellow fever. Historically, these viruses have been associated with tropical and subtropical regions, but some are increasingly appearing in the United States and European countries, as I discussed in my earlier article on dengue.
West Nile virus, another mosquito-transmitted virus, has long been established in the United States and parts of Europe, and it is making headlines right now. As of October 7, 2026, the European Centre for Disease Prevention and Control (ECDC) reported 1,715 locally acquired human infections and 130 deaths in the European Union. This makes 2026 the most intense West Nile season ever recorded in the EU, with Italy being particularly hard hit.
Some readers might wonder why they haven’t heard much about West Nile virus before, given that it has been around for such a long time. Others may have heard of it, but did you know that West Nile normally circulates between birds and mosquitoes, rather than humans and mosquitoes?
For anyone who wants to know more about West Nile virus, this article explains how the virus spreads, what happens when someone becomes infected, and what neuroinvasive West Nile disease is.
But before I start, here’s my usual disclaimer: this article is intentionally simplified. Its focus is on providing a basic understanding of West Nile virus rather than covering every scientific detail.
What Is West Nile Virus?
West Nile virus (WNV) was first identified in 1937 in the West Nile district of Uganda, which explains its name. Today, it is established in many parts of the world, including regions of Africa, Europe, the Middle East, Asia, and North America. In the United States, the virus was first detected in New York City in 1999. Since then, human infections have been reported in all 50 states, although sustained local transmission is established primarily across the 48 contiguous states.
West Nile virus is an RNA virus, meaning its genome is made of RNA rather than DNA. Other examples of RNA viruses include SARS-CoV-2 (the virus responsible for COVID-19) and dengue virus. WNV is actually related to dengue virus (but not SARS-CoV-2), as both belong to the family Flaviviridae, together with Zika and yellow fever viruses.
While both dengue virus and WNV use mosquitoes as vectors, their transmission cycles are quite different. Dengue primarily cycles between mosquitoes and humans, whereas West Nile primarily cycles between mosquitoes and birds.
A mosquito takes a blood meal from an infected bird and picks up the virus. The virus then multiplies inside the mosquito and can eventually be transmitted to another bird when the mosquito feeds again. And you guessed right, WNV then multiplies inside the newly infected bird, and the next mosquito feeding on that bird can pick up the virus.
The mosquitoes responsible for transmitting WNV belong mainly to the genus Culex, including Culex pipiens, a common mosquito species found across much of Europe and North America. These mosquitoes are particularly active around dusk and dawn.
But of course, these mosquitoes do not feed exclusively on birds. They also welcome a blood meal from other animals, including humans.
The “good news” is that most hosts other than birds are usually considered “dead-end hosts”: the virus can infect the host, but it will not be passed on to another mosquito feeding on that host.
This article will focus on what WNV can do to humans, although the virus can also cause serious illness in other animals, particularly horses.
What Illness Does West Nile Virus Cause?
Most people (approximately 80%) infected with West Nile virus will not experience any symptoms. They may have an itchy bite mark from the mosquito, but that’s it. They will not even know they have been infected with WNV, and the infection generally resolves on its own.
For the remaining ~20%, symptoms usually start anywhere between two and fourteen days after infection and may include fever, headache, muscle and joint pain, fatigue, nausea, and sometimes a skin rash. As these symptoms are not specific to WNV, patients often don’t realize what is causing them to feel unwell. In most cases, the illness resolves on its own. However, fatigue and weakness can sometimes continue for weeks or even months.
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But unfortunately, WNV can also be much more serious. In fewer than 1% of all infections, the virus affects the central nervous system, causing what is known as neuroinvasive West Nile disease.
If this happens, the virus can cause meningitis (an inflammation of the membranes surrounding the brain and spinal cord) or encephalitis (an inflammation of the brain itself). The virus can also damage nerve cells responsible for muscle movement, resulting in muscle weakness or even paralysis.
Neuroinvasive West Nile disease is rare, but it can happen, and particularly adults over 60 and people with weakened immune systems or certain underlying medical conditions are at greater risk. The initial flu-like symptoms do not allow us to predict who will develop severe complications, and are often not even associated with WNV. Neurological complications can develop rapidly, sometimes within 24–48 hours of the first symptoms, although their onset varies between patients. Symptoms including high fever, severe headaches, a stiff neck, confusion, tremors, seizures, and muscle weakness are warning signs that require immediate medical attention.
Doctors can help confirm WNV infection using a blood test that detects antibodies against the virus. However, there is currently no specific antiviral treatment for West Nile virus. Treatment is supportive, but crucial in cases involving complications such as seizures and breathing difficulties. Recovery can take weeks or months, and some patients are left with long-term neurological problems. Unfortunately, according to the CDC, approximately 10% of patients who develop neuroinvasive disease die.
There are currently no vaccines available for humans, so protecting ourselves from mosquito bites (repellents, long sleeves, trousers) and keeping mosquitoes out of our homes using window and door screens are our best defenses.
Why Is 2026 Such a Bad Year in Europe?
This year has seen the highest number of reported West Nile infections on record in the EU. Belgium even reported its first locally acquired human infections.
Scientists believe that one important contributing factor is temperature.
Researchers have shown that cooler conditions slow down the process by which infected mosquitoes become capable of transmitting the virus to another host. Warmer temperatures accelerate this process, making transmission more efficient.
Warmer temperatures earlier in the year, combined with longer summer-like conditions, can also extend the period during which mosquitoes are active. This provides more opportunities for the virus to circulate between birds and mosquitoes.
Another piece of the puzzle may be improved surveillance. The exact reasons for Europe’s unusually high number of reported cases in 2026 are not yet known. However, the ECDC expects West Nile to continue spreading into northern Europe.
In Summary
West Nile virus is not a new virus, and it has caused outbreaks before.
For the vast majority of people infected, the infection either goes completely unnoticed or causes only flu-like symptoms. However, a small percentage develop serious neurological disease, which can be fatal.
As we currently have no vaccine for humans and no specific antiviral treatment, understanding how the virus spreads, monitoring its circulation, and preventing mosquito bites remain our best defenses.
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