West Nile Virus (WNV) is a virus that spreads when people are bitten by infected mosquitoes. Luckily, most folks have no symptoms, but it’s worth knowing about, especially during the summer months and as mosquitoes are expanding their reach.
Wait, what? It’s called West Nile Virus, but I live nowhere near the Nile!
Yes, West Nile Virus was first detected in Uganda in 1937, in a region near the West Nile (aha, that’s why!). A first big outbreak in Europe happened in Romania in the 1990s and in 1999 it also spread to the US and has now been detected in all states.
🦟 What is it exactly and how does it spread? 🦟
WNV is a flavivirus (it’s in the same family as dengue and Zika) that cycles between birds and mosquitoes. The natural reservoir (science-speak for the source) is birds. Infected birds carry the virus in their blood, and when they get bitten by mosquitoes (especially Culex species), it in turn infects those pesky little biters. We call these mosquitoes “vectors”, because like an arrow that runs from A to B, they take a disease and bring it from A (the reservoir) to B (the host). They then pass it to the next thing they bite, including us. Humans are what epidemiologists call a “dead-end host”: we can get sick, but we don’t have enough virus in our blood to infect another mosquito.
(Source: Designed by author using AI)
It does not spread from person to person through coughing, sneezing, or close contact. Even if you touch an infected animal, you will not get infected. The primary way someone can get it is if an infected mosquito bites you.
There are very rare cases in which WNV can spread through blood transfusions, organ transplants, from a pregnant person to their baby during pregnancy, birth, or breastfeeding.
Most cases of West Nile Virus happen during the summer months, between July and September. August is usually the month with the most cases. If you want to nerd out on some data, you can check out data from ArboNet, the national arboviral surveillance system. You can find historic data here, and you can track this summer’s cases here.
Let’s talk about symptoms
The incubation period (the time between getting bitten and symptoms starting) is typically 2 to 14 days. But it can stretch to 21 days in people who are immunocompromised.
First of all, the good news: 80% of people who are infected with WNV never even know about it because they have no symptoms.
Those who do get symptoms can be split in two groups:
Those with mild illness: Folks may experience a sudden flu-like bout of fever, headache, and muscle aches (also called myalgia). Some may also develop lymphadenopathy (swollen lymph nodes) and a rash across the neck, trunk, or arms and legs that can look flat, bumpy, or measles-like.
Again, more good news: uncomplicated cases usually clear up within 3 to 6 days.
When it gets more serious: Fewer than 1% of infections (roughly 1 in 150) progress to a disease that affects the brain and nervous system. This can show up as meningitis (inflammation of the layers around the brain and spinal cord), encephalitis (inflammation of the brain itself), or acute flaccid paralysis. This is a sudden, often one-sided limb weakness that can, in rare cases, progress to affect breathing.
Warning signs for something more serious going on include high fever, neck stiffness, confusion, severe muscle weakness, tremors, seizures, or worsening paralysis. Other possible symptoms include unsteadiness, facial weakness, or vision changes.
Who’s most at risk?
Severe disease is more likely in people over 50 and in those with underlying conditions like cancer, diabetes, high blood pressure, or kidney disease. And if you are immunocompromised, you are also at higher risk.
Here is the reassuring part though: even when infection progresses to the type that affects the nervous system, it’s rarely fatal (and remember, getting to that point at all is already rare). Of the small group of people who do develop this severe form, the case fatality rate, which means the rate of infected people who end up dying, climbs with age and is around 20% in people over 70, compared to just around 2% in those under 50.
If you or a loved one falls into any of these groups, make sure you know the symptoms, and if you develop a fever, headache, or confusion after mosquito exposure, mention WNV to your clinician as it might be the cause.
And what about treatment?
There is no vaccine for West Nile virus, and the main treatment is supportive. This means meds like over-the-counter painkillers, resting up, and staying hydrated. For folks with more severe disease, hospitalization might be necessary.
How to protect yourself 🦟 🦟 🦟
Protecting yourself from getting bitten in the first place is the best strategy, not only against WNV, so here are some tips:
🔸 Use an EPA-registered repellent (look for DEET, picaridin, IR3535, or oil of lemon eucalyptus).
🔸 Wear long sleeves and pants during dawn and dusk, when Culex mosquitoes (the main WNV carriers) are most active.
🔸 Keep window and door screens intact. Or if you can, run your AC instead of leaving windows open.
🔸 Bed nets are also an option if you don’t have AC and want to keep your windows open.
🔸 Empty anything that holds standing water regularly. Think buckets, gutters, plant saucers, birdbaths (Mosquitoes love those places to lay their eggs!).
🔸 Consider permethrin-treated clothing if you spend a lot of time outdoors in areas with high mosquito activity.
Bottom line: It’s useful to know about WNV, especially if you live in a region where mosquitoes are abundant in the summer months. With a few simple habits (repellent, long sleeves at dawn/dusk, no standing water) you can cut your WNV risk significantly, and the bonus is they protect you from other mosquito-borne illnesses too 🦟
Stay well. Stay curious.
Love,
Those Nerdy Girls
Sources:
UK Health Security Agency – West Nile virus: epidemiology, diagnosis and prevention
CDC – About West Nile Virus [archived link]
CDC – Clinical Signs and Symptoms of West Nile Virus [archived link]
CDC ArboNET – West Nile Virus – current year data
CDC – West Nile Virus: Historic Data (1999-2025) [archived link]
Penn State Extension – Fight the Bite: How to Effectively Use Tick and Mosquito Repellent
