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Neuro Invasion: The Silent Siege of West Nile Virus
- Heather McSharry, PhD
- Sep 9
- 13 min read
Updated: Sep 16
West Nile virus has returned to the headlines, with new human cases emerging across the United States and steady activity in Europe. In this episode of Infectious Dose, I take you inside the current outbreak season, from CDC and ECDC surveillance to state-level reports, to understand where the virus is spreading and why it continues to matter.
But this story isn’t just about numbers—it’s about the science. Most people infected never develop symptoms, yet for some, West Nile launches a silent siege on the nervous system. We’ll look at how the virus cycles through birds and mosquitoes, what happens when it reaches the human brain, and why treatment options remain limited. From pathogenesis to prevention, this episode offers a concise guide to one of the most enduring mosquito-borne threats in the U.S. and beyond.
Listen here or scroll down to read full episode.
Full Episode
A warm summer evening. The hum of cicadas. And just beneath it, another sound—the high, thin whine of a mosquito. You swat, maybe miss, and think nothing of it. But for someone else, in another yard, another park, another city, the same bite will change everything.
Within days, fever. Crushing fatigue. Then confusion. A tremor in the hands. Muscles that no longer obey. Sometimes paralysis. Sometimes death.
This isn’t rare. It isn’t new. It’s West Nile virus—an infection that’s been haunting North America for more than two decades. Born in birds, carried by mosquitoes, it seeps quietly through communities each summer, leaving most people untouched, but striking a few with devastating force. And right now, it’s on the move again—from Massachusetts to Wyoming, from Italy to Greece.
So tonight, we’re diving in. Where cases are rising. How this virus works. The science behind its spread, its path into the brain, and the sobering truth about treatments and vaccines.
This is Neuro Invasion: The Silent Siege of West Nile Virus.
Let’s start with the global picture. Europe tracks West Nile very closely, and this summer has been no exception. Europe is seeing one of its most extensive West Nile seasons in recent years.
By 13 August, ECDC had confirmed 335 human cases and 19 deaths with a case-fatality rate of about 5.7 percent. The breakdown is as follows: Italy leads with 274 cases, followed by Greece (35), Serbia (9), France (7), Romania (6), Hungary (2), and one case each in Bulgaria and Spain. All recorded fatalities occurred in Italy. By 27 August, that list expanded to nine—now also including Albania—even as case counts remained centered in the south. Notably, this season has spread into new regions for the first time in years, such as Italy’s Latina and Frosinone provinces and Romania’s Sălaj County, marking the highest seasonal case burden Europe has seen in three years. Surveillance also shows continued outbreaks among horses, birds, and mosquitoes, with new regions reporting activity for the first time. With late-summer weather still favoring mosquitoes, numbers could rise further before the season winds down.”
First Detection of West Nile Virus in UK Mosquitoes
So far, there has never been evidence of locally-acquired human cases of WNV in the UK and as of June of this year only 7 confirmed travel-associated cases have been reported in the UK since the year 2000.
However, for the first time ever, UK scientists found fragments of West Nile virus in native mosquitoes this year—though it doesn’t signal an outbreak. Researchers with the UK Health Security Agency (UKHSA) and the Animal and Plant Health Agency (APHA), through their Vector-Borne RADAR (Real-time Arbovirus Detection and Response) surveillance, detected viral genetic material using PCR in two pools of Aedes vexans mosquitoes collected in July 2023 near the River Idle wetlands in Nottinghamshire—out of 200 pools tested at the site, the other 198 were negative.
Despite this groundbreaking finding, the public health risk remains very low: there is currently no evidence of West Nile circulating among UK birds, livestock, or humans. Since 2000, only seven travel-related human cases have been recorded, with no local cases reported—and none linked to this mosquito detection. The UKHSA is stepping up disease surveillance and urging healthcare professionals to consider West Nile testing in patients with unexplained brain infections.
Experts warn this development aligns with a broader pattern: climate change is making regions like the UK increasingly hospitable to mosquito-borne viruses once considered “tropical.” Continued surveillance and vigilance are the critical tools for staying ahead of potential emergence.
The Republic of Ireland remains free of West Nile virus. According to the Department of Agriculture, Food and the Marine, no local infections have ever been reported, and surveillance efforts—covering wild birds and horses—continue to show no presence of the virus. Human cases in Ireland have only been travel-related, with just one confirmed returnee infection noted in 2023. Mosquito species capable of spreading West Nile, like Culex pipiens, are present across Ireland, but so far there has been no evidence of local transmission. Authorities maintain active surveillance and robust reporting procedures to ensure that any potential incursion would be caught early. I'd like to give a shoutout to Ireland as the newest country to join my listeners. We're up to 21 countries now I think and I appreciate every one of you!
OK, so from Ireland let's look at the situation in the United States. It's not great. West Nile virus is present in at least 15 states so far this year. In the Northeast, Massachusetts confirmed its 4th human case of the year, Connecticut announced its first, Allegheny County, Pennsylvania reported its second and New York City has begun reporting cases. The Midwest / Great Plains are also seeing activity—Minnesota has logged 20 cases with two deaths, while Wyoming officials have tallied 11 cases so far, including 6 with severe disease and one death. Illinois had its first case in DuPage County.
Down South, Texas is seeing a scattered but growing number of human infections across multiple counties, including Brazos, Tarrant, Dallas, Hays, El Paso, and Harris, with San Antonio detecting the virus in mosquitoes and Midland reporting infected traps. In Louisiana, New Orleans confirmed its first neuroinvasive human case. So far, the cases reported across the U.S. in 2025 have all been locally acquired, with no evidence that travel-related infections are driving the numbers. Update: There have been reports of more WNV cases in Louisiana but I couldn't find confirmation on the Louisiana state public health pages.
Outside North America, the UK, and Europe, West Nile’s picture is mixed because surveillance is uneven. In the Philippines, the Department of Health’s Research Institute for Tropical Medicine, or RITM, has PCR capacity for West Nile alongside Japanese encephalitis, but there haven’t been recent public reports of West Nile activity—so the lab capacity exists, but routine national reporting appears limited. In Mexico, human cases have been rare, yet a 2023 border study in Chihuahua detected West Nile-positive field mosquitoes and explicitly called for intensified vector surveillance; regionally, the Pan American health organization or PAHO’s RELDA network supports arbovirus lab surveillance that includes West Nile. RELDA is The Arbovirus Diagnosis Laboratory Network of the Americas and its main goal is to strengthen laboratory surveillance and ensure countries have the capacity to respond quickly to arbovirus outbreaks and epidemics. And just as a reminder from the chikungunya episode, “arbovirus” is simply short for arthropod-borne viruses—viruses spread by mosquitoes, ticks, and other biting insects.
All right, in South America, Brazil has confirmed equine infections and at least one human detection, but peer-reviewed work notes West Nile remains a low surveillance priority nationally, with guidance and sentinel animal surveillance varying by state. Across Africa, West Nile is endemic in many countries but a 2025 review highlights major genomic surveillance gaps and sparse sequencing data, while South Africa’s NICD maintains seasonal human surveillance and typically detects a small number of cases each year.
On the innovation front, researchers are turning surveillance into science fiction—or at least its close cousin. In Maryland, an AI-driven mosquito monitoring system is being piloted to flag West Nile activity as it happens. In Germany, scientists are using genomic sequencing of blood donations to quietly monitor for silent community transmission. West Nile RNA also shows up in wastewater when the virus is active in the community. And down in the Caribbean, a smart blending of antibody testing and modeling has uncovered recurring yet undetected circulation on an island. These methods underscore how 21st-century tech is giving public health new eyes to spot threats before they strike. Please keep in mind that preprints have not yet gone through the review process.
Transmission and Pathogenesis (behind paywall so here's the PDF)
So what makes West Nile tick? At its core, West Nile virus is a single-stranded RNA virus in the Flaviviridae family, making it a close relative of Zika, yellow fever, and Japanese encephalitis. Genetic studies show there are at least nine distinct lineages of West Nile, but only lineages 1 and 2 have historically been linked to human disease. A new twist came in 2023, when U.S. doctors documented the first human infection with lineage 3, in a patient who was also co-infected with lineage 1.
The virus is sustained in nature through a classic mosquito–bird–mosquito cycle. When mosquitoes feed on infected birds, they pick up the virus and can pass it on when they bite again. Certain passerine, or perching, birds are the primary amplifying hosts because they develop high enough levels of virus in their blood to infect new mosquitoes. Among them, members of the crow family—crows, jays, and ravens—are especially vulnerable, often dying in large numbers during outbreaks. In fact, bird die-offs can act as an early warning that human cases may soon follow.
Transmission of WNV varies regionally depending on which mosquito species dominate—for example, Culex pipiens dominates in much of Europe and North America, Culex quinquefasciatus in southern U.S. states, Culex tarsalis in the west, and other Culex species across Africa and Asia. Humans, horses, and most other mammals are considered “dead-end hosts” because the levels of virus in our blood are too low, and too short-lived, to infect another mosquito. Even so, once we’re infected, the virus can still cause serious disease. The graphic is from an overview at JAMA.
Almost all transmission happens through mosquito bites, but there are some uncommon alternative routes. These include blood transfusions, organ transplants, and rarely, from mother to child during pregnancy, delivery, or breastfeeding. Occupational exposures—through accidental needle sticks or splashes into the eye—have also been reported. Blood transfusion–related spread was first recognized in the U.S. in 2002, which led to nationwide blood donor screening beginning in 2003. That testing has been very effective, and only 14 transfusion-related cases have been documented since, the last in 2016.
Pathogenesis
When a mosquito carrying West Nile virus bites, it injects saliva into the skin. That’s where the virus gets its start, multiplying in skin cells and in immune cells that usually act as the body’s first line of defense.
From there, the virus spreads to nearby lymph nodes, slips into the bloodstream, and moves throughout the body, reaching organs like the spleen and liver. In some people, the infection goes even further, breaking into the brain and spinal cord.
The time between a mosquito bite and the start of symptoms usually ranges from 2 to 14 days, though in people with weaker immune systems it can stretch to about three weeks. Most infections never cause any symptoms at all. Among those who do feel sick, the most common form—often called West Nile fever—comes on suddenly with fever, tiredness, headache, loss of appetite, muscle aches, stomach upset, and sometimes eye pain. A flat, reddish rash can appear just as the fever starts to fade.
But for some people, the story isn't so simple. The virus is notorious for its ability to invade the nervous system in roughly 1 in 150 infected people. Those unfortunate folks will develop neuroinvasive disease, meaning the virus has crossed into the brain or spinal cord—likely through a combination of direct penetration of the blood–brain barrier, “Trojan horse” entry by infected immune cells, or retrograde travel along nerves—it causes inflammation and destruction of neurons, especially in the spinal cord and brainstem gray matter. Clinically, this can appear as meningitis with stiff neck and sensitivity to light, or encephalitis with disorientation, tremors, seizures, and coma. In some cases, the virus targets the spinal cord’s motor neurons, leading to acute flaccid paralysis that mimics polio. People over 50 or those who are immunocompromised are most at risk, and while many survivors are left with lasting problems such as weakness, difficulty walking, or memory loss, about 1 in 10 patients with neuroinvasive disease die.
West Nile doesn’t always stay confined to the brain—it has also been linked to inflammation in the eyes, heart, pancreas, and liver, as well as severe muscle breakdown in rare cases.
When it comes to treating West Nile virus in humans, there are still no specific antiviral drugs with proven effectiveness. Researchers have tested treatments such as antibody infusions, antiviral drugs, and immune-modifying medicines, but so far none have proven effective. Medical care remains supportive—hydration, monitoring, hospitalization for severe cases, and intensive care when needed. Prevention continues to be our strongest defense.
Recent research has explored new therapeutic strategies targeting the virus’s key proteins—the structural Envelope (E) protein and the non-structural NS3 and NS5 proteins, which are essential for viral replication. Promising leads include peptide inhibitors, monoclonal antibodies, and small molecules designed to disrupt viral assembly or replication. However, these are largely at the in silico (computer modeling) or in vitro (lab dish) stage, with few yet advanced to testing in living organisms.
Vaccines:
Equine (horse) vaccines for West Nile virus are licensed and widely used in veterinary medicine.
Human vaccines have seen progress, including traditional inactivated-virus designs and experimental platforms, but none has yet cleared the final regulatory path to approval. Despite decades of effort, there still isn’t a licensed human WNV vaccine—highlighting how challenging and complex vaccine development can be for some pathogens.
With WNV spreading into new areas and climate-driven risk rising, experts underscore an urgent need for validated antiviral therapies and scalable human vaccines as part of our long-term strategy.
We’ve covered where West Nile is spreading, how it moves through birds and mosquitoes, and what happens when it invades the human body. But I wanted to answer some questions I got from my friends and family about this virus so The Hot Zone hotline is open!
Hot Zone hotline:
Q: How common is West Nile virus in the U.S. each year? A: It varies. Most years, the U.S. reports somewhere between 600 and 2,500 human cases. The worst year on record was 2003, with nearly 10,000 cases and more than 260 deaths. So while year-to-year numbers swing, West Nile is now a fixture of late summer in much of the country.
Q: Do most people who get West Nile virus get really sick? A: No. In fact, about 80% of people infected never develop symptoms at all. Around 20% get what’s called West Nile fever—basically a flu-like illness with fever, fatigue, and aches. But less than 1% develop the neuroinvasive form that affects the brain or spinal cord. Those cases are rare—but they can be devastating.
Q: When is the risk highest? A: In the U.S. and Europe, West Nile cases peak in late summer and early fall, usually in August and September. That’s when mosquito populations are at their highest, and when the bird–mosquito transmission cycle is in full swing.
Q: How do horses fit into the West Nile story? And what about other farm animals? A: Horses are highly vulnerable to West Nile. In fact, they can develop severe neurological disease much like humans, and outbreaks in horses often alert veterinarians and health officials to local transmission. That’s why vaccines for horses are licensed and widely used—they’ve been a real success story in protecting equine populations.
As for other farm animals—cattle, sheep, goats, pigs—they can be bitten by mosquitoes and infected, but they usually don’t get sick and don’t develop high enough levels of virus in their blood to pass it back to mosquitoes. In other words, they don’t play much of a role in the virus’s life cycle. Horses are the main farm animal we worry about when it comes to West Nile.
Q: If someone survives neuroinvasive disease, do they fully recover? A: Not always. Studies show that more than half of survivors are left with long-term problems—anything from tremors and memory loss to depression, fatigue, or trouble walking. In other words, recovery can be a long road.
Q: Are some people at higher risk than others? A: Yes. Older adults, people with weakened immune systems, and people with certain chronic illnesses are most at risk for severe disease. And on a community level, areas with less mosquito control, fewer resources, or limited healthcare access may be hit harder than wealthier neighborhoods, even in the same city.
Mosquito Control & Personal Protection
When it comes to West Nile, prevention is everything—and that starts with controlling mosquitoes and protecting yourself from their bites.
At the community level, the most effective strategies focus on reducing mosquito breeding sites. Mosquitoes need standing water to lay their eggs, and even a small puddle in a flowerpot tray or a clogged gutter can do the trick. Simple measures like emptying and scrubbing birdbaths, tipping over buckets, clearing gutters, and changing pet water bowls every few days can dramatically cut mosquito numbers around your home. Communities also use larvicide treatments in storm drains or ponds to stop mosquitoes before they emerge, but those approaches work best when paired with residents eliminating small water sources that are easy to overlook.
On the personal side, protective clothing makes a difference: light-colored, long sleeves and pants help reduce bites. Mosquitoes are most active at dawn and dusk, so avoiding outdoor activity at those times, or adding a head net when camping or gardening, can help too. Screens on windows and doors are a simple but powerful barrier—one torn screen can invite an entire swarm inside.
Chemical repellents—like DEET, picaridin, oil of lemon eucalyptus, or IR3535—are highly effective, but if you want non-chemical or lower-impact approaches, there are options:
Fans on porches and patios create air currents mosquitoes can’t fly through.
Mosquito traps can reduce local populations if maintained correctly.
Encouraging natural predators, such as bats, dragonflies, and some bird species, can provide supplemental control in certain environments.
Landscaping choices—like keeping grass short, trimming shrubs, and avoiding dense vegetation near the house—reduce shady resting spots mosquitoes love.
Together, these steps don’t eliminate risk entirely, but they stack the odds in your favor. The fewer mosquitoes around, the fewer opportunities for bites, and the lower the chance West Nile virus can find its way from birds to humans.
So where does this leave us? Globally, West Nile virus has established itself as a recurring seasonal threat—especially in North America and Europe, but with growing concern in other regions as well. In the U.S., we’re already seeing scattered cases across multiple states this season. Scientifically, we have a solid understanding of transmission—we know the birds, the mosquitoes, and the cycles that keep this virus in play. But when it comes to pathogenesis, especially why only a small fraction of infections become neuroinvasive and so devastating, there are still big gaps in knowledge. And despite decades of effort, we still lack both proven antiviral treatments and a licensed human vaccine.
Climate change only raises the stakes. Warmer temperatures, shifting rainfall patterns, and milder winters expand mosquito habitats and lengthen the season when transmission can occur. That means West Nile’s footprint is likely to grow in the years ahead, putting more people at risk in places that once considered it a distant problem.
For you, the listener, the message is simple: West Nile is still here, it’s still dangerous for some, and the best defense remains protecting yourself from mosquito bites while supporting the public health systems that monitor and control this virus.
That’s it for today’s episode—a snapshot of the global situation, the U.S. numbers, and the science behind West Nile virus. Until next time, stay healthy, stay informed, and spread knowledge not diseases.
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