West Nile Virus: When a Mosquito-Borne Infection Reaches the Nervous System

Most people infected with West Nile virus will never know they had it. Approximately 80 per cent of infections produce no symptoms whatsoever, and about 20 per cent cause West Nile fever, a relatively mild illness of fever, headache, fatigue, muscle and joint pain and sometimes rash.¹ In a small minority, however, the virus crosses into the nervous system, and the result can be meningitis, encephalitis, acute flaccid paralysis, coma and sometimes death.

That contrast between silent infection and devastating neurological disease is what makes West Nile so instructive, both as a One Health problem and as a question in neuroimmunology. It also demonstrates, more plainly than most pathogens manage, why surveillance cannot begin and end with human cases. The virus is maintained in nature through a cycle involving birds and mosquitoes, with humans and horses infected only incidentally, which means that changes in bird infection, mosquito populations, weather and environmental conditions can all signal rising risk well before the first patient with severe neurological disease reaches a hospital. The system is connected, and surveillance has to be connected too.

A virus moving between birds and mosquitoes

West Nile virus is a flavivirus, closely related to other important neurotropic viruses including Japanese encephalitis virus, and WHO describes it as being maintained in nature in a mosquito–bird–mosquito transmission cycle, with mosquitoes of the genus Culex generally considered the principal vectors.¹ Mosquitoes acquire the virus when feeding on infected birds, the virus replicates within the mosquito, and subsequent bites transmit it onward to other birds and occasionally to humans, horses and other mammals.

Birds are therefore not incidental to West Nile epidemiology; they are fundamental to it. Humans and horses sit at the other end of that relationship, because WHO is explicit that horses, just like humans, are dead-end hosts: they become infected but do not spread the infection onward, since the concentration of virus in their blood is usually insufficient to infect a feeding mosquito.¹ That single fact turns West Nile into a One Health disease by definition rather than by argument, because human infections simply cannot be understood without reference to bird populations, vector ecology and animal disease.

Most infections are silent

The central difficulty for West Nile surveillance is that infection is usually invisible. With roughly four in five infections producing no symptoms at all, reported human cases represent only a fraction of the transmission actually occurring in a population, and the visible caseload is therefore a poor guide to how much virus is circulating.¹

A much smaller proportion of infected people develop something far more serious. CDC puts it at less than one per cent developing severe illness that affects the central nervous system, requires hospitalisation or results in death, while WHO expresses the same thing as approximately one in 150 infections progressing to a severe form of the disease.^1,2 Those patients may present with meningitis, encephalitis or acute flaccid myelitis, and the recorded clinical picture includes high fever, neck stiffness, stupor, disorientation, tremors, convulsions, muscle weakness, vision loss, numbness, paralysis and coma.²

The outcome for that group is serious. About 10 per cent of people who develop severe illness affecting the central nervous system die, and among those who survive, recovery can take several weeks or months with some effects possibly permanent.² Set against a population in which most infections pass entirely unnoticed, that produces one of the more interesting questions in neuroinfectious disease: why does the same infection remain clinically silent in one person and invade the nervous system in another?

The neuroimmunology question

West Nile does not only provide a model of infection; it provides a model of host susceptibility. Older age and immunocompromising conditions are recognised risk factors for neuroinvasive disease, and other underlying conditions appear to influence neurological risk as well, which suggests that outcome is determined by considerably more than exposure to the pathogen.

Before the virus can cause encephalitis it has to overcome a sequence of host defences. Following infection at the site of a mosquito bite it can replicate locally and in lymphoid tissue before entering the bloodstream, and in most people immune control prevents any progression to severe neurological disease; in a minority, the virus eventually reaches the central nervous system. That creates a balance familiar across neuroinfectious disease, in which the immune response must control viral replication aggressively enough to prevent neuroinvasion, while inflammation within the brain can itself contribute to neurological injury once infection is established there.

The research questions follow directly from that balance, and they are uncomfortable ones. Why does protective immunity fail in particular patients? Which immune responses prevent neuroinvasion in the first place? Which inflammatory pathways contribute to damage once the virus has reached the brain? And why do some patients recover fully while others are left with long-term deficits? These place West Nile at the intersection of virology, immunology, neurology and health-data science, and they are the same questions we raised about Nipah, asked of a pathogen that infects far more people far more quietly.

Horses and birds as early warning

West Nile offers one of the clearest demonstrations of why veterinary surveillance matters to human medicine. Horses develop neurological disease after infection, including potentially fatal encephalomyelitis, and unlike humans they can be protected: no vaccine is available for people, but vaccines have been developed for horses.¹

More importantly, the animal signal arrives first. WHO states plainly that because West Nile outbreaks in animals precede human cases, establishing an active animal-health surveillance system to detect new cases in birds and horses is essential in providing early warning for veterinary and human public health authorities.¹ That is not a One Health aspiration expressed by advocates; it is the standing recommendation of the human-health agency, and it describes a chain of signals running from infected birds through mosquito activity and equine cases to human infection.

None of those datasets is sufficient on its own, and each is held by a different profession. The value of One Health surveillance here is not simply that veterinarians and public-health teams collect different information, but that the information becomes substantially more useful when it is interpreted together — which is the argument we set out at length in Surveillance Before the Outbreak.

Europe is already living with West Nile

West Nile is not a theoretical emerging threat for Europe. It is established as an endemo-epidemic mosquito-borne zoonosis across parts of southern, eastern and western Europe, and ECDC publishes seasonal surveillance covering both human and animal data across EU and EEA countries.³

The 2025 season makes the point concrete. As of 3 December 2025, 14 European countries had reported 1,112 locally acquired human West Nile infections and 97 deaths. Italy accounted for the overwhelming majority with 779 cases and 72 deaths — the highest annual total the country has recorded — concentrated in the Lazio region, which reported 267 cases, and Campania, which reported 133. Greece, France, Serbia, Romania and Spain each reported dozens more, and cases also appeared in Hungary, Croatia, Albania, Germany, North Macedonia, Bulgaria, Kosovo and Türkiye.³

The significance of that distribution is easy to underestimate. West Nile is no longer a disease associated with Africa or with distant tropical settings; it is part of the European infectious-disease landscape, and because transmission depends on mosquitoes, birds, climate and environmental conditions, its epidemiology is inherently dynamic rather than settled. A country reporting one case this year may report dozens the next, as Italy’s record season demonstrates.

Climate, water and landscape

Vector-borne diseases respond to environmental conditions, and West Nile responds to several at once. Mosquito abundance, the availability of breeding habitat, temperature and seasonal patterns all influence transmission; bird migration can move the virus considerable distances; urban environments create standing water in which Culex breeds readily; and agricultural landscapes, wetlands and water management alter the frequency with which birds and mosquitoes meet.

That is why surveillance for a disease like this increasingly needs information from well outside traditional healthcare systems. Meteorological data matters, vector surveillance matters, bird migration matters, veterinary cases matter and environmental conditions matter — and only at the very end of that chain does a hospital see a patient with encephalitis. The same logic applies to vectors arriving where they were not previously established, which is the subject of Aedes aegypti Is Breeding in Britain.

The health-data opportunity

West Nile presents an unusually rich opportunity for integrated health-data work, because the relevant signals are numerous, routinely collected and almost never combined. A surveillance system capable of relating bird surveillance to mosquito testing, equine neurological cases, weather and environmental data and human infections with their neurological outcomes could, in principle, identify combinations of signals associated with rising risk rather than waiting for confirmed human cases to accumulate.

The same principle applies at the level of the individual patient. For people who do become infected, the question is whether combinations of age, immune status, comorbidities, routine inflammatory markers, virological data, cerebrospinal-fluid findings, neurological presentation and longitudinal clinical measurement can help predict who is at greatest risk of neuroinvasive disease or of poor neurological recovery. The interesting question, in other words, is shifting from who has West Nile virus towards who is likely to develop neurological disease, and why.

Surveillance has to cross professional boundaries

West Nile creates a practical test of One Health rather than a rhetorical one. An ornithologist observing bird mortality holds one part of the evidence; an entomologist monitoring mosquito populations holds another; a veterinarian treating a horse with neurological disease sees a third; and a hospital clinician diagnosing viral encephalitis sees the final manifestation. Those observations traditionally sit in different organisations, different databases and different professional communities, while the pathogen moves freely between all of them.

A genuinely integrated surveillance system therefore has to connect wildlife, vector, veterinary, environmental and human-health evidence rather than merely collect it in parallel. Collection is the easy half; the difficulty, and the value, lies in being able to put a dead bird, a positive mosquito pool, a sick horse and a cluster of human fever cases beside one another quickly enough for the comparison to change a decision.

The One Health lesson

West Nile demonstrates something important about emerging infectious disease, which is that the human neurological case is usually the end of a much longer ecological process. Before a patient develops encephalitis, the virus may have been circulating between birds and mosquitoes for weeks or months, and before a neurologist recognises neuroinvasive disease, veterinarians, vector-control teams or wildlife surveillance programmes may already hold signals indicating that transmission is rising.

That is an opportunity rather than a criticism. If One Health is implemented properly, prevention does not have to begin when a patient reaches intensive care: it can begin in a mosquito population, in a bird surveillance programme, in an equine neurological case, or in environmental data showing that the conditions for transmission are changing. West Nile therefore reinforces the lesson that Nipah introduced — the place where a disease is ultimately diagnosed is not necessarily the place where it can first be detected — and for zoonotic neuroinfectious disease, understanding that difference may be among the most valuable tools we have for preventing neurological illness before it occurs.

This is the second article in our Zoonotic Neuroinfectious Disease pathway, which follows pathogens that cross from animals to people and then attack the nervous system.

Questions & Answers

References

1. World Health Organization, West Nile virus, fact sheet. Geneva: WHO.
2. Centers for Disease Control and Prevention, West Nile virus: signs and symptoms. Atlanta: CDC.
3. European Centre for Disease Prevention and Control, Historical data on local transmission of West Nile virus in Europe, and the West Nile fever surveillance pages. Stockholm: ECDC. Figures for the 2025 season as reported at 3 December 2025.