Causes and Risk Factors of West Nile Virus Infection

Infection with the West Nile virus is almost exclusively spread through contact with mosquitoes that carry the virus, although other modes of infection have also been identified. Understanding how this virus is spread is the best way to avoid a West Nile virus infection.

West Nile Virus causes
© Verywell, 2018


The West Nile virus is an RNA virus, classified by infectious disease experts as a member of the Japanese encephalitis virus group. It was first isolated from a blood sample stored in the 1930s from the West Nile area of Uganda.

In recent decades the virus has spread virtually worldwide, and today is found in Africa, the Middle East, Europe, Asia, Australia, and North and South America.

While it was initially thought to be of no special consequence, the West Nile virus is now known to be responsible for a particularly dangerous form of meningitis and encephalitis in a small proportion of those who become infected.

Common Causes of Infection

West Nile virus is an arbovirus, that is, a virus transmitted by arthropods. It is almost exclusively spread by mosquitoes. The virus is acquired by the mosquitoes when they feed on birds, the chief host of the West Nile virus.


Over 60 species of mosquitoes have been shown to be infected with West Nile virus. Mosquitoes that spread the virus to humans are usually one of the multitudes of Culex species, insects which are prevalent in many parts of the world. West Nile virus has also been isolated from ticks, but it is not clear that ticks are a vector of infection.

The Role of Birds

Many species of birds have been identified as the hosts which harbor the virus, and are the means by which West Nile virus has spread around the globe. Usually, the birds infected with West Nile virus have high concentrations of the virus in their blood for long periods of time but have no symptoms. This means that an infected bird is able to pass the virus to mosquitoes for a long time.

However, some species of crows, ravens, and jays have had high rates of death from West Nile virus, and several localized areas have experienced widespread bird death.

Furthermore, humans who live in proximity to areas where a lot of birds have died from the virus appear to have a higher incidence of West Nile virus infection.

Other Means of Infection

While by far the major means of human infection is by contact with infected mosquitoes, West Nile virus can also be acquired by contact with blood or blood products from people who have the virus in their bloodstream.


Infection with West Nile virus has been identified as occurring with blood transfusions and with transfusions of red blood cells, plasma, and platelets. This form of transmission has been greatly reduced now that universal screening is done in many countries on blood products. This screening is not perfect, however, since it may not detect West Nile virus if it is in very low concentrations.


Rarely, West Nile virus infection has also occurred with organ transplantation from infected donors. In these cases screened serum from the donors has been negative for West Nile virus, strongly suggesting that live virus was still present in the donated organs.


There have also been a few cases of congenital West Nile virus infection, caused by the spread across the placenta from the mother to the baby late in the second trimester. In these cases, babies developed illness from the virus shortly after birth.

Despite these reports, transplacental transmission of the West Nile virus is thought to be quite rare.

Causes of Symptoms

When West Nile virus enters the bloodstream and begins multiplying, the body’s immune system quickly reacts to get rid of the virus.

Typically, antibodies to the virus appear rapidly. These antibodies bind to the virus particles and cause them to be destroyed. In addition, immune cells quickly adapt to attack the virus. The immune response leads to the production of various interferons and cytokines, which fight the virus but which often produce inflammation, leading to the symptoms characteristic of West Nile fever. By these means, the body’s immune system typically gets rid of the virus within a few days.

In some people, however, the West Nile virus is able to cross the blood-brain barrier and gain a foothold within the nervous system. These people are the ones who develop the most feared consequences of West Nile virus—meningitis or encephalitis.

Risk Factors

Any person who is bitten by a mosquito in an area where the bird population carries West Nile virus is susceptible to infection. Since these areas now cover a large portion of the globe, almost any mosquito bite could potentially transmit the virus, to any person. The more mosquito bites you receive, the higher your risk.

Most people who are infected with the West Nile virus suffer only a self-limited illness, or no symptoms at all. However, a small proportion of infected individuals (less than one percent) will develop the serious, life-threatening neurological form of the infection.

While this severe result can affect anyone infected with West Nile virus, some appear to have a higher risk of developing meningitis or encephalitis. Factors that increase this risk include: 

  • Advanced age
  • Cancer
  • Recent chemotherapy
  • Diabetes 
  • Alcohol abuse
  • Kidney disease

In these scenarios, it's important to work with your healthcare provider if you notice anything out of the ordinary, even if it seems like a typical cold.

Frequently Asked Questions

  • What are the symptoms of West Nile virus?

    About eight out of 10 people who have West Nile virus don't have any symptoms. When symptoms do appear, they include:

    • Fever
    • Joint pain
    • Body aches
    • Headache
    • Nausea or vomiting
    • Diarrhea
    • Rash

    Signs of a severe illness, such as meningitis or encephalitis, include high fever, neck stiffness, disorientation, convulsions, vision loss, numbness, and paralysis.

  • How deadly is the West Nile virus?

    A few people will have severe symptoms. About one out of 150 people with West Nile virus develop a serious illness affecting the central nervous system. Out of those serious cases, about one out of 10 will die.

  • Can you get West Nile virus a second time?

    Researchers don't think it's likely. That's because once you're infected with West Nile, you develop immunity that protects you from another infection. However, that immunity may decrease over time, or you may be susceptible if you have a health condition that affects your immune system.

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  9. South Dakota Department of Health. West Nile virus.

Additional Reading
  • Busch Mp, Caglioti S, Robertson Ef, Et Al. Screening The Blood Supply For West Nile Virus Rna By Nucleic Acid Amplification Testing. N Engl J Med; 353:460.

  • Johnson Gd, Eidson M, Schmit K, Et Al. Geographic Prediction Of Human Onset Of West Nile Virus Using Dead Crow Clusters: An Evaluation Of Year 2002 Data In New York State. Am J Epidemiol; 163:171.

  • O'leary Dr, Kuhn S, Kniss Kl, Et Al. Birth Outcomes Following West Nile Virus Infection Of Pregnant Women In The United States: 2003-2004. Pediatrics; 117:E537.

  • Petersen Lr, Brault Ac, Nasci Rs. West Nile Virus: Review Of The Literature. JAMA 2013; 310:308.
  • Rizzo C, Napoli C, Venturi G, Et Al. West Nile Virus Transmission: Results From The Integrated Surveillance System In Italy, 2008 To 2015. Euro Surveill 2016; 21.

By Richard N. Fogoros, MD
Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology.