Epidemiology, Infectious Diseases, Viruses

West Nile Fever: How to Control This Mosquito-Borne Disease?

Author Chandana Balasubramanian , 27-Jun-2023

West Nile Virus is a critical public health concern. This mosquito-borne virus can cause severe, fatal disease. The disease is transmitted to humans and animals through infected mosquitoes. Unfortunately, there are no vaccines for humans against the West Nile Virus (WNV), so controlling their vectors and raising awareness about precautionary measures is the best way of fighting this infectious disease.


Here, we will cover the history of this disease, the epidemiology, how it’s spread, symptoms, diagnosis, treatment, and West Nile virus prevention. We will also cover how the virus infects the human body.



The West Nile Virus was first discovered in the West Nile district of Uganda in 1937; this is where the disease and virus get their names. The disease continued to be found in Africa, Europe, the Middle East, and Russia for decades until the nineties. In the mid-1990s, a new strain of the WNV virus was found in Eastern Europe, Russia, and Israel.  In 1999, the virus traveled to the Americas, causing an outbreak in the State of New York. After interviewing patients and their families, mosquitoes were suspected as the source of infection. At around the same time, a large number of crows began to die in the region, as well as exotic birds in the Bronx Zoo. Brain tissues from these birds were analyzed, and ultimately, the virus was identified as a WNV strain (lineage 1). During this outbreak, researchers also analyzed brain samples from people who died of encephalitis at the time and confirmed their deaths to be caused by WNV. The virus was finally isolated from Culex mosquitoes, confirming them as vectors for West Nile Fever.



The West Nile Virus (WNV) is a mosquito-borne disease that has been reported in various parts of the world, including Africa, Europe, the Middle East, North America, and West Asia. This widespread geographical distribution highlights the importance of understanding WNV transmission patterns and implementing effective prevention strategies.

Presence across continents

First identified in Uganda in 1937, WNV has since spread to multiple continents due to its ability to infect different species of mosquitoes and birds. The primary vectors for transmitting WNV are Culex mosquitoes which can be found globally. As infected birds migrate between regions or continents, they contribute to the spread of this virus among local mosquito populations.

There are two main lineages for the West Nile Virus:

  • Lineage 1: Central and North Africa, Europe, and Australia
  • Lineage 2: Southern Africa and Madagascar


Notable outbreaks


  • In 1999, an outbreak occurred in New York City, marking the first appearance of WNV in North America. Since then, it has become widely established from Canada down through Venezuela. Migratory birds are presumed to be the source of this outbreak.
  • Between 1999-2006: 23,000 cases (1,000 fatal) were reported in the United States.
  • In 2009: The first fatality in South America (Mexico) was reported.



  • From 1995-2015: 629 cases (82 fatal) of West Nile Fever were reported in the Eastern Mediterranean Region (WHO-designated region)
  • 2010: 375 cases (34 fatal) of human WNV cases reported by 17 countries in the Mediterranean region
  • 2011: 282 cases reported
  • 2012: 762 cases reported
  • 2013: 395 cases reported
  • 2014: 74 cases reported
  • 2015: 108 cases reported
  • 2017: 203 cases (26 fatal)
  • From 2018-2023, a large-scale outbreak took place across Europe during the summer months, with several countries reporting increased cases compared to previous years.
  • Outbreaks have also been documented throughout Africa and West Asia over recent decades with varying levels of severity depending on factors such as local vector control measures and population immunity levels.

Source of outbreak data: GIDEON infectious diseases database. To get more data on outbreaks, outbreak maps, country notes, and more, subscribe to the GIDEON app

High-risk populations for severe illness

While anyone can be infected with West Nile virus (WNV), certain individuals are at a higher risk of developing severe illness upon infection. Older folks and those with weakened immune systems, like organ transplant recipients or people living with HIV/AIDS, may be more at risk.

Age-related vulnerability

The risk of developing severe disease from West Nile virus, including West Nile neuroinvasive disease, increases significantly in individuals aged 50 and above. As we age, our immune system becomes less effective in combating infections such as WNV, thus making older adults more susceptible to severe illness or complications. As a result, they may experience more serious symptoms or complications if infected.

Immunocompromised individuals at increased risk

In addition to older adults, people with weakened immune systems due to underlying health conditions or medical treatments also face an elevated risk of severe illness from WNV infection. For example, cancer patients undergoing chemotherapy and individuals taking medications that suppress the immune system have a reduced ability to combat the virus effectively. Consequently, these populations may develop life-threatening complications such as encephalitis or meningitis when exposed to WNV.


How is it spread?

The West Nile virus (WNV) is a mosquito-borne disease that poses significant health risks to humans and animals. The Culex mosquito, in particular, Culex pipiens (aka the common house mosquito), is the primary vector for West Nile Virus. Birds are reservoirs, while humans and horses are “dead-end” hosts, meaning they can get infected but do not continue the virus’s cycle.

Mosquito bites as the primary mode of transmission

The most common way for people to contract West Nile virus is through the bite of an infected mosquito. When a Culex mosquito feeds on an infected bird, it ingests the virus, which then multiplies in its salivary glands. The next time this mosquito bites another animal or human, it injects them with WNV, potentially causing illness.

Contact with infected animals or their tissues

In some uncommon instances, people may become infected with the West Nile virus by direct contact with other contaminated creatures or their body parts. For example, handling dead birds without proper protection could expose someone to WNV if they have cuts or abrasions on their skin.

Organ transplants and blood transfusions

Transmission via organ transplantations and blood transfusions has been documented, although these occurrences are extremely rare due to stringent screening processes for donors. In addition, breast milk from mothers who have contracted WNV can also transmit the infection to infants during breastfeeding; however, such instances are infrequent.

Key Takeaway: West Nile virus is transmitted primarily through mosquito bites but can also be contracted through contact with infected animals or their tissues, organ transplants, and blood transfusions. Healthcare professionals can play a crucial role in preventing the spread of WNV by educating patients on ways to minimize exposure risks, such as using insect repellent, wearing protective clothing, and maintaining screens on windows and doors.


Biology of the disease

The West Nile Virus is an enveloped, single-strand RNA virus. It is a member of the Flaviviridae family of viruses. It is an arbovirus, which means it spreads to people through the bites of arthropods like mosquitoes or ticks. 80% of humans who contract the West Nile Virus remain asymptomatic, while 20% begin to exhibit symptoms. When an infected mosquito bites a human, the virus enters the bloodstream and infects fibroblasts, endothelial cells of blood vessels, and begins to replicate. Eventually, this infection may spread to the central nervous system. The risk of developing a neuroinvasive infection increases with age and compromised immune systems.



According to WHO, the symptoms of West Nile Fever are:

  • Fever
  • Headache
  • Tiredness
  • Body aches
  • Nausea and vomiting
  • Skin rash (sometimes)
  • Lymphadenopathy (swollen lymph glands)


If the infection is severe and infects the central nervous system (neuroinvasive disease), symptoms can include:

  • West Nile encephalitis
  • Meningitis
  • Fever
  • Headache
  • Neck stiffness
  • Coma
  • Tremors
  • Muscle weakness
  • Convulsions
  • Paralysis, and more



Detection of WNV infections is primarily done through laboratory testing of blood or cerebrospinal fluid samples.

Commonly used tests to detect WNV are:

  • IgG antibody test by enzyme-linked immunosorbent assay (ELISA)
  • IgM antibody capture ELISA test
  • Reverse transcription polymerase chain reaction (RT-PCR)
  • Cell culture


These tests look for specific IgM antibodies produced by the immune system in response to WNV infection. Early identification of cases allows healthcare professionals to implement appropriate treatment plans as well as initiate public health interventions aimed at reducing further transmission risks.

Given the global spread of West Nile Virus, healthcare professionals must take into account geographical distribution and outbreaks to better protect those most at risk for severe illness. However, understanding which populations are at a higher risk for severe illness from this virus can help healthcare professionals better prepare and protect those most vulnerable.



For humans infected with West Nile Fever, treatment is supportive. Infected individuals exhibiting symptoms may be hospitalized and provided intravenous fluids, respiratory support, and infection control, depending on their needs.  There is no vaccine available for humans.

There is a WNV vaccine for horses, so preventive vaccination can help prevent infection in horses. If a horse is infected, treatment is supportive.

Implementing effective prevention strategies is crucial in reducing the risk of West Nile virus (WNV) infections. These measures can be categorized into personal protection, water management for mosquito breeding site reduction, and chemical and biological control methods.



Personal Protection Measures

Taking steps to minimize exposure to infected mosquitoes is essential in preventing West Nile virus infection. Some key precautions include:

  • Avoiding outdoor activity during peak biting times such as dawn and dusk.
  • Covering up with long-sleeved garments, pants, socks, and mitts when out in the open.
  • Applying insect repellent containing DEET or other EPA-approved ingredients on exposed skin.
  • Maintaining window screens to prevent mosquitoes from entering homes.


Water Management for Mosquito Breeding Site Reduction

Controlling mosquito populations is essential to preventing the transmission of diseases like West Nile fever. Reducing potential breeding sites involves proper water management practices such as:

  • Eradicating standing water around homes by emptying containers that collect rainwater or irrigation runoff.
  • Cleaning gutters regularly to prevent stagnant water accumulation.
  • Maintaining swimming pools with appropriate chemicals and circulation systems to deter mosquito larvae growth.


Chemical and Biological Control Methods

In addition to personal protection measures and water management techniques, chemical, and biological control methods can be employed to combat West Nile virus infections. These approaches include:

  • Using larvicides in stagnant water sources to target mosquito larvae.
  • Applying adulticides like pyrethroids and organophosphates to kill adult mosquitoes during outbreaks of West Nile virus.
  • Introducing natural predators, such as fish that feed on mosquito larvae, into ponds and other bodies of water where Culex mosquitoes breed.


Incorporating these prevention strategies can significantly reduce the risk of contracting West Nile virus. By staying informed about local West Nile virus activity and implementing appropriate protective measures, individuals can safeguard themselves against this potentially severe disease.

Key Takeaway: To prevent West Nile virus infection, personal protection measures such as wearing long-sleeved clothing and applying insect repellent are crucial. Water management practices like eliminating standing water and maintaining swimming pools can also reduce mosquito breeding sites, while chemical and biological control methods, including larvicides and natural predators, can combat outbreaks of the disease. By implementing these strategies, individuals can significantly lower their risk of contracting West Nile virus.



In conclusion, healthcare professionals play a critical role in West Nile Virus prevention by educating the public on personal protection measures and implementing disease control strategies. It is important to understand the primary mode of transmission through mosquito bites, geographical distribution and outbreaks, high-risk populations for severe illness, impact on horses as dead-end hosts, and prevention strategies against West Nile Virus.


The GIDEON difference

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Learn more about West Nile Fever on the GIDEON platform.


Chandana Balasubramanian

Chandana Balasubramanian is an experienced healthcare executive who writes on the intersection of healthcare and technology. She is the President of Global Insight Advisory Network, and has a Masters degree in Biomedical Engineering from the University of Wisconsin-Madison, USA.

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