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
- 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.
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.