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West Nile Virus: Another Pathogen Going Global

Mosquito Culex sp larvae
Photo: James Gathany / CDC - Larvae of 'Culex' mosquitoes make dense groups in standing water

[This article was written by Natalia Rodríguez, an ISGlobal researcher and doctor who works at the Hospital Clínic’s International Health Department, and ISGlobal’s Communications Coordinator Pau Rubio.]


At the time of writing, 12 cases of viral meningoencephalitis caused by the West Nile virus have been confirmed in the Spanish province of Seville, and 7 other suspected cases are under study. West Nile virus is a flavivirus and a close relative of other viruses transmitted by arthropod vectors, such as dengue, yellow fever and Japanese encephalitis virus.

Humans and other mammals, such as horses, can also be infected incidentally as the result of bites from an infected mosquito

It takes its name from the West Nile district of Uganda where it was first isolated in a woman in 1937. The virus usually infects birds and is transmitted from one host to the next through the bites of infected mosquitoes. Humans and other mammals, such as horses, can also be infected incidentally as the result of bites from an infected mosquito. However, human-to-human transmission does not occur except in rare cases of mother-to-child transmission and infrequently through organ transplant or blood transfusion. Both humans and horses are considered dead-end hosts because, while they can be infected, they do not spread the infection.

Severe Disease Develops in 1 in Every 150 Cases

Around 80% of West Nile virus infections in humans are completely asymptomatic. In most of the patients who develop symptoms, the clinical presentation West Nile fever, a disease typically associated with fever, headache, body aches, fatigue, nausea and skin rashes. However, according to the World Health Organisation, 1 in every 150 people infected will develop acute meningoencephalitis or flaccid paralysis, characterised by multiple symptoms, including headache, high fever, muscle weakness, neck stiffness, disorientation, tremors, convulsions and even nerve or muscle paralysis. In about 0.1% of cases, the disease results in coma or death.

While severe disease can develop in patients of any age, older and immunocompromised individuals are at greater risk.

The risk of spread is increased when migratory birds carrying the virus are found in conjunction with mosquitoes capable of transmitting the pathogen

Watch Out for Mosquitoes!

Over the past 20 years, West Nile virus has been detected with increasing frequency throughout Europe and North America. In Spain, it was isolated in 2007 in a golden eagle. Since then, several outbreaks have been detected in horses and there have also been at least 7 sporadic cases in humans. In 2018, over 2000 West Nile virus infections in humans were reported in the European Union, a number far exceeding the total number of cases reported over the previous seven years.

Since the first recorded outbreak in the United States in 1999, the virus has spread rapidly throughout the country. Between 2002 and 2003, the US reported more than 14,000 cases and over 500 deaths.

The risk of spread is increased when migratory birds carrying the virus are found in conjunction with mosquitoes capable of transmitting the pathogen. Scientists have identified up to 40 species of mosquitoes that can transmit the virus. This list of vectors includes common mosquito species widely present in the Mediterranean area, such as Culex pipiens and Culex perexiguus.

Infograhic about Culex species, that is one of the three major types of mosquito inhabiting the planet

No Treatment or Vaccine

There is currently no treatment for West Nile infection and no vaccine against the disease in humans. A vaccine has been developed for horses.

The lack of a pharmacological solution makes preventive measures and epidemiological and vector surveillance programmes essential. For the individual, the key preventive measure is to avoid mosquito bites. Recommended preventive measures include the use of mosquito nets, insect repellents and long-sleeved clothing and avoiding outdoor activities in the evening and at night. It is also important to eliminate all stagnant water close to the home by emptying flower pots and any other containers where rainwater can accumulate.

The lack of a pharmacological solution makes preventive measures and epidemiological and vector surveillance programmes essential

Climate Change

The West Nile virus is yet another case of a disease originally endemic to warmer regions that is now finding ideal conditions in other latitudes due to climate change. It is also an example that serves to illustrate the need for and desirability of adopting a One Health multisectoral approach to health. Zoonotic diseases like West Nile virus clearly show the impossibility of dealing with this type of pathogen without taking into account a whole range of factors, including mosquito populations, birds and their migratory patterns and, in this case, even horses. They also underscore the need to take into account the environment where they occur and the effects of global warming.