West Nile Virus (WNV)

West Nile virus belongs to the flaviviruses. The name for this group was influenced by the yellow fever virus, another member of the flavivirus group, as Latin “flavus” means “yellow”. Dengue fever, Japanese encephalitis, Tick-borne encephalitis (TBE), and the Zika virus, for example, also belong to this genus of pathogens.

The virus was discovered in 1937 in the West Nile district of Uganda. In the following decades it appeared in Israel, Egypt and finally also in France. Since then it has spread to all five continents and is now one of the most widespread flaviviruses worldwide. Today, it is the most commonly transmitted virus to humans by mosquitoes in the U.S.

The West Nile Virus transmission cycle

The virus circulates between birds and mosquitoes. When an infected bird is bitten by a mosquito, the mosquito can transmit the virus to another bird, spreading the disease further. While WNV has been detected from 65 species of mosquitoes in 10 genera, Culex mosquitoes, Cx. pipiens (L.), Cx. quinquefasciatus, and Cx. tarsalis (Coquillett), are the primary vector species and most important in maintaining the cycle. WNV has also been isolated multiple times in field-collected Ae. aegypti and Ae. albopictus, but they appear to be of only minor importance with respect to human infections.

Sometimes a virus-carrying mosquito (vector) bites another animal instead of a bird for its next meal, e.g. a mammal such as horses or humans. These are not the actual target of the virus, but they can also get infected with WNV (they are therefore called incidental hosts). Humans and horses are also considered “dead end” hosts because they don’t develop a high enough viremia to infect biting mosquitoes (see Figure 1). In rare cases, transmission from human to human is possible by blood transfusion, organ transplantation, breastfeeding or by transmission from an infected pregnant woman to her unborn child.

The Disease

Fortunately, a WNV infection is not a problem for most people, as 8 out of 10 infected don’t develop symptoms. According to Centers for Diseases Control and Prevention (CDC), about 1 in 5 infected develop the following symptoms: fever, headache, body aches, joint pains, rash, vomiting, diarrhea. However, WNV can also lead to serious, neurologic illness in about 1 per 150 infected, and some of these cases are fatal. The main victims are people with a weakened immune system, such as elderly persons or people with pre-existing illness. The situation is aggravated by the fact that there is no specific antiviral treatment or vaccine available. A vaccine against the virus is currently only available for horses.

Therefore, the best way to avoid the disease is to protect yourself from mosquito bites: Eliminate all standing water on your property, change bird bath water weekly, cover rain barrels, maintain pools, use mosquito repellents with an active ingredient of proven efficacy (DEET, PMD, IR3535 or Icaridin), use mosquito nets, use powerful mosquito traps.

The WNV situation in Europe

Since WNV was first detected in France in 1960, there have been numerous human infections, including fatal cases in Europe. Especially in Southern and Eastern Europe, the virus occurs more frequently in humans. According to ECDC, the European Centre for Disease Prevention and Control, more cases of WNV were registered in Europe in 2018 alone (2083 cases, 181 deaths) than in the previous seven years (1832 cases). Compared to 2017, the average number of cases in Europe was 7.2- times higher. The highest increase compared to the previous transmission season was observed in Bulgaria (15-fold) followed by France (13.5-fold) and Italy (10.9).

Current figures on reported cases of WNV in humans and horses in Europe can be found on the website of the ECDC (European Centre for Disease Prevention and Control).

Climate change seems to play a crucial role in the spread of the virus. The mild winters and hotter summers favor the spread of mosquitoes to many regions where mosquito-associated diseases were previously absent or scarce.

Distribution map of human WNV cases in EU member states and neighboring countries in 2018.

The map is provided by ECDC (European Centre for Disease Prevention and Control).

Distribution map of equine WNV cases in EU member states and neighboring countries in 2018.

The map is provided by ECDC (European Centre for Disease Prevention and Control).

The WNV situation in North America

The West Nile virus was first detected in the USA in New York in 1999, when inhabitants of the city noticed many dead birds in Central Park. The virus subsequently spread throughout the U.S. and into Canada, Mexico, and several Central and South American countries. Cases have been reported from every state except Alaska. It took only four years for the virus to spread across the entire country causing major epidemics along the way. In 2002/2003 a total of over 14,000 cases and 548 deaths were reported and in 2012, nearly 5700 WNV cases and 286 deaths were registered. On the webpages of the CDC, you find WNV statistics and maps for cases in the US.

In Canada, the virus was first registered in Ontario in 2002. Almost 1500 cases were reported in 2003 and over 2200 cases in 2007. However, due to the climate, the distribution area has so far been essentially limited to a few southern states of the country.

What is done by the authorities?

Many research institutes and health authorities around the world run complex monitoring programs to assess the spread of the virus and the potential risks for humans. For example, the Bernhard Nocht Institute for Tropical Diseases and the Friedrich Löffler Institute for Animal Diseases, together with Biogents AG and several other institutes, carried out a nationwide mosquito monitoring programme between 2015 and 2017 in which the collected mosquitoes were also tested for various viruses. For such monitoring programs, mosquito traps such as the BG-Sentinel are often used because they have proven to be particularly effective. The results are then given to higher authorities such as the ECDC. Together with incidence data of infected birds, horses and humans, we can get a good picture of the spread of the virus. To some extent, it is even possible to make predictions about possible future developments.