West Nile in Europe: an increasing public health problem

West Nile virus (WNV) is a mosquito-borne flavivirus that is related to Japanese encephalitis, St. Louis encephalitis and Murray Valley encephalitis viruses. As the names of the viruses imply, WNV can cause encephalitis, however, as with most infectious diseases, clinical disease is relatively rare. Approximately 75% of infections are asymptomatic, 25% will be an influenza-like fever (known as West Nile fever [WNF]), ~1% will be neurologic disease (known as West Nile neurologic disease [WNND]) and ~0.1% will proceed to a fatal neurologic infection. Like many flaviviruses, WNV has a transmission cycle involving mosquitoes and vertebrate hosts. In the case of WNV, it is Culex mosquitoes and birds are the amplifying host (note that the mosquito vector is different to that of chikungunya, dengue, yellow fever and Zika viruses that are transmitted by Aedes mosquitoes). Although Culex mosquitoes can transmit the virus to a variety of vertebrate hosts, these are dead-end hosts due to low viremias and cannot transmit the virus to other vertebrates nor can the virus be transmitted to uninfected mosquitoes. Nonetheless, vertebrate hosts, including human and horses, can get clinical disease. WNV originates in Africa and has spread from there to Asia, Europe and the Americas.1 There is relatively little disease in Asia but the virus has emerged in both Europe and Americas to cause periodic large outbreaks. It is difficult to predict where West Nile activity will take place as mosquito activity varies according to climatic conditions and, since the vertebrate hosts are birds, migratory birds have the potential to carry the virus long distances. With increasing global travel,2 the spread of West Nile infections in travelers will also increase. Regarding Europe, West Nile emerged in Romania in 1996 with 393 WNND cases and this was followed by another major outbreak in southern Russia in 1999. Since 1999, there has been WNV activity (i.e. reports of virus being detected in either mosquitoes, birds or mammals, including humans) recorded in Europe most years and every year since 2010 with the European West Nile season normally running from June to November. Figure 1 shows an overview of number of cases by year and number of European countries reporting cases. Human WNF and WNND cases In Europe since 2010. Data are taken from the European Centre for Disease Prevention and Control.3 As with most mosquito-borne virus infections, outbreaks tend to take place each year but differ greatly in magnitude and geographic location. This is exemplified by the USA where WNV activity is reported each year but large outbreaks are seen only every 8–10 years (https://www.cdc.gov/westnile/statsmaps/index.html). As shown in Figure 1, human WN cases in Europe were high in 2012–2013 (935 and 785 cases, respectively) and the highest in 2018 (1670 cases to date), interspersed by years of lower activity. Although WN cases are a record high in 2018, it is difficult to predict whether WN cases will increase even further in subsequent years in Europe as outbreaks are sporadic and cannot be predicted temporarily or geographically due to movement of wild birds. In addition, countries vary in reporting. There are now 50 countries in Europe, and West Nile activity has been reported from approximately 20 countries in southern and southeastern Europe with numbers of cases varying by country and year.3 Nonetheless, Figure 1 shows that the number of countries reporting WNV activity is fairly consistent at 8–16 countries each year, although the actual country, number of cases in that country and area in the particular country with WNV activity will vary by year. In addition, as in the USA, European WNND cases are mostly reported in older people and those who are immunocompromised with the median age of cases in the 60s. Gossner et al. have recently provided a review of WNV surveillance in Europe identifying the variable surveillance and the need to have improved and standardized surveillance.4 Notably, human WN cases are still reported as a combination of WNF and WNND cases making it difficult to evaluate the true incidence of disease in a particular geographic area. In 2018, there has been a large outbreak of WN activity in southern and southeastern Europe and it has been suggested that this is due to the early Spring and very hot summer in Europe, i.e. climatic conditions.5 Indeed, WNF cases were reported in Greece as early as May 2018 suggesting that this year would see record levels of WNV activity, and this has been the case.6,7 As of 27 September, there have been 1670 cases, including 124 deaths, reported from 12 countries representing a >5-fold increase in number of reported cases in 2017.3 As the ratio of neurologic to fatal infection is 10 to 1, therefore, 124 deaths would be expected to equate to around 1240 neurologic cases. This would suggest that the total of 1670 human WNF and WNND cases is an accurate reflection of West Nile clinical disease during 2018. Overall, based on data since 2010, it would appear that WNV activity in Europe has a similar profile to that in North America where there is activity reported every year and periodic major outbreaks, such as that in 2018 (see Figure 1). The caveat to this conclusion is that there is increasing awareness of WNV in Europe in recent years that has improved reporting and it remains to be seen what the annual ‘low’ level of WNV activity will be in southern and southeast European countries. Overall, it is clear the WNV has become a major public health problem in southern and southeastern Europe. WNV has joined other emerging arboviral diseases in Europe that were introduced via travelers or relevant vector mosquitoes.8–10 There are no human vaccines but there are veterinary vaccines used for horses and animals in zoological gardens in both North America and Europe. Development of a human live attenuated West Nile, DNA and other vaccine platforms for West Nile infections are underway. The European Centre for Disease Prevention and Control (ECDC) has been excellent in providing information on outbreaks and risks of West Nile activity with weekly updates during the West Nile season and is an outstanding resource for the current status of West Nile activity in Europe.3 Conflict of interest: None declared.