The 2020 arthropod vector season in Europe is approaching its end. Data from indicator-based
surveillance on vector-borne diseases in 2020 will be reported to the European Centre
for Disease Prevention and Control (ECDC) only in 2021, but we are already able to
draw a preliminary picture from data obtained via event-based surveillance and weekly
reporting of West Nile virus (WNV) infections.
In 2020, WNV caused remarkable outbreaks in certain areas in Europe, such as Spain
and the Netherlands. However, the largest outbreak of human WNV infections in European
Union/European Economic Area (EU/EEA) countries was recorded in 2018, when 11 countries
reported 1,548 locally acquired mosquito-borne infections [1]. The number of WNV infections
in 2018 exceeded the cumulative number of all reported infections between 2010 and
2017, and the highest number of newly affected areas (n = 45) was reported [2]. Even
though in 2019, the number of reported locally acquired human WNV infections dropped
by 73% compared with 2018, the total numbers were still the second highest ever recorded.
Most countries reported numbers of infections similar to before 2018, while Greece
continued to report a high number of infections [1].
Since the start of the 2020 transmission season on 1 June, and as at 12 November,
EU/EEA countries have reported 315 human cases of WNV infection with known place of
infection through the European Surveillance System (TESSy): Greece (n = 143), Spain
(n = 77), Italy (n = 66), Germany (n = 13), Romania (n = 6), the Netherlands (n = 6),
Hungary (n = 3) and Bulgaria (n = 1) [3] (Figure). It is noteworthy that some countries
reporting very low numbers of infections in 2020, ranging from none to six, had previously
detected higher numbers of human WNV infections (e.g. Austria, Hungary, Serbia, Romania).
Additionally, in other endemic countries reporting a large number of cases (i.e. Greece
and Italy), the proportion of the more severe manifestation of the infection, West
Nile neuroinvasive disease (WNND), was higher than the average of the previous 5 years.
Figure
Distribution of locally acquired human West Nile virus infections by affected areas
and transmission seasons, EU/EEA countries and EU neighbouring countries, 2011–2020a
(n = 3,876)
ECDC: European Centre for Disease Prevention and Control; EEA: European Economic Area;
EU: European Union.
a Data last updated on 16 November 2020.
Source: ECDC.
Within the last decade, the geographical spread of a genetic WNV lineage 2 strain
has been observed in Central Europe and in the Mediterranean region [4]. In Germany,
this EU-dominant strain was first detected in 2018 in resident birds and horses. The
first five locally acquired vector-borne human cases were reported in 2019 in the
country. In the current issue of Eurosurveillance, Pietsch et al. report an outbreak
of nine locally acquired cases of West Nile fever (WNF) and WNND in Leipzig, Germany,
in August and September 2020 [5]. The authors hypothesise endemic seasonal circulation
of WNV lineage 2 in the city of Leipzig in 2020 and also in the coming years; therefore,
they suggest population’s and healthcare workers’ WNV awareness should be further
increased as well as the surveillance in animals.
WNV lineage 2 was also reported in bird and mosquito samples in the Netherlands, for
the first time, at the end of August 2020 [6]. Hereafter, the first locally acquired
human WNV infections were diagnosed, in the region of Utrecht, in September and October
[3,7].
Since 2017, WNV lineage 2 has been spreading further west in the Mediterranean region,
via the south of France, reaching Catalonia in north-eastern Spain, where it only
caused sporadic cases in birds [8]. However, an unprecedented outbreak of WNV infections
occurred in the southern Spanish provinces of Seville, Cádiz and Badajoz, between
July and September 2020, comprising 77 infections diagnosed in humans and 137 documented
outbreaks among equids [3]. A lineage 1 WNV strain was detected in both humans and
animals, hence this outbreak has no epidemiological link to other concurrent WNV outbreaks
in Europe [8]. Of note, Spain did not report locally acquired WNV infections in humans
from 2017 to 2019.
Albeit numbers of human WNV infections in 2020 were lower than in previous years,
the geographic expansion of WNV has continued in Europe. The environmental and ecological
drivers of WNV are complex and not known in detail, yet. Nevertheless, ambient temperature
is known as one important determinant through its effect on mosquito reproduction
rates and the extrinsic virus period in mosquitoes [9]. According to the monthly climate
bulletins of the Copernicus Climate Change Service [10], positive surface air anomalies
were recorded in the southern regions of the Iberian Peninsula from July to August,
as well as in north-west Europe from August to October. The temporospatial overlap
with the WNV outbreaks might be a mere coincidence, but in the long term, the environmental
conditions tend to become more favourable for WNV establishment and seasonal circulation
in many European regions [9]. The European spread of another flavivirus, closely related
to WNV, the Usutu virus (USUV), may be regarded as an example [11,12]. The first cases
of USUV-associated wild bird mortality events were described in Italy in 1996 and
in Austria in 2001. Within two decades, USUV has spread all over Europe, except for
the Baltic countries and Scandinavia, and in 2020, USUV emerged in the United Kingdom
[13]. Although the ecology and epizootiology of USUV and WNV differ in several points
(e.g. higher genetic diversity of USUV in Europe indicates various, recent introductions
from Africa), the two viruses share mosquito vectors and avian hosts; therefore, the
environmental and ecological conditions suitable for USUV may indicate the same for
WNV (e.g. vector competences [14]).
Most of the diagnosed human WNV infections in 2020 were reported from areas with virus
activities already recorded in the previous years. This, together with genetic data,
indicates overwintering and local circulation of the virus. Therefore, once established,
the likelihood of maintenance and the risk of re-emergence of WNV infections in the
affected European areas are high. The geographical spread after recent WNV lineage
2 introduction may be limited e.g. as it was seen in the eastern federal states of
Germany, since 2018, but a massive geographical spread has been observed 4 to 5 years
after its first emergence in Europe [4].
The annual fluctuations in WNV activity can be influenced by several factors. In the
current issue of Eurosurveillance, Lourenço et al. have analysed West Nile virus epidemiology
in Israel [15]. WNV infections caused by diverse strains have been diagnosed in the
country in the past decade. In 2020, Israel reported 17 human cases as at 12 November.
Authors adapted a suitability index to WNV and found that it confirmed the geotemporal
estimation of transmission potential of WNV in the country. Several further studies
have been and are investigating the ecological and environmental drivers of WNV [16,17].
Factors associated to national health systems, such as diagnostic awareness and vigilance,
diagnostic capabilities and capacities, surveillance and reporting accuracies could
also contribute to the annual variations of diagnosed and reported cases in a country.
In 2020, the extraordinary and unprecedented burden on national health diagnostic
systems caused by the coronavirus disease (COVID-19) pandemic might have had an influence
on WNV surveillance in some countries.
The maintenance of national preparedness for seasonal WNV outbreaks in the forthcoming
years is particularly important in countries where – even if sporadic – cases have
been reported previously, as well as in countries where the ecological conditions
are suitable for WNV emergence and establishment. The risk and public health impact
of human WNV infections in the different European countries clearly and significantly
varies. In certain countries, surveillance of human WNV infections might be challenging
or of low priority. The natural cycle of WNV involves avian hosts and mosquito vectors,
while – besides humans – equids are also frequent, incidental hosts of the virus.
Therefore, integrated animal-human WNV surveillance with systematic response and control
measures e.g. for blood donation safety, vector control, awareness and information
campaigns, might allow a more efficient utilisation of national resources and capacities.
Animal WNV infections should be diagnosed timely and EU/EEA countries are encouraged
to report infections via the Animal Disease Notification System of the European Commission.
These data are visualised together with the TESSy data on human cases in ECDC’s weekly
updated maps on WNV infections in Europe [1]. The joint animal-human surveillance
data can provide more accurate information on WNV activity within the transmission
season than solely data from human infections. Joint efforts on WNV surveillance and
control could be one of the good examples for a One Health approach towards zoonotic
diseases.