West Nile virus has become endemic in many parts of Europe due to environmental drivers such as temperature, precipitation, landscape features, avian hosts, and human factors. Above average temperatures increase mosquito and virus reproduction rates, accelerating transmission. Both rural wetlands and urban areas support transmission cycles. Bird migration and changing migration patterns influence virus spread into new areas. A One Health approach integrating public health, veterinary and environmental interventions can help reduce human exposure and transmission risk.
Environmental Drivers of West Nile Virus Endemaization in Europe
1. Environmental Drivers of
West Nile Virus Endemaization
in Europe
Shlomit Paz1 and Jan C. Semenza2
1University
2ECDC,
of Haifa, Israel
Stockholm, Sweden
2nd GRF One Health Summit 2013
November 2013, Davos, Switzerland
2. Background
West Nile virus (WNV), member of the
Flavivirus genus, is responsible for West Nile fever (WNF),
an emerging arbovirosis with a complex epidemiological
cycle.
The transmission cycle involve wild birds as the principal
hosts and mosquitoes (especially Culex), largely birdfeeding species, as the primary vectors.
Humans and horses do not produce significant viraemia and
3. WNV exists in rural ecosystems as well as in urban
areas where mosquitoes breed in organic-rich water in
artificial containers.
WNV is widely distributed throughout the tropical and
temperate regions of the world where it infects
birds, humans, horses and other mammals.
In fact, today it is a vector-borne pathogen of global
importance.
4. Humans are mainly infected through mosquito bites, but
infection can occur through organ transplantation and
blood.
An incubation period of 3-14 days precedes symptoms.
While most human infections are asymptomatic, the
majority of clinical cases are mild and present with flu-like
symptoms.
Severe cases with signs of encephalitis, meningoencephalitis or meningitis, are most often observed
among elderly.
No specific therapy or vaccine are available.
The main preventive measures are aimed at reducing
exposure to mosquito bites.
5. WNF in Europe
Over the last years, sporadic human cases or limited
outbreaks of WNF have been reported in Europe.
In summer 2010, the number of WNF cases in new areas in
Europe and its neighboring countries (particularly in South
Eastern Europe) was the highest ever reported.
Later, WNF outbreaks in Europe and Eurasia during the
summers of 2011, 2012 and 2013 occurred in most of the
disease locations in 2010.
6. How WNV has become an
endemic pathogen in many
parts
of Europe and Eurasia?
7. The epidemiology of WNF is multifactorial and
complex.
Abiotic and biotic conditions are both important
determinants of WNF epidemiology.
The transmission of WNV is determined by many
factors.
8. Temperature
Ambient temperature plays an important role in viral
replication rates and transmission of WNV, affecting the
length of extrinsic incubation, seasonal phenology of
mosquito host populations and geographical variation in
human case incidence.
Elevated ambient temperatures increase growth rates of
vector populations, decrease the interval between blood
meals, and accelerate the rate of virus evolution.
9. A recent example was an outbreak in the summer
of 2010, when southeastern Europe and Eurasia
experienced an unprecedented upsurge in the
number of human WNF cases, which was preceded
by hot spells (Paz et al., 2013).
12. Temperature anomalies during summer 2010 in selected
stations
Constanta, Romania
Volgograd, Russia
Min
C
Min
C
-
mean
-
mean
-
max
-
max
-
-
-
week #
week #
Thessaloniki, Greece
Ben Gurion Airport, Israel
Min
C
Min
C
-
mean
-
mean
-
max
-
max
-
-
-
week #
week #
13. Results of Pearson and lag correlations between the weekly
average temperature and the dates of WNF onset, at
selected stations
(bold values = significant results)
Volgograd, Russia
Constanta, Romania
Temperatur
e
min
mean
max
min
mean
max
Lag 0
0.16
0.25
0.26
0.17
0.17
0.12
lag 1 week
0.20
0.28
0.29
0.38
0.39
0.33
lag 2 weeks
0.38
0.44
0.44
0.59
0.61
0.56
lag 3 weeks
0.63
0.63
0.61
0.52
0.55
0.51
lag 4 weeks
0.61
0.65
0.64
0.37
0.40
0.38
Thessaloniki, Greece
Ben Gurion Airport, Israel
Temperatur
e
min
mean
max
min
mean
max
Lag 0
0.49
0.37
0.31
0.35
0.36
0.22
lag 1 week
0.45
0.36
0.32
0.33
0.27
0.16
lag 2 weeks
0.37
0.30
0.24
0.34
0.29
0.18
lag 3 weeks
0.29
0.24
0.19
0.37
0.26
0.08
lag 4 weeks
0.28
0.19
0.13
0.18
0.18
0.08
14. Results of the Multinomial Logistic Regression
between summer temperatures and
WNF occurrences
No-lag
Lag1
Lag2
Lag3
Lag4
p-val
estimate p-val
estimate p-val
estimate p-val
estimate p-val
estimate
Rostov, Russia
min
0.0562
0.1737
0.0134
1.32 0.0006
2.27
0.0011
2.05
mean
0.042
0.935 0.0053
1.41 0.0002
2.3
max
0.087
0.2636
0.009
1.29 0.0004
2.12
0.0003
2.25
Constanta, Romania
min
0.3067
0.1091
0.0013
2.13 0.0021
1.93
0.0362
1.036
mean
0.007 12.0496 0.0007
2.203 0.0017
1.85
0.03
1.03
max
0.0019
2.022 0.0043
1.758
0.028
1.19
Thessaloniki, Greece
min
0.0148
1.46 0.0377
1.05 0.0175
1.42 0.0236
1.345
0.0415
1.18
mean
0.0173
1.28
max
Haifa, Israel
min
0.0309
3.5871 0.0366
1.72 0.0108
2.19 0.0385
1.7
mean
max
0.0026
1.68
0.0298
1.11
0.014
1.29
0.0227
1.18
15. Reported cases of West Nile fever for the EU and neighboring countries.
Transmission season 2012 and previous transmission seasons. Source:
ECDC.
“WNV always dispersed into new areas during years with above-normal
temperatures and that amplification during the following year occurred during
summers with above- or normal temperatures ” (Reisen et al. , J. Med.
Entomol. 2006).
17. Precipitation
The scientific literature presents inconsistent results.
on one hand, heavy rainfall during spring may increase
standing water resources at the beginning of the hot
season.
In contrast, during drought conditions, standing water pools
-
June
July
Cordoba
Ben Gurion Airp
August
Venezia -Tessera
May
Mugla
April
Thessaloniki
March
Budapest
Constanta
Anomalies of Monthly
Surface Gauss
Precipitation Rate of
spring-summer
2010, relative to the 30
years period in selected
stations (per each
country)
Volgograd
needs to thrive.
Surface Gauss Precipitation Rate
Anomaly
become richer in the organic material that mosquitoes
September
18. Landscape Features and Land-Use
In Europe and Eurasia, circulation of WNV is mainly
confined to two land-use patterns:
1. rural areas where wetlands serve as bird-nesting
areas that provide ideal conditions for establishing
endemic cycles of WNV
(the Rhone delta, southern France; the
Volga delta, southern Russia; and the Danube delta in Romania).
2. urban areas where mosquitoes feed both on birds
and on humans and thus act as bridge vectors
(Bucharest, Romania; Volgograd, Russia and in Thessaloniki, Greece).
19. Overall, the urban environment in which the WNF
outbreaks
occur
is
a
diverse
mix
of
buildings, transportation routes, vegetation, land
uses and people associated with many aspects of
urban life, from economic activity to patterns of
illness.
20. Avian Hosts
• Bird migration influences new areas of spread.
• As a reaction to increasing spring temperature in recent
years, populations of migratory birds have advanced the
seasonal start of their reproduction.
• Changes
in
tropical
storms
may
influence
WNV
dispersal, by impacting the dynamics of storm-driven
birds.
21. A comparison between bird migration tracks during
spring (based on Leshem, 2010) and WNV infection
main locations
22. Other Drivers
Transportation
In a world that is ever more interconnected through rapid
transportation systems, such as passenger air travel and
global trade, the virus is now being dispersed
internationally.
Socio-demographic level
It can be expected that epidemiology of WNV-induced
disease in people will vary between regions as a result of
differences at the socio-demographic level. Populations
living in poverty might be less likely to have secure, airconditioned homes, and thus are more exposed to biting
mosquitoes.
23. Main conclusions
Abiotic and biotic environmental factors have both
contributed to the endemization of WNF in Europe, such as:
favorable climatic conditions (while temperature is the most
important climatic driver); hospitable habitats for high
densities of competent mosquitoes; infected migratory birds
for the dispersion of WNV; susceptible human and equine
populations.
24. ADDED VALUE TO THE ONE HEALTH APPROACH
Reduced human exposure to mosquitoes infected with
WNV, through vector abatement measures, is currently the
most effective way to prevent transmission.
Ecologically based land-use planning, combined with improved
development and sanitation, could reduce contact with and the
abundance
of
human-commensal
species
and
hence
transmission of their pathogens. A critical component of a
vector control program is concurrent public education and
health promotion to prevent and reduce risk of exposure.
Understanding
the
environmental
drivers
of
the
epidemiology will help guide these public health efforts.
WNF
25. For more details:
Paz, S., Malkinson, D., Green, M.S., Tsioni, G., Papa, A., Danis, K., Si
rbu, A., Ceianu, C., Katalin, K., Ferenczi, E., Zeller, H., Semenza, J.C.
, 2013. Permissive Summer Temperatures of the 2010 European
West Nile Fever Upsurge. PLOS ONE, 8(2): e56398.
Paz, S., Semenza, J.C., 2013. Environmental Drivers of West Nile
Fever Epidemiology in Europe and Eurasia - Review. International
Journal of Environmental Research and Public Health, 10, 3543-3562.