Gert van der Hoek
In Memoriam - Editor, Senior Moderator
From ProMED
ENCEPHALITIS - GREECE (11): (CENTRAL MACEDONIA), WEST NILE VIRUS, COMMENT
***********************************************
Date: Thu 26 Jul 2010
From: Miguel Angel Jimenez-Clavero
In a recent commentary in ProMED on the current situation regarding
West Nile virus [WNV] in Greece (ENCEPHALITIS - GREECE (10): (CENTRAL
MACEDONIA) WEST NILE VIRUS, COMMENT [ProMED-mail archive no.
20100824.2972], where some aspects of wild bird mortality in European
WNV outbreaks are discussed, the moderator wonders whether there are
differences in virulence between European and American strains of
WNV, and about the susceptibility of local birds to West Nile virus
disease. We have been investigating these issues recently, and the
results obtained could be of interest for this discussion.
- snip -
These studies are currently being extended to other European wild
bird species, and also to other WNV strains, but to date, what we can
conclude with the above results is 1) at least some European wild
bird species are susceptible to WNV disease in experimental
conditions; 2) despite the close similarity of the Western
Mediterranean WNV strains used, significant differences in virulence
are found between them; and 3) some Western Mediterranean strains
appear to be as virulent as the highly virulent American strain,
while others are not.
In the context of the striking differences observed between Europe
and North America with regard to bird mortality due to WNV, many
hypotheses have been raised.
In the ProMED comment above cited, host
resistance in areas of long term circulation of the virus is
presented as a likely explanation for this observation. But it does
not fit with the above results: in an area such as the Western
Mediterranean, where WNV has been found since the 1960s and has been
circulating continuously for at least 14 years (1), at least one wild
bird species, abundant and autochthonous to this area, is not
resistant.
Moreover, wild bird mortality has been found in a number
of outbreaks in Europe, not only in Hungary and Austria (cited in the
comment), but also in Spain (3), France (4) and Israel (5). So wild
bird mortality in Europe and the Mediterranean should not be
considered an "exception" but rather, its apparent absence seems to
be a consequence of the difficulties to detect birds dying from WNV.
Although host resistance could possibly play a role in some
susceptible species, it should not be considered as the sole
explanation to the observed differences of WNV virulence between the
Old and New World.
In a disease with such a complex, essentially
multifactorial, eco-epidemiology, it is more wise to think of a
number of different factors as contributing to the appearance of
clinical cases in a given area.
Not only host resistance, but
virulence of the strains circulating in each area, co-circulation of
variants with different virulence (as it appears to be the case in
Europe), and local host heterogeneity or vector competence can also
contribute in different instances to different extents to the outcome
of disease outbreaks.
The study of virus-host interactions using
local viral variants as well as autochthonous wild bird species and
vectors would contribute much to the understanding of this eco-epidemiology.
--
Miguel Angel Jimenez-Clavero
Valdeomos (Madrid) Spain
http://www.promedmail.org/pls/apex/..._BACK_PAGE,F2400_P1001_PUB_MAIL_ID:1000,84443
ENCEPHALITIS - GREECE (11): (CENTRAL MACEDONIA), WEST NILE VIRUS, COMMENT
***********************************************
Date: Thu 26 Jul 2010
From: Miguel Angel Jimenez-Clavero
In a recent commentary in ProMED on the current situation regarding
West Nile virus [WNV] in Greece (ENCEPHALITIS - GREECE (10): (CENTRAL
MACEDONIA) WEST NILE VIRUS, COMMENT [ProMED-mail archive no.
20100824.2972], where some aspects of wild bird mortality in European
WNV outbreaks are discussed, the moderator wonders whether there are
differences in virulence between European and American strains of
WNV, and about the susceptibility of local birds to West Nile virus
disease. We have been investigating these issues recently, and the
results obtained could be of interest for this discussion.
- snip -
These studies are currently being extended to other European wild
bird species, and also to other WNV strains, but to date, what we can
conclude with the above results is 1) at least some European wild
bird species are susceptible to WNV disease in experimental
conditions; 2) despite the close similarity of the Western
Mediterranean WNV strains used, significant differences in virulence
are found between them; and 3) some Western Mediterranean strains
appear to be as virulent as the highly virulent American strain,
while others are not.
In the context of the striking differences observed between Europe
and North America with regard to bird mortality due to WNV, many
hypotheses have been raised.
In the ProMED comment above cited, host
resistance in areas of long term circulation of the virus is
presented as a likely explanation for this observation. But it does
not fit with the above results: in an area such as the Western
Mediterranean, where WNV has been found since the 1960s and has been
circulating continuously for at least 14 years (1), at least one wild
bird species, abundant and autochthonous to this area, is not
resistant.
Moreover, wild bird mortality has been found in a number
of outbreaks in Europe, not only in Hungary and Austria (cited in the
comment), but also in Spain (3), France (4) and Israel (5). So wild
bird mortality in Europe and the Mediterranean should not be
considered an "exception" but rather, its apparent absence seems to
be a consequence of the difficulties to detect birds dying from WNV.
Although host resistance could possibly play a role in some
susceptible species, it should not be considered as the sole
explanation to the observed differences of WNV virulence between the
Old and New World.
In a disease with such a complex, essentially
multifactorial, eco-epidemiology, it is more wise to think of a
number of different factors as contributing to the appearance of
clinical cases in a given area.
Not only host resistance, but
virulence of the strains circulating in each area, co-circulation of
variants with different virulence (as it appears to be the case in
Europe), and local host heterogeneity or vector competence can also
contribute in different instances to different extents to the outcome
of disease outbreaks.
The study of virus-host interactions using
local viral variants as well as autochthonous wild bird species and
vectors would contribute much to the understanding of this eco-epidemiology.
--
Miguel Angel Jimenez-Clavero
Valdeomos (Madrid) Spain
http://www.promedmail.org/pls/apex/..._BACK_PAGE,F2400_P1001_PUB_MAIL_ID:1000,84443