ProMED-mail |

Published Date: 2003-08-03 23:50:00
Subject: PRO/AH> West Nile virus, birds: antibody - UK (02)
Archive Number: 20030803.1903

WEST NILE VIRUS, BIRDS: ANTIBODY - UK (02)
******************************************
A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>
Date: Wed 30 Jul 2003
From: Graeme Kirk <Graeme_Kirk@ipcmedia.com>
Source: Cage and Aviary Birds, Sat 2 Aug 2003 [edited]

Research Suggests West Nile Virus is Endemic in the UK
---------------------------------------------------
[The following are edited extracts from an article written by Graeme Kirk
for the 2 Aug 2003 edition of the weekly "Cage and Aviary Birds" - Mod.CP]
The UK bird population would appear to be safe from attack by West Nile
virus (WNV), according to new research, because the vast majority of birds
have already been exposed. A new study [see: West Nile virus, birds:
antibody - UK 20030719.1772], carried out by researchers from the Centre
for Ecology and Hydrology (CEH), examined more than 350 wild-caught and
farm birds of 30 different species, from blackbirds and carrion crows to
song thrushes and robins. Most of the birds had blood samples taken before
being released, while brain tissue samples were also taken from crows and
magpies caught and destroyed as part of pest control programmes.
The samples were tested for antibodies against up to 3 WNV types: NY,
isolated in New York State, USA, in 1999; DAK, isolated in Central African
Republic in 1967; and Is, isolated in Israel in the early 1980s. The
results were startling, showing that 84 (59.6 percent) of the 141 samples
tested for WNV-Is, 233 (66 percent) of the 353 samples tested for WNV-NY
and a staggering 157 (91.3 percent) of the 172 samples tested for WNV-DAK
had antibodies against the virus and had, therefore, been exposed to WNV at
some time. Tests were also carried out to try and find active WNV in the
samples taken -- without success -- although researchers were confident
that it would be found if enough birds were tested.
While it will take some time for the full repercussions of the work to be
explored, one initial conclusion is that the UK bird population is not at
risk from WNV by virtue of its herd immunity, built up over many years.
While no work has been done to verify this, it is also likely that all
captive bird populations with access to open-air flights will have been
challenged by WNV at some stage by virtue of the virus's transmission
methods. Whether or not imported birds from the Americas could prove
vulnerable if exposed to WNV from the UK population is yet to be seen, but
the current high numbers of bird deaths in the USA and Canada show that
there is no in-built immunity to WNV on that continent, leaving American
stock open to challenge from the virus.
The apparent herd immunity against WNV found in UK birds is hardly
surprising, considering the annual migration patterns of the many species
that overwinter in Africa. This is backed up by the fact that antibodies to
the Central African WNV-DAK strain proved the most prevalent in the tests
carried out by CEH. With migratory birds arriving back in the UK each
spring, the transfer of the disease to the native wild -- and captive --
bird population was just a matter of the presence of the appropriate
methods of transmission, in this case mosquitoes and, to a lesser extent,
ticks and mites.
It is impossible to tell when this process began, but it is clear that
migration has been going on for many thousands of years, and it's therefore
possible that WNV was in the UK bird population well before any of the
virus strains were isolated in the laboratory. Dr Ernie Gould of CEH also
suggests it's likely that some antibodies to WNV are passed from mother to
chick: "The egg is a rich source of antibodies, but the levels of antibody
we have recorded in adult birds mean this is not likely to be the only
source of antibodies in the chicks. I would go as far as to say that the
initial antibodies received from the egg are sufficient to allow the young
birds to fight off the virus when they are exposed to it. This could be the
reason why bird deaths from WNV have never been seen as a problem in the UK."
While Dr Gould and his team were unable to isolate any active virus in any
of the samples tested, he is positive it is there. Tests on 69 juvenile
spring-born birds caught and sampled during the summer of 2002 showed that
35 of these had been exposed to the virus and had developed antibodies. In
each of these cases the source of infection had to be in the UK.
"We have to presume that the virus is circulating harmlessly in the bird
population at a sub-clinical (showing no symptoms) level and is building
herd immunity," Dr Gould said, "while we have not physically isolated the
live virus, we have found RNA from the virus which suggests it is present.
To draw another analogy with the human population, measles virus is known
to persist in the jejunum (part of the small intestine) of people who have
immunity to the virus. If you know where to look you will find it. Purists
say that until the virus has actually been isolated, it cannot be
considered to be in a population, but I am convinced that if we do enough
sampling, it will be found."
One of the most worrying points about WNV is that it can prove fatal to
humans. Indeed it was deemed responsible for the deaths of about 270 people
in the USA during 2002. To date there have been no human cases of WNV found
in the UK. The Public Health Laboratories recently studied brain samples
from 150 unexplained brain encephalopathies and found no evidence of the
virus. Dr Gould said, however, that would be surprised if there has been no
cross-infection of WNV to humans in the UK. "Research in the Volgograd
region of Russia has shown that ticks can be important in the transmission
of WNV. While it's likely that this is one method of infection at work in
the UK, the spread rate is too high for ticks to be responsible for the
level of antibodies we have seen. The primary route of infection has to be
the mosquito."
So, if UK mosquitoes are carrying the virus (something Dr Gould hopes to
start studying shortly), why haven't we seen cases in the human and animal
population? "Possibly because we've not looked hard enough," Dr Gould
says. In July, the Chief Medical Officer -- who had seen an early draft of
the paper produced by Dr Gould and his team -- suggested that medical
professionals should step up their efforts to test for WNV in the human
population.
--
Graeme Kirk
Acting Deputy Editor
Cage and Aviary Birds
<Graeme_Kirk@ipcmedia.com>

See Also