Back to the West Nile Virus listserv archive.

Date: 4 Aug 2003
Posted by: Graeme Kirk <Graeme_Kirk@ipcmedia.com>
Ron Rees Davies <Ron_vet@lineone.net>

Excerpted from ProMED-mail (Id: 20030803.1903, August 3, 2003, and 
Id: 20030804.1916, August 4), "West Nile virus, birds: antibody - UK 
(02, 03).
See also WestNileVirus-L postings October 29, 2002 and July 21, 2003:

[1]
RESEARCH SUGGESTS WNV IS ENDEMIC IN THE UK

From: Graeme Kirk <Graeme_Kirk@ipcmedia.com>
Acting Deputy Editor
Cage and Aviary Birds
<Graeme_Kirk@ipcmedia.com>
---
Source: Cage and Aviary Birds, Sat Aug 2, 2003

The UK bird population would appear to be safe from attack by WNV, 
according to new research, because the vast majority of birds have 
already been exposed. A new study, 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 NYS 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.

---------
[2]
Date: Mon 4 Aug 2003
From: Ron Rees Davies <Ron_vet@lineone.net>
COMMENT ON WNV TRANSMISSION IN THE UK

The preceding ProMED-mail post  referred to the search for mosquito 
and/or tick vectors which could account for intra/inter-species 
spread of WNV within the UK bird population.

Dr Redig's work on raptors in the US with the virus has shown a 
possible correlation with Hippoboscid (Flat-fly; Louse-fly) 
population numbers, and it has been suggested that these insects 
rather than mosquitoes are
responsible for transmission.

Hippoboscids are common parasites on UK Hirundiniae (i.e. UK-Africa 
migratory swallows/swifts/martins) as well as corvids and birds of 
prey, amongst many others.

Ron Rees Davies
BVSc., CertZooMed., MRCVS
The Exotic Animal Centre
12, Fitzilian Avenue
Harold Wood
Romford, Essex - UK RM3 0QS
<Reesdavies@AAV.org>

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