http://www.eurekalert.org/pub_releases/2007-03/bpl-mbi032307.php
Migratory birds: Innocent scapegoats for the dispersal of the H5N1 virus
Public release date: 26-Mar-2007
Migratory birds, such as these gulls seen here at a resting staging
post, are too readily blamed for the spread of H5N1 without any
evidence to support such claims
A review to be published shortly in the British Ornithologists' Union's
journal, Ibis, critically examines the arguments concerning the role of
migratory birds in the global dispersal of the highly pathogenic avian
influenza virus H5N1. Ecologists of the Station Biologique de la Tour
du Valat and of the GEMI-CNRS in the Camargue (France), Michel
Gauthier-Clerc, Camille Lebarbenchon and Frédéric Thomas conclude that
human commercial activities, particularly those associated with
poultry, are the major factors that have determined its global
dispersal.
Highly pathogenic avian influenza virus subtype H5N1 was first detected
in poultry in November 1996 in south-east China. The virus subsequently
dispersed throughout most of Asia, and also to Africa and Europe. From
mid-2005, migratory wild birds have been widely considered to be the
primary source of the dispersal of H5N1 outside Asia. This claim was
based on the discovery in May 2005 that hundreds of wild birds had died
on Lake Quinghaihu, on the high Asian plateau in China. It is however
clear that the trajectory of the virus does not correspond with to the
main migration routes of wild birds. The global network of migration
routes seemed to hide the globalisation - without strict health control
- of the exchanges of poultry, the more likely mechanism for disease
spread.
During the previous epizooties of highly pathogenic subtypes of H5 and
H7, it was shown that the expansion of these viruses was due to human
activities, in particular, movements of poultry or their products. This
commercial scenario is the one that explained the expansion and the
maintenance of the H5N1 virus in south-east Asia until 2004, via the
legal and illegal trade in poultry.
The cases in western Europe in February 2006 after a cold spell on the
Black Sea showed that virus can spread through infected wild birds
travelling short distances, but no evidence for long distance
transmission during seasonal migration has yet been found. The evidence
overwhelmingly supports the hypothesis that human movements of domestic
poultry have been the main agent of global dispersal of the virus to
date. The occurrence of an outbreak at a commercial turkey farm in
Suffolk, England, in February 2007 fits this wider pattern.
Wild birds, particularly waterfowl, are a key element of the viral
ecology of low pathogenic avian influenza. Very high densities of
domestic animals and increased stress factors are particularly
favourable for the maintenance and transmission of virulent agents, in
particular subtypes of highly pathogenic influenza. Paradoxically, the
H5N1 virus coupled with a fear of transmission by wild birds could lead
to a reversion to battery farming which increases risk of outbreaks.
This would stall the current trend to better animal welfare resulting
from free-range agriculture. Maintaining these trends, whilst
controlling disease through strong veterinary scrutiny and control of
trade, is more likely to be a successful strategy.
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