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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