in late July 2005, official reports to the OIE from government authorities
indicate that the H5N1 virus has expanded its geographical range.
Both Russia and Kazakhstan reported outbreaks of avian influenza in
poultry in late July, and confirmed H5N1 as the causative agent in
early August. Deaths in migratory birds, infected with the virus,
have also been reported. Outbreaks in both countries have been attributed
to contact between domestic birds and wild waterfowl via shared water
These are the first outbreaks of highly pathogenic H5N1 avian influenza
recorded in the two countries. Both countries were previously considered
free of the virus.
Since the initial reports, the Russian H5N1 outbreak in poultry, which
has remained confined to Siberia, has spread progressively westward
to affect 6 administrative regions. In Kazakhstan, several villages
bordering the initial outbreak site in Siberia are now known to have
experienced disease in poultry. To date, outbreaks in the two countries
have involved some large farms as well as small backyard flocks, with
close to 120,000 birds dead or destroyed in Russia and more than 9,000
affected in Kazakhstan.
In early August, Mongolia issued an emergency report following the
death of 89 migratory birds at two lakes in the northern part of the
country. Avian influenza virus type A has been identified as the cause,
but the virus strain has not yet been determined. Samples have been
shared with WHO reference laboratories and are currently being investigated.
Also in early August, an outbreak of H5N1 in poultry was detected
in Tibet, China.
In all of these recent outbreaks, authorities have announced control
measures in line with FAO and OIE recommendations for highly pathogenic
avian influenza. To date, no human cases have been detected, vigilance
is high, and rumours are being investigated by local authorities.
The outbreaks in Russia and Kazakhstan provide evidence that H5N1
viruses have spread beyond their initial focus in south-east Asian
countries, where outbreaks are now known to have begun in mid-2003.
Despite aggressive control efforts, FAO has warned that the H5N1 virus
continues to be detected in many parts of Viet Nam and Indonesia and
in some parts of Cambodia, China, Thailand, and possibly also Laos.
The south-east Asian outbreaks, which have resulted in the death or
destruction of more than 150 million birds, have had severe consequences
for agriculture and most especially for the many rural farmers who
depend on small backyard flocks for income and food. Human cases,
most of which have been linked to direct contact with diseased or
dead poultry in rural areas, have been confirmed in four countries:
Viet Nam, Thailand, Cambodia, and Indonesia. Only a few instances
of limited human-to-human transmission have been recorded. Poultry
outbreaks of H5N1 avian influenza in Japan, Malaysia, and the Republic
of Korea were successfully controlled.
WHO fully agrees with FAO and OIE that control of avian influenza
infection in wild bird populations is not feasible and should not
be attempted. Wild waterfowl have been known for some time to be the
natural reservoir of all influenza A viruses. Migratory birds can
carry these viruses, in their low pathogenic form, over long distances,
but do not usually develop signs of illness and only rarely die of
the disease. The instances in which highly pathogenic avian influenza
viruses have been detected in migratory birds are likewise rare, and
the role of these birds in the spread of highly pathogenic avian influenza
remains poorly understood.
Very large die-offs of migratory birds from avian influenza, such
as the one detected at the end of April at Qinghai Lake in central
China, in which more than 6,000 birds died, are considered unusual.
Research published in July indicates that H5N1 viruses in that outbreak
are similar to viruses that have been circulating in south-east Asia
for the last two years.
Analyses of viruses from the Russian outbreak, recently published
on the OIE website, show apparent similarity to viruses isolated from
migratory birds during the Qinghai Lake outbreak. Specimens from the
Mongolian outbreak in migratory birds should also prove useful in
shedding light on these recent developments. Monitoring the spread
and evolution of avian H5N1 viruses in birds and rapidly comparing
these results with previously characterized H5N1 viruses is an essential
activity for assessing the risk of pandemic influenza.
Implications for human health
The poultry outbreaks in Russia and Kazakhstan are caused
by a virus that has repeatedly demonstrated its ability, in outbreaks
in Hong Kong in 1997, in Hong Kong in 2003, and in south-east Asia
since early 2004, to cross the species barrier to infect humans, causing
severe disease with high fatality. A similar risk of human cases exists
in areas newly affected with H5N1 disease in poultry.
Experience in south-east Asia indicates that human cases of infection
are rare, and that the virus does not transmit easily from poultry
to humans. To date, the majority of human cases have occurred in rural
areas. Most, but not all, human cases have been linked to direct exposure
to dead or diseased poultry, notably during slaughtering, defeathering,
and food preparation. No cases have been confirmed in poultry workers
or cullers. No cases have been linked to the consumption of properly
cooked poultry meat or eggs.
Factors relating to poultry densities and farming systems seen in
different countries may also influence the risk that human cases will
occur. During a 2003 outbreak of highly pathogenic avian influenza,
caused by the H7N7 strain, in the Netherlands, more than 80 cases
of conjunctivitis were detected in poultry workers, cullers, and their
close contacts, and one veterinarian died. That event, which was contained
following the destruction of around 30 million poultry, underscores
the need for newly affected countries to follow FAO/OIE/WHO recommended
precautions when undertaking control measures in affected farms.
Pandemic risk assessment
The possible spread of H5N1 avian influenza to poultry in
additional countries cannot be ruled out. WHO recommends heightened
surveillance for outbreaks in poultry and die-offs in migratory birds,
and rapid introduction of containment measures, as recommended by
FAO and OIE. Heightened vigilance for cases of respiratory disease
in persons with a history of exposure to infected poultry is also
recommended in countries with known poultry outbreaks. The provision
of clinical specimens and viruses, from humans and animals, to WHO
and OIE/FAO reference laboratories allows studies that contribute
to the assessment of pandemic risk and helps ensure that work towards
vaccine development stays on course.
The expanding geographical presence of the virus is of concern as
it creates further opportunities for human exposure. Each additional
human case increases opportunities for the virus to improve its transmissibility,
through either adaptive mutation or reassortment. The emergence of
an H5N1 strain that is readily transmitted among humans would mark
the start of a pandemic.