Since 2003, a stable variant of the H5N1 avian influenza
virus has spread easily between various wild and domestic bird species in
many locations worldwide. It has been demonstrated that where biosecurity
is poor and disease control ineffective, these viruses can easily become endemic
in domestic birds.
There has been much concern that H5N1 could form a pandemic strain. However
it cannot be said whether or not the overall burden of circulating animal
influenza viruses with some pandemic potential (such as H5 and H2) has increased
and therefore whether the risk of a pandemic has actually increased. No
H5 virus is known to have adapted to humans in the past, and other viruses,
such as low pathogenic H2 and H3 viruses, may represent greater threats,
because they have formed the basis of pandemic strains. Exposure of humans
to H5N1 viruses has probably increased considerably in recent months, for
example in Africa. This exposure may increase the probability of H5N1 recombination
with human or other influenza viruses although this does not necessarily
change the pandemic potential of H5N1 viruses .
Knowledge of how humans are infected, the real level of human to human
transmission, the spectrum of disease presentation and the effectiveness
of treatment remains scanty. Human outbreaks of H5N1 have generally been
incompletely investigated, although there have been some improvements recently,
notably the investigation of the outbreak in north Sumatra .
Human to human transmission is known to have occurred, but there is no
evidence that transmission has become more efficient. All the human to human
infections with H5N1 to date seem not to have transmitted on further, or
at least they have not led to any prolonged or expanding chains of infection
[2,3]. Therefore, although the case fatality rate for human infection remains
high, (around 57% for cases reported to WHO with no suggestion of any changes
over time It seems that H5N1 avian viruses remain poorly adapted to humans.
That is given a high dose viral challenge and perhaps some genetic host
susceptibility the viruses can infect humans, where they are then lethally
pathogenic, but that then there is little transmission on to other humans.
Surveillance for H5N1 cases in humans is becoming harder where poultry
immunisation is widely but imperfectly implemented, because the marker of
local poultry deaths for human case detection is being lost. Declines in
the number of sporadic human cases in countries with poultry vaccination
programmes should therefore be interpreted cautiously. It is also unclear
whether large scale poultry vaccination programmes increase or decrease
the overall human population exposure to H5N1 viruses. The potential impact
on human health of poorly implemented poultry vaccination and experimental
poultry vaccines needs to be carefully considered.
Avian influenza worldwide
Surveillance of sick and dead birds (wild and domestic) for highly pathogenic
avian influenza (HPAI) is particularly strong. This mechanism in 2006 monitored
the extension of the H5N1 virus from the East in wild birds including non-migratory
species in 13 countries. Some domestic poultry have also been infected.
Surveys of healthy wild birds in the EU have so far indicated that H5N1
infection is very rare .
Other countries in Europe
There is a continuing risk of H5N1 appearing in parts of eastern, Europe,
especially around the Danube Basin. The role of wild birds in transmission
versus informal and formal trade is unclear. In Romania there was a very
large and concerning outbreak in poultry in May 2006 that involved both
commercial and domestic birds . No human cases occurred despite many
of the infected birds being in backyard flocks.
Human outbreaks in Turkey, Iraq and Azerbaijan have indicated the potential
for infection of humans from sick domestic and perhaps also wild birds in
Europe. There is no evidence of transmission to humans from casual contact
with infected wild birds . Uncertainty remains in Russia, which has few
data that have been subjected to international confirmation.
Indonesia is currently the most active site of animal (bird) H5N1 transmission
in the Asia Pacific region, and a greater number of human cases have been
detected here in the first half of 2006 than for the whole of 2005. China
and Cambodia have also reported human cases in 2006 but in general, activity
in the Asia Pacific region appears reduced when compared with 2004 and 2005.
Information about the overall H5N1 situation in China is difficult to obtain.
The introduction of widespread poultry vaccination is making surveillance
of H5N1 more difficult because the marker of local poultry deaths for human
case detection is being lost.
China has made major progress in strengthening its health sector response
since the shock of SARS in 2003, but major planning and logistical problems
in controlling H5N1 in birds remain. Poultry immunisation is used widely
but is a challenging task, with billions of birds being needed to be immunised
annually. Few details of preparations for pandemic influenza in China have
been publicised . In south Asia (India and Pakistan), there are have
only been sporadic outbreaks of infection in poultry to date but comprehensive
surveillance data are missing .
In south Asia (India and Pakistan), there have only been sporadic reports
of infection in poultry to date.
Vietnam and Thailand
There have been no official reports of poultry outbreaks in Vietnam for
nearly 6 months. After a nationwide vaccination campaign in autumn 2005,
there has been a targeted campaign in 35 provinces in 2006, and 118 million
birds have so far been vaccinated. Other control measures, such as banning
of sale of live birds and restrictions in raising and movement of poultry,
are being partially implemented at present .
Veterinary control measures, but without vaccination, have been highly
successful in Thailand . Therefore it would seem that both Vietnamese
and Thai authorities are showing considerable success in controlling the
infection in their poultry, using somewhat different strategies. In contrast,
Indonesia is failing to control outbreaks, due to a poorly implemented veterinary
control strategy, including poorly implemented vaccination. The outcome
in China remains to be seen.
Africa: a bleak outlook for H5N1 control
Surveillance in Africa is especially weak, and there is evidence of widespread
infection in domestic poultry in parts of north, west and central Africa.
Prospects of control are bleak here because of weaknesses in veterinary
services, and a number of competing animal and human health problems. There
is little evidence that migratory birds are playing a big role in transmission
here: trade and movement of poultry is likely to be the most important driver
. The outbreaks in Egypt have been well described. These involved both
commercial and backyard flocks, with considerable impact on economic life
and food security . It is probable that large numbers of people in African
countries are at risk of H5N1 infection. If that virus had pandemic potential
then a pandemic arising from Africa must be a possibility
The conclusions that arise from the epidemiological data and the national
reports are mixed . Some national authorities, as in Thailand in Vietnam,
are doing well in controlling outbreaks. However, others, such as in Indonesia,
are doing poorly. Control measures have been stepped up, but the virus has
already spread to Africa.
The global epidemiology, strategy and success of measures against
H5N1, and to a limited extent, preparations for a human pandemic due to
any influenza, were recently reviewed at a ‘Senior Officers Meeting’ organized
by the European Union and the International Partnership on Avian and Pandemic
Influenza (Vienna, 6-7 June 2006) . This was a follow-up to the January
Pledging Conference in Beijing.