AVIAN INFLUENZA A/H5N1,
DOMESTIC DUCKS - ASIA
*********************************************
A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of
the
International Society for Infectious
Diseases
<http://www.isid.org>
Date: Fri 29 Oct 2004
From: Marianne Hopp
<mjhopp12@yahoo.com>
Source: WHO - CSR (Communicable
Disease Surveillance & Response)
Outbreak News, 29 Oct
2004 [edited]
<http://www.who.int/csr/don/2004_10_29/en/>
Avian influenza - situation
in Asia: altered role of domestic ducks
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Countries experiencing outbreaks
of H5N1 in poultry need to be aware that domestic ducks may have acquired an important
role in the transmission of highly
pathogenic H5N1 to other poultry and,
possibly, to humans as well.
A new laboratory study [see
commentary] of domestic ducks infected with several 2004 H5N1 viruses shows that, when
compared with infections caused
by viruses from 2003, domestic ducks are shedding more virus for longer periods. The majority
are doing so without showing symptoms
of illness.
Findings from this study also
show that, compared to highly pathogenic
avian influenza [HPAI] viruses from previous outbreaks, the recent H5N1 viruses survive several days longer
in the environment.
The study found that the quantities
of virus excreted by healthy-looking
ducks approach those excreted by diseased -- and visibly very ill -- chickens. This suggests
that domestic ducks might
now be acting as a "silent"
reservoir for the H5N1 virus, which is highly pathogenic for chickens.
To date, no evidence has linked
human H5N1 cases to exposure to asymptomatic
domestic ducks. However, the laboratory findings come at a time when some human cases could not be traced
to contact with diseased or dead
poultry.
In terms of preventing further
human cases, it is of public health concern that ducks may be infected and shed
virus for long periods, yet issue
no "warning signal" in the form of visible signs and
symptoms that alert people
to take precautions. The concern is greatest in rural areas of affected countries,
where free-ranging ducks and chickens
often mingle, frequently sharing the same water
supplies.
The new findings expand on
recent evidence, based on characterization of H5N1 viruses from southern China, that domestic
ducks have played a central role
in generating and maintaining H5N1, in its highly pathogenic form, in parts of Asia. [Li KS, Wang
J, Smith GJD et al.
Genesis of a highly pathogenic
and potentially pandemic H5N1 influenza
virus in eastern Asia. Nature; 430 (8 July 2004): 209-13].
Findings pointing to an altered
role for domestic ducks join other recent evidence that the H5N1 virus circulating
in parts of Asia has increased its
pathogenicity in chickens and mice (a laboratory model for mammals), and has expanded its host range
to include mammals, such as felines,
not previously considered susceptible to infection.
[In view of the said findings,
the following] public health recommendations
for affected countries [should be applied]:
- Investigation of human cases
should now include possible exposure to apparently healthy domestic ducks, as well
as to diseased or dead poultry,
as a risk factor for infection.
- Advice for people living in an affected area
should be revised to include precautions
for apparently healthy ducks similar to those for visibly ill poultry.
- Ducks should not be kept
as pets or allowed to enter households.
- Water supplies for human
use should not be drawn from open ponds used by domestic ducks and should be stored
in ways that prevent contact with
ducks.
- Once prepared and properly
cooked, duck meat and eggs do not pose a risk to human health.
- Significant exposure can
occur during home slaughtering and preparation prior to cooking, and these risks
need to be addressed.<http://www.who.int/csr/don/2004_10_29/en/>
<http://www.who.int/foodsafety/micro/avian2/en/>
WHO, FAO, and OIE recognize
that the altered role of domestic ducks in the transmission cycle of H5N1 should be
addressed, as it may complicate
efforts to control the disease in poultry and to prevent further human cases.
Research on the prevalence
of asymptomatic infection in ducks in Asia is urgently needed. Such information will allow
a focused approach to the prevention
of human cases arising from exposure to apparently healthy domestic ducks.
The wider implications of the
role played by domestic ducks are being jointly considered by FAO and OIE in formulating
a long-term strategy for bringing
the current outbreaks in poultry under control. With the H5N1 virus now endemic in parts of Asia, significant
changes in farming practices may
be needed to control the disease, especially in rural areas, and thus reduce opportunities for
community-wide exposure.
In animals, surveillance remains
the key to prevention and control, and this activity needs to be adequately supported.
FAO and OIE have recently issued
comprehensive guidelines for responding
to the outbreaks of H5N1 in poultry in Asia. [See 20040927.2672 and
<http://www.oie.int/eng/AVIAN_INFLUENZA/FAO%20recommendations%20on%20HPAI.pdf>].
The 2 agencies are conducting
research on the epidemiology of H5N1
infection in ducks and on the
use of vaccination to control infection.
--
ProMED-mail
<promed@promedmail.org>
[According to the CSR notification,
the laboratory studies mentioned were completed last week [3rd week of October
2004] by researchers at the WHO
collaborating center for animal influenza viruses at St. Jude Children's Research Hospital in Memphis, Tennessee,
USA. The main findings are being
made public by the WHO, in advance of publication, because of their significance for human health.
<http://www.who.int/csr/disease/avian_influenza/labstudy_2004_10_29/en/>
The findings, and the subsequent
WHO recommendations, might have impact
upon the internationally prescribed measures to control HPAI. If (apparently) "healthy" ducks are present
in the vicinity of affected poultry
farms, the stamping out of the latter might not be sufficient to eradicate the outbreak. This new
situation might prescribe a need
to reassess existing provisions, as laid out in chapter 2.7.12 of OIE's terrestrial animal health
code
<http://oie.int/eng/normes/mcode/en_chapitre_2.7.12.htm>
It might also explain the
recurrence of the disease in countries like Thailand, where stamping-out policy without
vaccination has been
applied. - Mod.AS]
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