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1.
Introduction
Avian influenza is a contagious infectious disease of birds caused by
type A strains of the influenza virus. The disease, which was first
identified in Italy in 1878, occurs worldwide among poultry
populations. Birds are an especially important species because all
known subtypes of influenza A viruses circulate among wild birds,
which are considered the natural hosts for this variety of viruses.
However, avian influenza viruses usually do not make wild birds sick,
but can make domesticated birds very sick and often kill them. This
form of influenza is not usually known to infect humans, however once
transmitted the infection may lead to development of this disease with
symptoms of avian influenza ranging from typical influenza-like
symptoms (e.g., fever, cough, sore throat and muscle aches) to eye
infections, pneumonia, acute respiratory distress, viral pneumonia,
and other severe and life-threatening complications. Confirmed
instances of avian influenza viruses infecting humans have been
reported since 1997 mainly among the South East Asian countries known
as the Hong Kong outbreak. All genes are of avian origin, indicating
that the virus has not acquired human genes. The acquisition of human
genes is known to increase the likelihood that a virus of avian origin
can be readily transmitted from one human to another.
The current epidemic was first reported in October 2003 in Vietnam and
is now known to spread to South Korea, Thailand and Laos and in all
these countries it has been reported to be caused by the H5N1 strain
of Influenza Type A variety of viruses. The World Health Organization
(WHO) has reported that the H5N1 strain implicated in the outbreak has
been sequenced. According to the latest figures provided by the WHO,
that there have been 15 reported laboratory-confirmed cases of H5N1
strain in Vietnam,11 of which have been fatal. 5 cases have been
reported from Thailand with all of them dying from the condition.
2. Description
Infectious agent • H5N1 strain of Type A Influenza virus
Occurrence
Vietnam
Fifteen people have died among the 22 laboratory confirmed human cases.
Median age among the human cases is 16.5 years and half of the cases were
females. Since the beginning of the outbreak, nearly 3 million chickens have
died/culled in the country.
South Korea
First time reported in the country on 12th December 2003, and nearly 1.1
million chickens and ducks have died/culled. No human cases have been
reported
Thailand
• The 1st case was a 7-year-old boy from Suphanburi province who developed
fever and cough on 3 Jan 2004 and progressed to Acute Respiratory Distress
Syndrome (ARDS) on 13 Jan 2004. The 2nd case was a 6-year-old boy from
Kanchanaburi province who developed fever on 6 Jan 2004 followed by severe
pneumonia with ARDS a week later. Nearly 6 million chickens have died/culled
in this country. To date a total of 9 cases have been reported with 7 of
them dying from the disease. The median age of the human cases is 6 years
and 20% of them are females.
Japan
More than 10,000 chickens have died/culled. First reported during the
outbreak on the 12th of Jan 2004, there have been no reports of any human
case of avian influenza.
Taiwan
Nearly 50,000 chickens have died with no reports of any human cases to date
Cambodia
Thousands of chicken have died (the exact number is yet to be disclosed )
however there have been no human cases reported officially to date
Indonesia
Millions of chickens are reported to have died over the recent months and is
now confirmed to be caused by H5N1 strain of the Type A influenza virus.
China
Reports of avian influenza among bird populations (poultry chickens and
ducks) have been reported (suspected or confirmed) from 14 of the countries
31 provinces and regions. The exact numbers of affected population are
increasing and the outbreak is now confirmed to be cause by H5N1 strain. No
human cases have been reported to date by the Chinese authorities.
Reservoir
Till now the reservoir seems to be wild and domestic chickens and
turkeys, with humans being reported to only have symptomatic cases.
Mode of transmission
Route and mode of spread according to current evidences includes;
• Bird to bird
• Bird to person contact
• There is no scientific consensus on human to human spread of the H5N1
strain.
Incubation Period and characteristics of H5N1 virus
Incubation period is known to be of one week on an average. The virus is can
be killed by heat (56 0 C for 6 hours or 60 0 C for 30 minutes). The virus
is known to survive at cool temperatures, in contaminated manure of poultry
birds for at least three months. In water the virus is known to survive upto
4 days at 22 degrees Celsius and more than 30 days at zero degrees C.
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