<download a
printable factsheet>
Background
How does avian influenza spread among
birds?
History of the disease in
Australia
Outbreaks in Asia
Implications for human health
What are the factors leading to cross
infection?
So why all the worry?
Research and action in Australia
Avian
influenza background
Avian influenza is an infectious disease of birds caused by a
Type A Orthomyxoviridae virus. The disease occurs worldwide, and
was first identified in Italy more than 100 years ago (Tracey,
Woods, Roshier, West, & Saunders, 2004). Influenza A
virus has 24 known subtypes, and these strains may combine and
re-assort within a host animal. In wild waterbirds (which are
a common host, and may carry the virus without showing symptoms)
there is stability of the virus, with most strains being
non-virulent. However, when several strains are transmitted to
domestic poultry new virus combinations multiply readily, and a few
strains (H5, H7) appear to have the capacity to mutate and produce
severe disease or mortality in domestic bird populations.
The N1 variant of the H5 strain is virulent, and the one known
to have crossed the avian/human barrier to date.
How does
avian influenza spread among birds?
Infected birds shed influenza virus in their saliva, nasal
secretions, and faeces. Susceptible birds become infected when they
have contact with contaminated excretions or with surfaces that are
contaminated with excretions or secretions. Domesticated birds may
become infected with avian influenza virus through direct contact
with infected waterfowl or other infected poultry or through
contact with surfaces (such as dirt or cages) or materials (such as
water or feed) that have been contaminated with the virus.
<top>
History of the disease in Australia
There have been five known outbreaks of avian influenza in
commercial bird flocks in Australia. These were in Keysborough and
Bendigo, Victoria in 1976 (Turner 1976), 1985 (Bar et al. 1986) and
1992 (Selleck et al. 1997); in Queensland in 1994 (Westbury 1998);
and in Tamworth NSW in 1997 (Selleck et al. 2003). All viruses
identified at these sites were of subtype H7, with a relatively low
virulence, and not implicated in transmissions to human hosts. In
addition, Australia has had outbreaks of “Newcastle
disease” caused by a Paramyxovirus which also affects
poultry, and is carried in wild birds (particularly Psittacine)
throughout the world.
Claims by two Australian academics that the H5N1 strain of avian
influenza is already in Australia are unfounded according to
Australia’s Chief Veterminary Officer, Dr Gardner Murray.
(See DAFF website for full response to these claims).
On each occasion that an outbreak has occurred, poultry managers
in Australia and the US have immediately contacted relevant
authorities and control measures were implemented rapidly.
Successful eradication has been managed by slaughter of infected
birds, disinfection of premises, surveillance in the area and
movement controls of all domestic birds. Eggs, meat and poultry
products in Australia remain safe.
<top>
Outbreaks in Asia
From mid-December 2003 to September 2004, poultry outbreaks
caused by this fatal strain (H5N1) of the virus were reported in
nine Asian nations (listed in order of reporting): the Republic of
Korea, Vietnam, Japan, Thailand, Cambodia, Lao People’s
Democratic Republic, Indonesia, China and Malaysia. Most of these
countries had never before experienced an outbreak of highly
pathogenic avian influenza.
These outbreaks were followed by reports from Russia and nearby
Kazakhstan. Soon after, Mongolia reported the detection of H5N1 in
dead migratory birds, followed by outbreaks in poultry in Turkey
and Romania.
Japan, the Republic of Korea, and Malaysia have announced control
of their poultry outbreaks and are now considered free of the
disease. In the other affected areas, outbreaks are continuing with
varying degrees of severity (World Health Organisation, 2006).
Implications for human health
Unlike normal seasonal influenza, where infection causes only
mild respiratory symptoms in most people, the disease caused by
H5N1 follows an unusually aggressive clinical course, with rapid
deterioration and high fatality. Primary viral pneumonia and
multi-organ failure are common. More than half of those infected
with the virus have died. Most cases have occurred in previously
healthy children and young adults.
A second risk, of even greater concern, is that the virus
– if given enough opportunities – will change into a
form that is highly infectious for humans and spreads easily from
person to person. Such a change could mark the start of a global
outbreak (a pandemic).
To date approximately 190 cases over 9 years have been confirmed
(Centers for Disease Control, 2006), with 92 fatal cases. Only a
few of these cases involved direct human-human transmission. To put
this in context, at least 200 million infected domestic birds have
been culled or died from the disease. It is important to note that
human pandemic influenza and avian influenza are different
diseases. Human pandemic influenza has killed millions of people in
the same period.
<top>
Where have human cases occurred?
Hong Kong has experienced two outbreaks in the past. In 1997, in
the first recorded instance of human infection with H5N1, the virus
infected 18 people and killed 6 of them. In early 2003, the virus
caused two infections, with one death, in a Hong Kong family with a
recent travel history to southern China. Laboratory-confirmed human
cases have been reported in four countries since then: Cambodia,
Indonesia, Thailand, and Vietnam.
To read a more detailed analysis of these outbreaks visit
www.cdc.gov/flu/avian/gen-info/avian-flu-humans.htm
<top>
What are the factors leading to cross infection?
In almost all cases where humans have contracted the disease, a
number of common factors have been involved. These include:
- Probable interactions between local wild water birds and
domestic poultry
- Direct contact with infected poultry or their faeces
- Rural or peri-urban environment, where there are many small
poultry flocks roaming freely
- A high likelihood that ill birds are still sold, slaughtered
and/or consumed, rather than reported (socio-economic cause).
In no instance to date has the virus spread beyond a first
generation of close contacts or caused illness in the general
community.
So
why all the worry?
When compared with H5N1 viruses from 1997 and early 2004, H5N1
viruses now circulating are more lethal to experimentally infected
mice and to ferrets (a mammalian model) and survive longer in the
environment.
H5N1 appears to have expanded its host range, infecting and
killing mammalian species previously considered resistant to
infection with avian influenza viruses. A domestic cat,
tiger, leopard, civet cats and pigs have all contracted the H5N1
from eating infected poultry (UN FAO, 2006). The behaviour of
the virus in its natural reservoir, wild waterfowl, may be
changing. The spring 2005 die-off of upwards of 6,000
migratory birds at a nature reserve in central China, caused by
highly pathogenic H5N1, was highly unusual and probably
unprecedented. In the past, only two large die-offs in
migratory birds, caused by highly pathogenic viruses, are known to
have occurred: in South Africa in 1961 (H5N3) and in Hong Kong in
the winter of 2002–2003 (H5N1).
Ongoing large losses of domestic poultry are, of course, also
unacceptable from an economic perspective.
<top>
Research and action in Australia
There is no evidence that bird flu is affecting birds or humans
in Australia at the present time. However, there is the
possibility that the H5NI strain could enter the country through
illegal importations of birds and their products or through
contaminated visitors.
Since the discovery of avian influenza, the level of biosecurity
has been upgraded on Australian poultry farms to minimise the
exposure to risk factors like wild birds, contaminated water
supplies, other animals and humans. Contingency plans have been
developed to minimise the impact of an outbreak through early
detection and appropriate responses.
Another potential input source is from migratory wild
birds. Ducks and geese are the recognised spreaders of the
virus, but Australian ducks and geese (although known to carry the
non-fatal strains) are not migratory and rarely leave the
continent. (Much of the data implicating Anatids has come
from Europe and North America where these birds are
migratory). It is therefore considered unlikely that Anatids
will bring the disease to Australia.
We are, however, visited by a wide variety of migratory
shorebirds, many of which pass through Asia. Since the
emergence of the disease in SE Asia in 1997, despite the 3 million
migratory birds visiting our shores every year, Australia has so
far remained free of H5N1. Other countries on the Australian
migration route like NZ, New Guinea, Taiwan, and the Philippines
also remain free. The scientific data to date have not
implicated migratory birds in any of the five previous Australian
outbreaks of the lesser strain of avian influenza, but surveying of
populations has been inconsistent and inconclusive.
Given the increasing virulency of the H5N1 strain overseas and
the lack of data on the incidence of avian influenza in our
migratory shorebird populations, researchers at NSW Department of
Primary Industries are attempting to develop more systematic
surveying procedures and collate data on avian influenza incidence
and strain type in our wild bird populations.
With funding from the Wildlife and Exotic Diseases Preparedness
Program of the Commonwealth Department of Agriculture, Fisheries
& Forestry, and the Invasive Animals Cooperative Research
Centre; and valuable support from the Game Council of NSW (great
network, in-depth knowledge of the timing and sites of bird
movements, sampling data) researchers are attempting to
illuminate more clearly the role of our wild shore birds in hosting
and transmitting avian influenza.
The work involves:
- Identifying species most likely to carry the disease, and the
strains carried
- Identifying the most appropriate timing for sampling (when do
birds arrive and from where, seasonality of movements)
- Identifying priority areas for sampling (where wild birds come
in close contact with domestic ones, and where migratory shorebirds
are mixing with high density Anatid populations)
- Developing effective field sampling procedures (trapping
methods, protocols).
Other
research effort
The Australian Government announced through the National Health
& Medical Research Council on 20 February, 2006, funding
totaling $6.5million for 33 projects which are expected to lead to
important advances in Australia’s pandemic
planning. A full list of successful grant applicants
can be viewed at
http://www.nhmrc.gov.au/publications/_files/pandemic.rtf
<top>
Bibliography
Barr, D.A., Kelly, A.P., Badman, R.T., and Campey, A.R. (1986)
Avian influenza on a multi-age chicken farm. Australian Veterinary
Journal 63, 195-196.
http://www.cdc.gov/flu/avian/gen-info/facts.htm) Centers for
Disease Control (2006), Department of Health & Human Services,
Atlanta. Updated Feb 7, 2006.
http://www.fao.org/ag/AGAinfo/subjects/en/health/diseases-cards/avian_cats.html
Agriculture Department, Animal Production & Health Division,
Food & Agriculture Organisation of the United Nations. Updated
30 March, 2006.
Selleck, P.W., Arzey, G., Kirkland, P.D., Reece, R.L., Gould, A.R.,
Daniels, P.W., and Westbury, H.A. (2003). An outbreak of highly
pathogenic avian influenza in Australia in 1997 caused by an H7N4
virus. Avian Diseases 47, 806-811.
Tracey J.P, Woods R, Roshier D, West P & Saunders, G (2004)
Emu, Volume 104, pp. 109-124, CSIRO Publishing, 2004.
Turner, A.J. (1976) The isolation of fowl plague virus in Victoria.
Australian Veterinary Journal 52, 384.
World Health Organisation (2006) FAQs,
http://www.who.int/csr/disease/avian_influenza/avian_faqs/en/index.html#whatare.
Westbury, H.A. (1998) History of highly pathogenic avian influenza
in Australia. In “The 4th International Symposium on Avian
Influenza”. pp.23-30.
<top>
