Highly Pathogenic Avian
Influenza Virus H5N1 and Wild Birds
What are avian influenza viruses?
• Avian
influenza viruses (AIV) are Type A influenza viruses that are associated with
avian species. They have been isolated from more than 100 species of free-living
birds world-wide.
•
Classification of these viruses is based on their hemagglutinin (H) and
neuraminidase (N) subtypes. There currently are 16 H and 9 N recognized
subtypes, and all of these subtypes are represented in viruses isolated from
wild birds.
• Wild birds
represent the historic source for Type A influenza viruses affecting both
domestic bird and mammalian species.
• The host
adaptation that occurs after the movement of these viruses from wild birds to
domestic animals to humans often results in the evolution of “new” viruses,
which can become adapted to the new host population. These “new” viruses (which
include the human Type A influenza viruses) differ from the original viruses
found in wild birds and are no longer associated with wild avian populations.
• The movement
and adaptation of Type A influenza viruses from wild birds to new host species
(especially mammals) is not a common event, which is evident from the limited
number of human type A influenza viruses.
What is a Highly Pathogenic Avian
Influenza (HPAI) virus?
• Highly
pathogenic avian influenza viruses are influenza viruses that cause high
mortality in domestic poultry.
• Highly
pathogenic avian influenza viruses are associated with the H5 and H7 subtypes.
• Not all H5 and H7 subtypes are highly pathogenic.
What is “Bird Flu” and what is
“HPAI H5N1”?
•
“Bird Flu” is a nonscientific term that was coined to describe the
HPAI H5N1 viruses that have been present in Asia since 1997. This term has
caused a great deal of confusion because it is often used as a synonym for avian
influenza.
• HPAI
H5N1 is a highly pathogenic H5N1 virus that has persisted in Asia at
least since 1997. It is established in domestic poultry populations in Asia
(primarily chickens and domestic ducks).
• In 1997, a human death resulting from HPAI
H5N1 virus infection in Hong Kong was reported; there have been over 100 human
cases with approximately 60 fatalities since that time. All human cases have
occurred in Asia, and almost all of these cases have been linked to direct
contact with infected poultry.
• In 2002/2003, wild bird mortality in Hong
Kong was attributed to infection with HPAI H5N1 virus. Wild bird mortality
associated with HPAI H5N1 has continued through 2005, and the current
distribution suggests movement of this virus via migratory birds.
What do we know about avian influenza viruses
in wild birds?
• Our knowledge regarding the epidemiology of
avian influenza in wild birds is extensive but not complete.
• Most AIVs have been isolated from birds that
are associated with water, with most isolations originating from species in the
Anseriformes (ducks, geese, and swans) and Charadriiformes (gulls, terns, and
shorebirds).
• In ducks, the prevalence of AIV peaks in late
summer and early fall. Outside of this period, infection rates often are lower
than 1%.
• In gulls and shorebirds, peak infection rates
are associated with spring migration but these rates differ greatly between
species and are generally low.
• These temporal patterns result in consistent
spatial patterns. For example, avian influenza viruses can be isolated from
ducks on wintering grounds but the prevalence of infection is very low.
• Viruses recovered from wild birds include all
of the H and N subtypes but these subtypes are not equally represented. In North
America, viruses representing the H5 and H7 subtypes are present but these are
not HPAI viruses and they are not common.
• None of these naturally occurring North
American AIVs from wild birds have been associated with mortality or morbidity
in any wild bird species.
• Prior to 2004, when the HPAI H5N1 was linked
to wild bird deaths in Asia, there was only one historic case of any wild bird
mortality associated with AIV infection (an H5N3 from South Africa in the 1960s
caused mortality in common terns). This HPAI virus, may have originated from
infected poultry flocks and it did not persist in wild bird populations.
• Thousands of influenza isolates have been
made from ducks and other birds in North America during the last 30 years.
Despite this ongoing surveillance, there is no indication that any HPAI viruses
exist in North American wild bird populations.
Do we have HPAI H5N1 in North America?
• There is no evidence to suggest that an HPAI
H5N1 virus is present anywhere in North America.
Is there currently a public health risk
associated with HPAI H5N1 in wild birds?
• In the United States there currently is no
recognized public health risk associated with wild bird contact.
• All human deaths associated with
bird-to-human transmission of avian influenza viruses have occurred in Asia and
all have involved the HPAI H5N1 viruses. Human cases in Asia have occurred in
connection with extensive infections in domestic poultry.
• Other H5, H7, and H9 avian influenza viruses
have been transmitted directly from infected domestic birds to humans. These
events have involved HPAI and Low Pathogenic Avian Influenza (LPAI) viruses, but
all have involved contact with infected poultry.
• There has never been a single documented case
of avian influenza virus transmission directly from wild birds to humans.
• There is no indication that wild waterfowl
species hunted in North America are infected with HPAI H5N1.
• Although there currently is no recognized
risk associated with hunting waterfowl and HPAI H5N1 in North America, basic
hygiene, primarily hand-washing, when handling any wild animals or carcasses is
always recommended.
• With regard to pandemic influenza, the
primary public health risk associated with HPAI H5N1 in Asia relates to the
potential for genetic changes (mutations within the H5N1or recombination with
human influenza viruses) that would allow for efficient human-to-human
transmission. If this were to occur, transmission of this “new” virus would no
longer require an avian source.
Is there a domestic animal health risk
associated with HPAI in wild birds?
• There have been many documented cases of
avian influenza virus transmission from wild birds to domestic birds. This is
especially true for free-ranging domestic flocks that have direct contact with
wild ducks.
• In the United States, there are no documented
cases of HPAI transmission from a wild bird to domestic birds, and it is
believed that most HPAI viruses evolve after an H5 or H7 virus becomes
established in domestic bird populations.
• In Asia the recent expansion in distribution
of HPAI H5N1 Asia suggests that domestic flocks are being infected with this
virus through contact with migratory wild birds.
What is the possibility of HPAI H5N1 entering
North America via migratory wild birds?
• Some migratory bird species move between
North America, Asia, and Europe, however, genetic studies of avian influenza
viruses from Eurasia and North America suggest that there is very limited
exchange of AIVs between continents (even with very common influenza viruses).
• It is not possible to discount the
possibility of an HPAI H5N1 introduction, but such an event based on the known
epidemiology of other avian influenza viruses would likely be a very low
probability event.
What is the possibility of this virus being
maintained in wild bird populations?
• We have limited information on which to
evaluate this possibility.
• Experimental studies have demonstrated
bird-to-bird transmission of HPAI H5N1 in mallards, but these studies were
completed under confinement conditions that are not representative of natural
conditions.
• Experimental studies with HPAI H5N1 strains
have consistently demonstrated higher respiratory rather than cloacal shedding
of virus. In wild birds, low pathology avian influenza viruses generally are
associated with cloacal shedding, and transmission occurs via a fecal/oral route
through contaminated water. It is not clear if the extent of fecal shedding with
HPAI H5N1 is consistent with the naturally occurring AIVs that are maintained in
wild bird populations.
• It is known that other AIVs can persist for
extended periods of time in water. Information on environmental persistence of
HPAI H5N1 in water is lacking.
• In experimental trials of mallards with HPAI
H5N1, mortality and morbidity were common. Most isolates from wild birds in Asia
also have been associated with sick or dead birds. It is unclear if wild birds
can be infected with HPAI H5N1 and remain healthy.
Do we have surveillance for HPAI H5N1 in the
United States?
• Surveillance for AIV was taking place in the
United States and other North American countries prior to the emergence of HPAI
H5N1.
• Wild bird surveillance has and will be
expanded to include larger geographic areas and areas of potential introduction,
such as Alaska.
What do I do if I see or find sick or dead
waterfowl or water birds (cormorants, shore birds, gulls)?
-
If you see or find dead waterfowl or water
birds report your findings to the Kentucky Department Fish and Wildlife
Resources at 1-800-858-1549. ext. 352.
Additional information on
HPAI can be found at these websites:
The Centers for Disease Control
and Prevention (www.cdc.gov/flu/avian)
USGS
National Wildlife Health Center
http://www.nwhc.usgs.gov/disease_information/avian_influenza/index.jsp