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Avian influenza, also known as avian flu or bird flu, is a disease caused by the
influenza A virus ''Influenza A virus'' (''Alphainfluenzavirus influenzae'') or IAV is the only species of the genus ''Alphainfluenzavirus'' of the virus family '' Orthomyxoviridae''. It is a pathogen with strains that infect birds and some mammals, as well as c ...
, which primarily affects birds but can sometimes affect mammals including humans. Wild aquatic birds are the primary host of the influenza A virus, which is
enzootic Enzootic describes the situation where a disease or pathogen is continuously present in at least one species of non-human animal in a particular region. It is the non-human equivalent of endemic. In epizoology, an infection is said to be "''enzo ...
(continually present) in many bird populations. Symptoms of avian influenza vary according to both the strain of virus underlying the infection, and on the species of bird or mammal affected. Classification of a virus strain as either low pathogenic avian influenza (LPAI) or high pathogenic avian influenza (HPAI) is based on the severity of symptoms in domestic
chicken The chicken (''Gallus gallus domesticus'') is a domesticated subspecies of the red junglefowl (''Gallus gallus''), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is now one of the most common and w ...
s and does not predict severity of symptoms in other species. Chickens infected with LPAI display mild symptoms or are
asymptomatic Asymptomatic (or clinically silent) is an adjective categorising the medical conditions (i.e., injuries or diseases) that patients carry but without experiencing their symptoms, despite an explicit diagnosis (e.g., a positive medical test). P ...
, whereas HPAI causes serious breathing difficulties, significant drop in egg production, and sudden death. Domestic poultry may potentially be protected from specific strains of the virus by vaccination. Humans and other mammals can only become infected with avian influenza after prolonged close contact with infected birds. In mammals including humans, infection with avian influenza (whether LPAI or HPAI) is rare. Symptoms of infection vary from mild to severe, including fever, diarrhea, and cough. Influenza A virus is shed in the saliva, mucus, and feces of infected birds; other infected animals may shed bird flu viruses in respiratory secretions and other body fluids (e.g., cow milk). The virus can spread rapidly through poultry flocks and among wild birds. A particularly virulent strain,
influenza A virus subtype H5N1 Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus, which causes the disease avian influenza (often referred to as "bird flu"). It is enzootic (maintained in the population) in many bird populations, and also panzoo ...
(A/H5N1) has the potential to decimate domesticated poultry stocks and an estimated half a billion farmed birds have been slaughtered in efforts to contain the virus.


Highly pathogenic avian influenza

Because of the impact of avian influenza on economically important chicken farms, a classification system was devised in 1981 which divided avian virus strains as either highly pathogenic (and therefore potentially requiring vigorous control measures) or low pathogenic. The test for this is based solely on the effect on chickens – a virus strain is highly pathogenic avian influenza (HPAI) if 75% or more of chickens die after being deliberately infected with it. The alternative classification is low pathogenic avian influenza (LPAI). This classification system has since been modified to take into account the structure of the virus' haemagglutinin protein. Other species of birds, especially water birds, can become infected with HPAI virus without experiencing severe symptoms and can spread the infection over large distances; the exact symptoms depend on the species of bird and the strain of virus. Classification of an avian virus strain as HPAI or LPAI does not predict how serious the disease might be if it infects humans or other mammals. Since 2006, the World Organization for Animal Health requires all LPAI H5 and H7 detections to be reported because of their potential to mutate into highly pathogenic strains.


Virology

Avian influenza is caused by the ''
influenza A virus ''Influenza A virus'' (''Alphainfluenzavirus influenzae'') or IAV is the only species of the genus ''Alphainfluenzavirus'' of the virus family '' Orthomyxoviridae''. It is a pathogen with strains that infect birds and some mammals, as well as c ...
'' which principally affects birds but can also infect humans and other mammals. Influenza A is an
RNA virus An RNA virus is a virus characterized by a ribonucleic acid (RNA) based genome. The genome can be single-stranded RNA (ssRNA) or double-stranded (Double-stranded RNA, dsRNA). Notable human diseases caused by RNA viruses include influenza, SARS, ...
with a genome comprising a
negative-sense In molecular biology and genetics, the sense of a nucleic acid molecule, particularly of a strand of DNA or RNA, refers to the nature of the roles of the strand and its complement in specifying a sequence of amino acids. Depending on the context, ...
, RNA segmented genome that encodes 11 viral genes. The virus particle (also called the virion) is 80–120 nanometers in diameter and elliptical or filamentous in shape. There is evidence that the virus can survive for long periods in freshwater after being excreted in feces by its avian host, and can withstand prolonged freezing. There are two
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...
on the surface of the
viral envelope A viral envelope is the outermost layer of many types of viruses. It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes. A viral envelope protein or E protein is a protein in the en ...
; hemagglutinin and neuraminidase. These are the major antigens of the virus against which neutralizing antibodies are produced. Influenza virus epidemics and epizootics are associated with changes in their antigenic structure.
Hemagglutinin The term hemagglutinin (alternatively spelt ''haemagglutinin'', from the Greek , 'blood' + Latin , 'glue') refers to any protein that can cause red blood cells (erythrocytes) to clump together (" agglutinate") ''in vitro''. They do this by bindin ...
(H) is an
antigenic In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. Anti ...
glycoprotein Glycoproteins are proteins which contain oligosaccharide (sugar) chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known a ...
which allows the virus to bind to and enter the host cell.
Neuraminidase Exo-α-sialidase (, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: : Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glycosidic linkag ...
(N) is an antigenic glycosylated
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
which facilitates the release of progeny viruses from infected cells. There are 18 known types of hemagglutinin, of which H1 thru H16 have been found in birds, and 11 types of neuraminidase.


Subtypes

Subtypes of influenza A are defined by the combination of H and N proteins in the
viral envelope A viral envelope is the outermost layer of many types of viruses. It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes. A viral envelope protein or E protein is a protein in the en ...
; for example, "
H5N1 Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus, which causes the disease avian influenza (often referred to as "bird flu"). It is enzootic (maintained in the population) in many bird populations, and also panzoo ...
" designates an influenza A subtype that has a type-5 hemagglutinin (H) protein and a type-1 neuraminidase (N) protein. The subtyping scheme only takes into account the two envelope proteins, not the other proteins coded by the virus' RNA. Almost all possible combinations of H (1 thru 16) and N (1 thru 11) have been isolated from wild birds. Further variations exist within the subtypes and can lead to very significant differences in the virus's ability to infect and cause disease.


Influenza virus nomenclature

To unambiguously describe a specific isolate of virus, researchers use the internationally accepted ''Influenza virus nomenclature,'' which describes, among other things, the species of animal from which the virus was isolated, and the place and year of collection. As an example, A/chicken/Nakorn-Patom/Thailand/CU-K2/04(H5N1): * A stands for the genus of influenza ( A, B or C) * chicken is the animal species the isolate was found in (note: human isolates lack this component term and are thus identified as human isolates by default) * Nakorn-Patom/Thailand is the place this specific virus was isolated * CU-K2 is the laboratory reference number that identifies it from other influenza viruses isolated at the same place and year * 04 represents the year of isolation 2004 * H5 stands for the fifth of several known types of the protein
hemagglutinin The term hemagglutinin (alternatively spelt ''haemagglutinin'', from the Greek , 'blood' + Latin , 'glue') refers to any protein that can cause red blood cells (erythrocytes) to clump together (" agglutinate") ''in vitro''. They do this by bindin ...
* N1 stands for the first of several known types of the protein
neuraminidase Exo-α-sialidase (, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: : Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glycosidic linkag ...
. Other examples include: A/duck/Hong Kong/308/78(H5N3), A/avian/NY/01(H5N2), A/chicken/Mexico/31381-3/94(H5N2), and A/shoveler/Egypt/03(H5N2).


Genetic characterization

Analysis of the virus' genome enables researchers to determine the order of its nucleotides. Comparison of the genome of a virus with that of a different virus can reveal differences between the two viruses. Genetic variations are important because they can change amino acids that make up the influenza virus’ proteins, resulting in structural changes to the proteins, and thereby altering properties of the virus. Some of these properties include the ability to evade immunity and the ability to cause severe disease. Genetic sequencing enables influenza strains to be further characterised by their
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
or
subclade In genetics, a subclade is a subgroup of a haplogroup. Naming convention Although human mitochondrial DNA (mtDNA) and Y chromosome DNA (Y-DNA) haplogroups and subclades are named in a similar manner, their names belong to completely separate syst ...
, revealing links between different samples of virus and tracing the evolution of the virus over time.


Species barrier

Humans can become infected by the avian flu if they are in close contact with infected birds. Symptoms vary from mild to severe (including death), but as of December 2024 there have been no observed instances of sustained human-human transmission. There are a number of factors that generally prevent avian influenza viruses from causing epidemics in humans or other mammals. * The viral HA protein of avian influenza binds to alpha-2,3
sialic acid Sialic acids are a class of alpha-keto acid sugars with a nine-carbon backbone. The term "sialic acid" () was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this group is ''N''-acetylneuraminic acid ...
receptors, which are present in the
respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of conducting air to the alveoli for the purposes of gas exchange in mammals. The respiratory tract is lined with respiratory epithelium as respirato ...
and intestines of avian species, while human influenza HA binds to alpha-2,6 sialic acid receptors, which are present in the human upper respiratory tract. * The myxovirus resistance protein ( Mx1) is an important antiviral restriction factor that inhibits the replication of avian influenza viruses in particular. Human-adapted strains of IAV display reduced sensitivity to human Mx1 compared with avian strains. * Other factors include the ability to replicate the viral RNA genome within the host cell nucleus, and to transmit between individuals. Influenza viruses are constantly changing as small genetic
mutations In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosi ...
accumulate, a process known as
antigenic drift Antigenic drift is a kind of genetic variation in viruses, arising from the accumulation of mutations in the virus genes that code for virus-surface proteins that host antibodies recognize. This results in a new strain of virus particles that is ...
. Over time, mutation may lead to a change in antigenic properties such that host
antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...
(acquired through vaccination or prior infection) do not provide effective protection, causing a fresh outbreak of disease. The segmented genome of influenza viruses facilitates genetic reassortment. This can occur if a host is infected simultaneously with two different strains of influenza virus; then it is possible for the viruses to interchange genetic material as they reproduce in the host cells. Thus, an avian influenza virus can acquire characteristics, such as the ability to infect humans, from a different virus strain. The presence of both alpha 2,3 and alpha 2,6 sialic acid receptors in
pig The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...
tissues allows for co-infection by avian influenza and human influenza viruses. This susceptibility makes pigs a potential "melting pot" for the reassortment of influenza A viruses.


Epidemiology


History

Avian influenza (historically known as fowl plague) is caused by bird-adapted strains of the influenza type A virus. The disease was first identified by
Edoardo Perroncito Edoardo Bellarmino Perroncito (10 March 1847, Viale, Piedmont, Viale in the Asti (province), Province of Asti – 4 November 1936) was an Italian parasitologist. He was the father of pathologist Aldo Perroncito (1882–1929). He earned his degree ...
in 1878 when it was differentiated from other diseases that caused high mortality rates in birds; in 1955 it was established that the fowl plague virus was closely related to human influenza. In 1972, it became evident that many subtypes of avian flu were
endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
in wild bird populations. Between 1959 and 1995, there were 15 recorded outbreaks of highly pathogenic avian influenza (HPAI) in poultry, with losses varying from a few birds on a single farm to many millions. Between 1996 and 2008, HPAI outbreaks in poultry have been recorded at least 11 times and 4 of these outbreaks have resulted in the death or culling of millions of birds. Since then, several virus strains (both LPAI and HPAI) have become endemic among wild birds with increasingly frequent outbreaks among domestic poultry, especially of the H5 and H7 subtypes.


Transmission and prevention

Birds – Influenza A viruses of various subtypes have a large reservoir in wild waterbirds of the orders
Anseriformes Anseriformes is an order (biology), order of birds also known as waterfowl that comprises about 180 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest f ...
(for example, ducks, geese, and swans) and
Charadriiformes Charadriiformes (, from '' Charadrius'', the type genus of family Charadriidae) is a diverse order of small to medium-large birds. It includes about 390 species and has members in all parts of the world. Most charadriiform birds live near water ...
(for example, gulls, terns, and waders) which can infect the respiratory and gastrointestinal tract without affecting the health of the host. They can then be carried by the bird over large distances, especially during annual migration. Infected birds can shed avian influenza A viruses in their saliva, nasal secretions, and feces; susceptible birds become infected when they have contact with the virus as it is shed by infected birds. The virus can survive for long periods in water and at low temperatures, and can be spread from one farm to another on farm equipment. Domesticated birds (chickens, turkeys, ducks, etc.) may become infected with avian influenza A viruses through direct contact with infected waterfowl or other infected poultry, or through contact with contaminated feces or surfaces. Avian influenza outbreaks in domesticated birds are of concern for several reasons. There is potential for low pathogenic avian influenza viruses (LPAI) to evolve into strains which are high pathogenic to poultry (HPAI), and subsequent potential for significant illness and death among poultry during outbreaks. Because of this, international regulations state that any detection of H5 or H7 subtypes (regardless of their pathogenicity) must be notified to the appropriate authority. It is also possible that avian influenza viruses could be transmitted to humans and other animals which have been exposed to infected birds, causing infection with unpredictable but sometimes fatal consequences. When an HPAI infection is detected in poultry, it is normal to
cull Culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is removing or segregating animals from a breeding stock based on a specific trait. This is done to exagge ...
infected animals and those nearby in an effort to rapidly contain, control and eradicate the disease. This is done together with movement restrictions, improved hygiene and biosecurity, and enhanced surveillance. Humans – Avian flu viruses, both HPAI and LPAI, can infect humans who are in close, unprotected contact with infected poultry. Incidents of cross-species transmission are rare, with symptoms ranging in severity from no symptoms or mild illness, to severe disease that resulted in death. As of February 2024 there have been very few instances of human-to-human transmission, and each outbreak has been limited to a few people. All subtypes of avian Influenza A have potential to cross the species barrier, with
H5N1 Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus, which causes the disease avian influenza (often referred to as "bird flu"). It is enzootic (maintained in the population) in many bird populations, and also panzoo ...
and H7N9 considered the biggest threats. In order to avoid infection, the general public are advised to avoid contact with sick birds or potentially contaminated material such as carcasses or feces. People working with birds, such as conservationists or poultry workers, are advised to wear appropriate personal protection equipment. Other animals – a wide range of other animals have been affected by avian flu, generally due to eating birds which had been infected. There have been instances where transmission of the disease between mammals, including seals and cows, may have occurred.


Pandemic potential

Influenza viruses have a relatively high mutation rate that is characteristic of
RNA viruses ''Orthornavirae'' is a kingdom of viruses that have genomes made of ribonucleic acid (RNA), including genes which encode an RNA-dependent RNA polymerase (RdRp). The RdRp is used to transcribe the viral RNA genome into messenger RNA (mRNA) and t ...
. The segmentation of the influenza A virus
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
facilitates
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...
by segment
reassortment Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. The product of reassortment is called a reassortant. It is particularly used when two similar viruses that are infecting the same cell ...
in hosts who become infected with two different strains of influenza viruses at the same time. ''Figure 1 shows a diagramatic representation of the genetic relatedness of Asian H5N1
hemagglutinin The term hemagglutinin (alternatively spelt ''haemagglutinin'', from the Greek , 'blood' + Latin , 'glue') refers to any protein that can cause red blood cells (erythrocytes) to clump together (" agglutinate") ''in vitro''. They do this by bindin ...
genes from various isolates of the virus''
With reassortment between strains, an avian strain which does not affect humans may acquire characteristics from a different strain which enable it to infect and pass between humans – a
zoonotic A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a virus, bacterium, parasite, fungi, or prion) that can jump from a non-human vertebrate to a human. When h ...
event. It is thought that all influenza A viruses causing outbreaks or
pandemics A pandemic ( ) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic dis ...
among humans since the 1900s originated from strains circulating in wild aquatic birds through reassortment with other influenza strains. It is possible (though not certain) that pigs may act as an intermediate host for reassortment. As of June 2024, there is concern about two subtypes of avian influenza which are circulating in wild bird populations worldwide,
H5N1 Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus, which causes the disease avian influenza (often referred to as "bird flu"). It is enzootic (maintained in the population) in many bird populations, and also panzoo ...
and H7N9. Both of these have potential to devastate poultry stocks, and both have jumped to humans with relatively high case fatality rates.


Surveillance

The
Global Influenza Surveillance and Response System The Global Influenza Surveillance and Response System (GISRS) is a global network of laboratories that has the purpose to monitor the spread of influenza with the aim to provide the World Health Organization with influenza control information. It wa ...
(GISRS) is a global network of laboratories that monitor the spread of
influenza Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These sympto ...
with the aim to provide the
World Health Organization The World Health Organization (WHO) is a list of specialized agencies of the United Nations, specialized agency of the United Nations which coordinates responses to international public health issues and emergencies. It is headquartered in Gen ...
with influenza control information and to inform vaccine development. Several millions of specimens are tested by the GISRS network annually through a network of laboratories in 127 countries. As well as human viruses, GISRS monitors avian, swine, and other potentially
zoonotic A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a virus, bacterium, parasite, fungi, or prion) that can jump from a non-human vertebrate to a human. When h ...
influenza viruses.


Vaccines

Poultry – it is possible to vaccinate poultry against specific strains of HPAI influenza. Vaccination should be combined with other control measures such as infection monitoring, early detection and biosecurity. Humans – Several "candidate vaccines" are available in case an avian virus acquires the ability to infect and transmit among humans. There are strategic stockpiles of vaccines against the H5N1 subtype, which is considered the biggest risk. A vaccine against the H7N9 subtype, which has also infected humans, has undergone a limited amount of testing. In the event of an outbreak, the "candidate" vaccine would be rapidly tested for safety as well as efficacy against the zoonotic strain, and then authorised and distributed to vaccine manufacturers. Zoonotic influenza vaccine Seqirus is authorized for use in the European Union. It is an H5N8 vaccine that is intended to provide
acquired immunity The adaptive immune system (AIS), also known as the acquired immune system, or specific immune system is a subsystem of the immune system that is composed of specialized cells, organs, and processes that eliminate pathogens specifically. The ac ...
against H5 subtype influenza A viruses. In May 2025, the
United States Department of Health and Human Services The United States Department of Health and Human Services (HHS) is a cabinet-level executive branch department of the US federal government created to protect the health of the US people and providing essential human services. Its motto is ...
cancelled a $590 million contract with
Moderna Moderna, Inc. ( ) is an American pharmaceutical and biotechnology company based in Cambridge, Massachusetts, that focuses on RNA therapeutics, primarily mRNA vaccines. These vaccines use a copy of a molecule called messenger RNA (mRNA) to carry inst ...
for the development of an mRNA bird flu vaccine, though Moderna indicated that it would seek other options to move forward, citing successful early clinical trials.


Influenza A virus subtype H5N1

The highly pathogenic influenza A virus subtype
H5N1 Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus, which causes the disease avian influenza (often referred to as "bird flu"). It is enzootic (maintained in the population) in many bird populations, and also panzoo ...
is an emerging avian influenza virus that is causing global concern as a potential
pandemic A pandemic ( ) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic (epi ...
threat. It is often referred to simply as "bird flu" or "avian influenza", even though it is only one of many subtypes. A/H5N1 has killed millions of poultry in a growing number of countries throughout Asia, Europe, and Africa. Health experts are concerned that the coexistence of human flu viruses and avian flu viruses (especially H5N1) will provide an opportunity for genetic material to be exchanged between species-specific viruses, possibly creating a new virulent influenza strain that is easily transmissible and lethal to humans. Influenza A/H5N1 was first recorded in a small outbreak among poultry in Scotland in 1959, with numerous outbreaks subsequently in every continent. The first known transmission of A/H5N1 to a human occurred in
Hong Kong Hong Kong)., Legally Hong Kong, China in international treaties and organizations. is a special administrative region of China. With 7.5 million residents in a territory, Hong Kong is the fourth most densely populated region in the wor ...
in 1997, when there was an outbreak of 18 human cases resulting in 6 deaths. It was determined that all the infected people had been exposed to infected birds in poultry markets. As the disease continued to spread among poultry flocks in the territory, the decision was made to cull all 1.6 million poultry in the area and to impose strict controls on the movement and handling of poultry. This terminated the outbreak. There is weak evidence to support limited human-to-human transmission of A/H5N1 in 139 outbreaks between 2005 and 2009 in
Sumatra Sumatra () is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the list of islands by area, sixth-largest island in the world at 482,286.55 km2 (182,812 mi. ...
. The reproduction number was well below the threshold for sustained transmission.


Influenza A virus subtype H7N9

A significant outbreak of influenza A virus subtype H7N9 (A/H7N9) started in March 2013 when severe influenza affected 18 humans in China; six subsequently died. It was discovered that a low pathogenic strain of A/H7N9 was circulating among chickens, and that all the affected people had been exposed in poultry markets. Further cases among humans and poultry in mainland China continued to be identified sporadically throughout the year, followed by a peak around the festival season of
Chinese New Year Chinese New Year, or the Spring Festival (see also #Names, § Names), is a festival that celebrates the beginning of a New Year, new year on the traditional lunisolar calendar, lunisolar Chinese calendar. It is one of the most important holi ...
(January and February) in early 2014 which was attributed to the seasonal surge in poultry production. Up to December 2013, there had been 139 cases with 47 deaths. Infections among humans and poultry continued during the next few years, again with peaks around the new year. In 2016 a virus strain emerged which was highly pathogenic to chickens. In order to contain the HPAI outbreak, the Chinese authorities in 2017 initiated a large scale vaccination campaign against avian influenza in poultry. Since then, the number of outbreaks in poultry, as well as the number of human cases, dropped significantly. In humans, symptoms and mortality for both LPAI and HPAI strains have been similar. Although no human H7N9 infections have been reported since February 2019, the virus is still circulating in poultry, particularly in laying hens. It has demonstrated antigenic drift to evade vaccines, and remains a potential threat to the poultry industry and public health. Genetic and evolutionary analyses have shown that the A(H7) viruses in the Chinese outbreak probably transferred from domestic duck to chicken populations in China and then reassorted with poultry influenza A(H9N2) to generate the influenza A(H7N9) strain that affected humans. The genetic characteristics of A(H7N9) virus are of concern because of their pandemic potential, e.g. their potential to recognise human and avian influenza virus receptors which affects the ability to cause sustained human-to-human transmission, or the ability to replicate in the human host. Between February 2013 and February 2019, there were 1,568 confirmed human cases and 616 deaths associated with the outbreak in China. The majority of human cases have reported contact with poultry in markets or farms. Transmission between humans remains limited with some evidence of small family clusters. However, there is no evidence of sustained human-to-human transmission of A/H7N9 influenza. During early 2017, outbreaks of avian influenza A(H7N9) occurred in poultry in the USA. The strain in these outbreaks was of North American origin and is unrelated to the Asian lineage H7N9 which is associated with human infections in China.


Domestic animals

Several domestic species have been infected with and shown symptoms of H5N1 viral infection, including cats, dogs, ferrets, pigs, and birds.


Poultry

Attempts are made in the United States to minimize the presence of HPAI in poultry through routine surveillance of poultry flocks in commercial poultry operations. Detection of a HPAI virus may result in immediate
culling Culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is removing or segregating animals from a breeding stock based on a specific trait. This is done to exagge ...
of the flock. Less pathogenic viruses are controlled by vaccination.


Dairy cows

During April 2024, avian influenza was first detected in dairy cows in several US states and subsequently spread more widely through the year. Influenza A(H5N1) was found to be present at high levels in the
mammary glands A mammary gland is an exocrine gland that produces milk in humans and other mammals. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, human ...
and in the milk of affected cows. It was shown that the virus can persist on milking equipment, which provides a probable transmission route for cow-to-cow and cow-to-human spread. A number of humans who had been in contact with cows tested positive for the virus, with mild symptoms. According to CDC, 7% of 115 dairy workers had evidence of recent infection in a study from Michigan and Colorado from June to August 2024 – half of them asymptomatic. This is higher than estimates from prior transmission studies in poultry. All dairy workers had worked in cleaning the milk parlor and none had used
personal protective equipment Personal protective equipment (PPE) is protective clothing, helmets, goggles, or other garments or equipment designed to protect the wearer's body from injury or infection. The hazards addressed by protective equipment include physical, elect ...
. In February 2025, a second type of avian flu, named D1.1, was confirmed in cattle in Nevada, besides the B3.13 circulating in cattle since late 2023, which has infected more than 950 herds in 16 states. This new strain had been present in poultry and "more than a dozen people exposed to poultry" causing mild symptoms but was also found in a dairy worker in
Churchill County, Nevada Churchill County is a county in the western U.S. state of Nevada. As of the 2020 census, the population was 25,516. Its county seat is Fallon. Named for Mexican–American War hero brevet Brigadier General Sylvester Churchill, the county wa ...
.


Cats


Global aspects


Global measures

In 2005, the formation of the International Partnership on Avian and Pandemic Influenza was announced in order to elevate the importance of avian flu, coordinate efforts, and improve disease reporting and surveillance in order to better respond to future pandemics. New networks of laboratories have emerged to detect and respond to avian flu, such as the Crisis Management Center for Animal Health, the Global Avian Influenza Network for Surveillance, OFFLU, and the Global Early Warning System for major animal diseases. After the 2003 outbreak,
WHO The World Health Organization (WHO) is a specialized agency of the United Nations which coordinates responses to international public health issues and emergencies. It is headquartered in Geneva, Switzerland, and has 6 regional offices and 15 ...
member states have also recognized the need for more transparent and equitable sharing of vaccines and other benefits from these networks. Cooperative measures created in response to HPAI have served as a basis for programs related to other emerging and re-emerging infectious diseases.


Impact on national policies

HPAI control has also been used for political ends. In Indonesia, negotiations with global response networks were used to recentralize power and funding to the Ministry of Health. In Vietnam, policymakers, with the support of the
Food and Agriculture Organization The Food and Agriculture Organization of the United Nations; . (FAO) is a specialized agency of the United Nations that leads international efforts to defeat hunger and improve nutrition and food security. Its Latin motto, , translates ...
of the United Nations (FAO), used HPAI control to accelerate the industrialization of livestock production for export by proposing to increase the portion of large-scale commercial farms and reducing the number of poultry keepers from 8 to 2 million by 2010.


Traditional Asian practices

Backyard poultry production was viewed as "traditional Asian" agricultural practices that contrasted with modern commercial poultry production and seen as a threat to biosecurity. Backyard production appeared to hold greater risk than commercial production due to lack of biosecurity and close contact with humans, though HPAI spread in intensively raised flocks was greater due to high density rearing and genetic homogeneity. Asian culture itself was blamed as the reason why certain interventions, such as those that only looked at place-based interventions, would fail without looking for multifaceted solutions.


Economic impact

Avian influenza (AI), or bird flu, has significantly impacted the poultry industry with economic ramifications felt globally. The virus primarily affects poultry species, and when outbreaks occur, large-scale culling of infected birds is often necessary to prevent further spread. These measures disrupt egg production, causing supply shortages that directly lead to higher egg prices. For instance, during the 2022 avian influenza outbreaks, particularly in the U.S., millions of egg-laying hens were culled. As egg production diminished, prices surged, placing a financial burden on consumers and businesses reliant on eggs for food production. When egg prices increase, this low-cost source of animal proteins become less available to low-income consumers, making them the hardest hit. In February 2025, to combat avian influenza, the USDA created a five-pronged strategy to combat highly pathogenic avian influenza and lower egg prices. This included strengthening biosecurity measures, expediting relief for farmers to accelerate repopulation, reducing regulatory burdens to expand supply and lower prices, investing $100 million in avian flu research and vaccine development, and exploring temporary import-export adjustments to stabilize supply. Through this, the New York wholesale egg prices that peaked at $8.53 per dozen steadily declined to $4.08 from 26 February, to 19 March 2025. Approximately 20% of the protein consumed in developing countries come from poultry. A report by FAO totalled economic losses caused by avian influenza in South East Asia up to 2005 around US$10 billion. This had the greatest impact on small scale commercial and backyard producers. As poultry serves as a source of food security and liquid assets, the most vulnerable populations were poor, small scale farmers. The loss of birds due to HPAI and culling in Vietnam led to an average loss of 2.3 months of production and US$69–108 for households where many have an income of $2 a day or less. The loss of food security for vulnerable households can be seen in the stunting of children under five in Egypt. Women are another population at risk as in most regions of the world, small flocks are tended to by women. Widespread culling also resulted in the decreased enrollment of girls in school in Turkey.


See also

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Influenza pandemic An influenza pandemic is an epidemic of an influenza virus that spreads across a large region (either multiple continents or worldwide) and infects a large proportion of the population. There have been five major influenza pandemics in the l ...
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Influenza Genome Sequencing Project The Influenza Genome Sequencing Project (IGSP), initiated in early 2004, seeks to investigate influenza evolution by providing a public data set of complete influenza genome sequences from collections of isolates representing diverse species dist ...


References


Further reading

* * * {{DEFAULTSORT:Avian Influenza Bird diseases Poultry diseases Agricultural health and safety Zoonotic viral diseases