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 envelope, which may be acquired by the capsid from an infected host cell.
102-405: Avian influenza , also known as avian flu or bird flu , is a disease caused by the influenza A virus , 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 (continually present) in many bird populations. Symptoms of avian influenza vary according to both the strain of virus underlying
204-420: A cold , although usually more severe and less likely to include a runny nose . The onset of symptoms is sudden, and initial symptoms are predominately non-specific: a sudden fever; muscle aches; cough; fatigue; sore throat; headache; difficulty sleeping; loss of appetite; diarrhoea or abdominal pain; nausea and vomiting. Humans can rarely become infected with strains of avian or swine influenza , usually as
306-400: A zoonotic event. It is thought that all influenza A viruses causing outbreaks or pandemics 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. The Global Influenza Surveillance and Response System (GISRS)
408-452: A HPAI virus may result in immediate culling of the flock. Less pathogenic viruses are controlled by vaccination. 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 and in the milk of affected cows. It was shown that the virus can persist on milking equipment, which provides
510-582: A basis for programs related to other emerging and re-emerging infectious diseases. 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 of the United Nations (FAO), used HPAI control to accelerate
612-593: A common ancestor around 8,000 years ago. Outbreaks of influenza-like disease can be found throughout recorded history. The first probable record is by Hippocrates in 142 BCE. The historian Fujikawa listed 46 epidemics of flu-like illness in Japan between 862 and 1868. In Europe and the Americas, a number of epidemics were recorded through the Middle Ages and up to the end of the 19th century. In 1918-1919 came
714-428: A dry, hacking cough, depression, loss of appetite and weakness. EI is caused by two subtypes of influenza A viruses: H7N7 and H3N8, which have evolved from avian influenza A viruses. Most animals infected with canine influenza A will show symptoms such as coughing, runny nose, fever, lethargy, eye discharge, and a reduced appetite lasting anywhere from 2–3 weeks. There are two different influenza A dog flu viruses: one
816-430: A factor in determining longevity of a virus on inanimate surfaces. Enveloped viruses possess great adaptability and can change in a short time in order to evade the immune system. Enveloped viruses can cause persistent infections . Vaccination against enveloped viruses can function by neutralizing the glycoprotein activity with antibodies. Eliminating the virus's ability to form an envelope—by removing or inactivating
918-462: A few people. All subtypes of avian Influenza A have potential to cross the species barrier, with H5N1 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 –
1020-418: 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
1122-462: A layer between the nucleoprotein and the envelope, called the capsid . Envelope - The viral envelope consists of a lipid bilayer derived from the host cell. Two viral proteins; hemagglutinin (HA) and neuraminidase (NA), are inserted into the envelope and are exposed as spikes on the surface of the virion. Both proteins are antigenic ; a host's immune system can react to them and produce antibodies in response. The M2 protein forms an ion channel in
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#17327761409581224-669: A lipid envelope. The virus particle (also called the virion ) is 80–120 nanometers in diameter such that the smallest virions adopt an elliptical shape; larger virions have a filamentous shape. Core - The central core of the virion contains the viral RNA genome, which is made of eight separate segments. The nucleoprotein (NP) coats the viral RNA to form a ribonucleoprotein that assumes a helical (spiral) configuration. Three large proteins (PB 1 , PB 2 , and PA), which are responsible for RNA transcription and replication, are bound to each segment of viral RNP. Capsid - The matrix protein M1 forms
1326-543: 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 (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
1428-404: A mechanism called cap-snatching . It consists in the hijacking and cleavage of host capped pre-mRNAs . Host cell mRNA is cleaved near the cap to yield a primer for the transcription of positive-sense viral mRNA using the negative-sense viral RNA as a template. The host cell then transports the viral mRNA into the cytoplasm where ribosomes manufacture the viral proteins. Replication of
1530-593: A pandemic in 2009. Because of its close resemblance to some strains circulating in pigs, this became known as " Swine flu " Influenza A virus continues to circulate and evolve in birds and pigs. Almost all possible combinations of H (1 thru 16) and N (1 thru 11) have been isolated from wild birds. As of June 2024, two particularly virulent IAV strains - H5N1 and H7N9 - are predominant in wild bird populations. These frequently cause outbreaks in domestic poultry, with occasional spillover infections in humans who are in close contact with poultry. Influenza viruses have
1632-400: A potential "melting pot" for the reassortment of influenza A viruses. 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 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
1734-455: 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
1836-441: A process known as antigenic drift . Over time, mutation may lead to a change in antigenic properties such that host antibodies (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
1938-452: A protein shell known as the capsid, from the Latin capsa, meaning "box," in order to shield it from this hostile environment. Similar to how numerous bricks come together to form a wall, the capsid is made up of one or more distinct protein types that repeatedly repeat to form the whole capsid. This repetitive pattern creates a robust but rather flexible capsid. The nucleic acid inside the capsid
2040-467: A relatively high mutation rate that is characteristic of RNA viruses . The segmentation of the influenza A virus genome facilitates genetic recombination by segment reassortment in hosts who become infected with two different strains of influenza viruses at the same time. 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 -
2142-407: A result of close contact with infected animals or contaminated material; symptoms generally resemble seasonal flu but occasionally can be severe including death. Some species of wild aquatic birds act as natural asymptomatic carriers of a large variety of influenza A viruses, which they can spread over large distances in their annual migration. Symptoms of avian influenza vary according to both
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#17327761409582244-474: A result of close contact with infected animals; symptoms range from mild to severe including death. Bird-adapted strains of the virus can be asymptomatic in some aquatic birds but lethal if they spread to other species, such as chickens. IAV disease in poultry can be can be prevented by vaccination, however biosecurity control measures are preferred. In humans, seasonal influenza can be treated in its early stages with antiviral medicines. A global network,
2346-454: A result, flaviviruses and pseudo typed retroviruses have a harder time infecting TAM-deficient DCs, albeit infection can be brought back by type I IFN antibodies. A TAM kinase inhibitor, meanwhile, prevents infection of wild-type DCs. TAM receptors, which are potential targets for therapy, are thereby activated by viruses to reduce type I IFN signaling. Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on
2448-534: 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 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
2550-644: A type-1 hemagglutinin (H) protein and a type-1 neuraminidase (N) protein. 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, as well as to the severity of symptoms. Symptoms of human seasonal flu usually include fever, cough, sore throat , muscle aches , conjunctivitis and, in severe cases, breathing problems and pneumonia that may be fatal. Humans can rarely become infected with strains of avian or swine influenza , usually as
2652-491: A type-5 hemagglutinin (H) protein and a type-1 neuraminidase (N) protein. Further variations exist within the subtypes and can lead to very significant differences in the virus's behavior. By definition, the subtyping scheme only takes into account the two outer proteins, not the at least 8 proteins internal to the virus. Almost all possible combinations of H (1 thru 16) and N (1 thru 11) have been isolated from wild birds. H17 and H18 have only been discovered in bats. Due to
2754-488: A virus buds often dies or is weakened, and sheds more viral particles for an extended period. The lipid bilayer envelope of these viruses is relatively sensitive to desiccation , heat, and amphiphiles such as soap and detergents , therefore these viruses are easier to sterilize than non-enveloped viruses, have limited survival outside host environments, and typically must transfer directly from host to host. Viral envelope persistence, whether it be enveloped or naked, are
2856-531: 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. Influenza viruses have a relatively high mutation rate that is characteristic of RNA viruses . The segmentation of the influenza A virus genome facilitates genetic recombination by segment reassortment in hosts who become infected with two different strains of influenza viruses at
2958-576: Is a global network of laboratories that monitor the spread of influenza with the aim to provide the World Health Organization 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 influenza viruses. Flu season
3060-447: Is a global network of laboratories that monitor the spread of influenza with the aim to provide the World Health Organization 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 influenza viruses. Poultry – it
3162-453: Is an enveloped negative-sense RNA virus , with a segmented genome. Through a combination of mutation and genetic reassortment the virus can evolve to acquire new characteristics, enabling it to evade host immunity and occasionally to jump from one species of host to another. Subtypes of IAV are defined by the combination of the antigenic H and N proteins in the viral envelope ; for example, " H1N1 " designates an IAV subtype that has
Avian influenza - Misplaced Pages Continue
3264-455: Is an H3N8 virus and the other is an H3N2 virus. The H3N8 strain has evolved from an equine influenza avian virus which has adapted to sustained transmission among dogs. The H3N2 strain is derived from an avian influenza which jumped to dogs in 2004 in either Korea or China. It is likely that the virus persists in both animal shelters and kennels, as well as in farms where dogs are raised for meat production. The first bat flu virus, IAV(H17N10),
3366-588: Is an annually recurring time period characterized by the prevalence of an outbreak of influenza , caused either by Influenza A or by Influenza B . The season occurs during the cold half of the year in temperate regions; November through February in the northern hemisphere and May to October in the southern hemisphere. Flu seasons also exist in the tropics and subtropics , with variability from region to region. Annually, about 3 to 5 million cases of severe illness and 290,000 to 650,000 deaths from seasonal flu occur worldwide. There are several possible reasons for
3468-422: Is appropriately protected by its modest size and physical difficulty in opening it. The nucleocapsid of the virion is made up of the nucleic acid and the capsid. Remember that the genomes of most viruses are very small. Genes code for instructions to make proteins, so small genomes cannot code for many proteins. Therefore, the virion capsid consists of one or only a few proteins that repeat over and over to form
3570-676: 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, or if it is genetically similar to such a strain. The alternative classification is low pathogenic avian influenza (LPAI). Classification of a virus strain as either LPAI or HPAI is based on the severity of symptoms in domestic chickens and does not predict severity of symptoms in other species. Chickens infected with LPAI display mild symptoms or are asymptomatic , whereas HPAI causes serious breathing difficulties, significant drop in egg production, and sudden death. Since 2006,
3672-513: 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
3774-555: Is caused by the influenza A virus which principally affects birds but can also infect humans and other mammals. Influenza A is an RNA virus with a genome comprising a negative-sense , RNA segmented genome that encodes for 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 on
3876-463: Is caused by the viral fusion protein . Many enveloped viruses only have one protein visible on the surface of the particle, which is required for both mediating adhesion to the cell surface and for the subsequent membrane fusion process. To create potentially protective vaccines for human pathogenic enveloped viruses for which there is currently no vaccine, it is essential to comprehend how antibodies interact with viral envelope proteins, particularly with
3978-511: Is concern about two subtypes of avian influenza which are circulating in wild bird populations worldwide, H5N1 and H7N9 . Both of these have potential to devastate poultry stocks, and both have jumped to humans with relatively high case fatality rates . H5N1 in particular has infected a wide range of mammals and may be adapting to mammalian hosts. As of June 2024, the influenza viruses which circulate widely in humans are IAV subtypes H1N1 and H3N2, together with Influenza B. Annual vaccination
4080-456: 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
4182-677: Is detected in poultry, it is normal to cull 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
Avian influenza - Misplaced Pages Continue
4284-443: 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 and H7N9 . Both of these have potential to devastate poultry stocks, and both have jumped to humans with relatively high case fatality rates . The Global Influenza Surveillance and Response System (GISRS)
4386-412: 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 tissues allows for co-infection by avian influenza and human influenza viruses. This susceptibility makes pigs
4488-461: 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
4590-596: Is possible to vaccinate poultry and pigs against specific strains of influenza. Vaccination should be combined with other control measures such as infection monitoring, early detection and biosecurity. The main treatment for mild influenza is supportive; rest, fluids, and over-the-counter medicines to alleviate symptoms while the body's own immune system works to recover from infection. Antiviral drugs are recommended for those with severe symptoms, or for those who are at risk of developing complications such as pneumonia. The symptoms of seasonal flu are similar to those of
4692-630: 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
4794-438: Is spread either through close contact between animals or by the movement of contaminated equipment between farms. Humans who are in close contact with pigs can sometimes become infected. Equine influenza can affect horses, donkeys, and mules; it has a very high rate of transmission among horses, and a relatively short incubation time of one to three days. Clinical signs of equine influenza include fever, nasal discharge, have
4896-588: Is that the HA protein of avian influenza binds to alpha-2,3 sialic acid receptors, which are present in the respiratory tract 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. Other factors include the ability to replicate the viral RNA genome within the host cell nucleus, to evade host immune responses, and to transmit between individuals. Influenza viruses are constantly changing as small genetic mutations accumulate,
4998-443: Is the primary and most effective way to prevent influenza and influenza-associated complications, especially for high-risk groups. Vaccines against the flu are trivalent or quadrivalent, providing protection against the dominant strains of IAV(H1N1) and IAV(H3N2), and one or two influenza B virus strains; the formulation is continually reviewed in order to match the predominant strains in circulation. Poultry and other animals - it
5100-572: The Global Influenza Surveillance and Response System (GISRS) monitors the spread of influenza with the aim to inform development of both seasonal and pandemic vaccines. 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 influenza viruses. IAV vaccines need to be reformulated regularly in order to keep up with changes in
5202-509: The World Health Organization has recorded 948 cases of confirmed H5N1 influenza, leading to 464 deaths. The true fatality rate may be lower because some cases with mild symptoms may not have been identified as H5N1. 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
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#17327761409585304-584: The World Organization for Animal Health requires all detections of LPAI H5 and H7 subtypes to be reported because of their potential to mutate into highly pathogenic strains. Signs of swine flu in pigs can include fever, depression, coughing (barking), discharge from the nose or eyes, sneezing, breathing difficulties, eye redness or inflammation, and going off feed. Some pigs infected with influenza, however, may show no signs of illness at all. Swine flu subtypes are principally H1N1, H1N2, and H3N2; it
5406-541: The ligands Gas6 and Protein S . The phospholipid phosphatidylserine may be seen on the membranes of several enveloped viruses, which they employ to bind Gas6 and Protein S to activate TAM receptors. Ligand-coated viruses stimulate type I IFN signaling, activate TAM receptors on dendritic cells (DCs), and suppress type II interferon signaling to circumvent host defenses and advance infection.TAM-deficient DCs exhibit type I IFN responses that are more pronounced than those of wild-type cells in response to viral exposure. As
5508-445: The viral core (capsid). For a very long time, it was thought that the spike proteins, which are necessary for infectivity, were directly incorporated into the viral core through their cytoplasmic domains. Recent research suggests that while such direct interactions may be what causes the budding of alphaviruses, this may not be the case for retroviruses and negative strand RNA viruses . These viruses can form bud particles even in
5610-649: 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 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
5712-507: The European Union. It is an H5N8 vaccine that is intended to provide acquired immunity against H5 subtype influenza A viruses. The highly pathogenic influenza A virus subtype H5N1 is an emerging avian influenza virus that is causing global concern as a potential pandemic 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
5814-617: The absence of spike proteins by relying only on viral core components. The spike proteins can occasionally be produced as virus-like particles without the viral core. Therefore, optimal budding and release may be dependent on a coordinated "push-and-pull" action between core and spike, where oligomerization of both components is essential. They may help viruses avoid the host immune system . TAM receptor tyrosine kinases increase phagocytic clearance of apoptotic cells and inhibit immunological responses brought on by Toll-like receptors and type I interferons (IFNs) when they are activated by
5916-427: The core of new virions. The predominant natural reservoir of influenza viruses is thought to be wild waterfowl. The subtypes of influenza A virus are estimated to have diverged 2,000 years ago. Influenza viruses A and B are estimated to have diverged from a single ancestor around 4,000 years ago, while the ancestor of influenza viruses A and B and the ancestor of influenza virus C are estimated to have diverged from
6018-522: 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. Birds – Influenza A viruses of various subtypes have a large reservoir in wild waterbirds of the orders Anseriformes (for example, ducks, geese, and swans) and Charadriiformes (for example, gulls, terns, and waders) which can infect
6120-409: The envelope and is responsible for uncoating the virion once it has bound to a host cell. The table below presents a concise summary of the influenza genome and the principal functions of the proteins which are encoded. Segments are conventionally numbered from 1 to 8 in descending order of length. PB2 also inhibits JAK1/STAT signaling to inhibit host innate immune response It also degrades
6222-402: The evolution of the virus over time. Rarely, 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 these instances have not sustained transmission from one person to another. There are a number of factors that generally prevent avian flu from causing epidemics in humans or other mammals. One of them
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#17327761409586324-497: The expression of cell surface proteins implicated in adaptive immunity. Being made up mostly of host membrane, the viral envelope can also have the proteins associated with the host cell within their membrane after budding. Many enveloped viruses mature by budding at the plasma membrane, which allows them to be discharged from infected cells. During this procedure, viral transmembrane proteins, also known as spike proteins , are integrated into membrane vesicles containing components of
6426-475: The first flu pandemic of the 20th century, known generally as the " Spanish flu ", which caused an estimated 20 to 50 million deaths worldwide. It is now known that this was caused by an immunologically novel H1N1 subtype of influenza A. The next pandemic took place in 1957, the " Asian flu ", which was caused by a H2N2 subtype of the virus in which the genome segments coding for HA and NA appeared to have derived from avian influenza strains by reassortment, while
6528-465: The fusion protein to the cell surface but may also happen extracellularly. So far, structural studies have revealed two kinds of viral fusion proteins. These proteins are believed to catalyze the same mechanism in both situations, resulting in the fusing of two bilayers. In other words, these proteins operate as enzymes, which while having various structural variations catalyze the same chemical reaction. The envelopes are typically derived from portions of
6630-475: The fusion protein, and how antibodies neutralize viruses. Enveloped viruses enter cells by joining a cellular membrane to their lipid bilayer membrane. Priming by proteolytic processing, either of the fusion protein or of a companion protein, is necessary for the majority of viral fusion proteins. The priming stage then gets the fusion protein ready for triggering by the processes that go along with attachment and uptake, which frequently happens during transport of
6732-584: 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 or subclade , revealing links between different samples of virus and tracing
6834-516: The greatest impact on small scale commercial and backyard producers. Influenza A virus Influenza A virus (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 causing seasonal flu in humans. Mammals in which different strains of IAV circulate with sustained transmission are bats, pigs, horses and dogs; other mammals can occasionally become infected. IAV
6936-486: The high variability of the virus, subtyping is not sufficient to uniquely identify a strain of influenza A virus. To unambiguously describe a specific isolate of virus, researchers use the Influenza virus nomenclature, which describes, among other things, the subtype, year, and place of collection. Some examples include: The influenza A virus has a negative-sense , single-stranded, segmented RNA genome , enclosed in
7038-498: The host cell membranes ( phospholipids and proteins), but include some viral glycoproteins . One of the main parts of human pathogenic viruses is glycoprotein. They have been shown to play significant roles in immunity and infection. Viral glycoproteins, a new class of cellular inhibitory proteins has been discovered. These include the E3 ubiquitin ligases of the membrane-associated RING-CH (MARCH) family, which among other things, inhibits
7140-575: The host cell's mitochondrial antiviral signaling protein At the early stage of infection, the RNP binds to the host cell's importin-α which transports it into the host cell nucleus, where the viral RNA is transcribed and replicated. At a later stage of infection, newly manufactured viral RNA segments assemble with the NP protein and polymerase (PB1, PB2 and PA) to form the core of a progeny virion NA also facilitates
7242-419: The host cell. There are three main types of viral glycoproteins: Envelope proteins, membrane proteins, and spike proteins (E, M, and S). The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host. All enveloped viruses also have a capsid , another protein layer, between the envelope and the genome. The virus wraps its delicate nucleic acid with
7344-441: The host's membrane. The particular set of viral proteins are engaged in a series of structural changes. When these changes are set/finished, there is then and only then, fusion with the host membrane. These glycoproteins mediate the interaction between virion and host cell, typically initiating the fusion between the viral envelope and the host's cellular membrane. In some cases, the virus with an envelope will form an endosome within
7446-562: 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. 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
7548-452: 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. Avian influenza
7650-554: 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 chickens and does not predict severity of symptoms in other species. Chickens infected with LPAI display mild symptoms or are asymptomatic , whereas HPAI causes serious breathing difficulties, significant drop in egg production, and sudden death. Domestic poultry may potentially be protected from specific strains of
7752-457: 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
7854-498: The milk parlor and none had used personal protective equipment. 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
7956-531: The movement of infective virus particles through mucus, enabling them to reach host epithelial cells. M1 also assists with the function of the NEP protein. Three viral proteins - PB1, PB2, and PA - associate to form the RNA-dependent RNA polymerase (RdRp) which functions to transcribe and replicate the viral RNA. Viral messenger RNA Transcription - The RdRp complex transcribes viral mRNAs by using
8058-654: 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,
8160-456: 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
8262-446: The potential to decimate domesticated poultry stocks and an estimated half a billion farmed birds have been slaughtered in efforts to contain the virus. 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
8364-513: The relationship between influenza viruses and bacteria, how influenza symptoms progress, and why some influenza viruses are deadlier than others. Past pandemics, and especially the 1918 pandemic, are the subject of much research to understand and prevent flu pandemics. The World Health Organization has published a Research Agenda with five streams: Viral envelope Numerous human pathogenic viruses in circulation are encased in lipid bilayers , and they infect their target cells by causing
8466-459: 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 of influenza A are defined by the combination of H and N proteins in the viral envelope ; for example, " H5N1 " 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
8568-465: The remainder of the genome was descended from the 1918 virus. The 1968 pandemic (" Hong Kong flu " ) was caused by a H3N2 subtype in which the NA segment was derived from the 1957 virus, while the HA segment had been reassorted from an avian strain of influenza. In the 21st century, a strain of H1N1 flu (since titled H1N1pdm09 ) which was antigenically very different from previous H1N1 strains, leading to
8670-840: 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
8772-435: The same time. 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 event. It is thought that all influenza A viruses causing outbreaks or pandemics among humans since the 1900s originated from strains circulating in wild aquatic birds through reassortment with other influenza strains. It
8874-414: 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) : 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). Analysis of the virus' genome enables researchers to determine the order of its nucleotides. Comparison of
8976-492: The strain of virus underlying the infection, and on the species of bird affected. Symptoms of influenza in birds may include swollen head, watery eyes, unresponsiveness, lack of coordination, respiratory distress such as sneezing or gurgling. Because of the impact of avian influenza on economically important chicken farms, avian virus strains are classified as either highly pathogenic (and therefore potentially requiring vigorous control measures) or low pathogenic. The test for this
9078-431: The structure. The viral nucleic acid would be physically too large to fit inside the capsid if it consisted of more than a few proteins. The capsid, having a focused role of protecting the genome in addition to immune recognition evasion. The viral capsid is known for its protection of RNA before it is inserted into the host cell, unlike the viral envelope which protects the protein capsid. The cell from which
9180-452: The surface of the viral envelope ; 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 (H) is an antigenic glycoprotein which allows the virus to bind to and enter the host cell. Neuraminidase (N) is an antigenic glycosylated enzyme which facilitates
9282-487: 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. The reproduction number was well below the threshold for sustained transmission. Between 2003 and November 2024,
9384-493: 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. To unambiguously describe a specific isolate of virus, researchers use the internationally accepted Influenza virus nomenclature, which describes, among other things,
9486-417: The viral RNA - The replication of the influenza virus, unlike most other RNA viruses , takes place in the nucleus and involves two steps. The RdRp first of all transcribes the negative-sense viral genome into a positive-sense complimentary RNA (cRNA), then the cRNAs are used as templates to transcribe new negative-sense vRNA copies. These are exported from the nucleus and assemble near the cell membrane to form
9588-460: The viral envelope and cell membrane to fuse. Although there are effective vaccines against some of these viruses, there is no preventative or curative medicine for the majority of them. In most cases, the known vaccines operate by inducing antibodies that prevent the pathogen from entering cells. This happens in the case of enveloped viruses when the antibodies bind to the viral envelope proteins. The membrane fusion event that triggers viral entrance
9690-673: 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 (A/H5N1) has
9792-480: 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
9894-622: The virus. There are two methods of classification, one based on surface proteins (originally serotypes ), and the other based on its behavior, mainly the host animal . There are two antigenic proteins on the surface of the viral envelope, hemagglutinin and neuraminidase . Different influenza virus genomes encode different hemagglutinin and neuraminidase proteins. Based on their serotype , there are 18 known types of hemagglutinin and 11 types of neuraminidase. Subtypes of IAV are classified by their combination of H and N proteins. For example, " H5N1 " designates an influenza A subtype that has
9996-949: The winter peak in temperate regions: A zoonosis a disease in a human caused by a pathogen (such as a bacterium , or virus ) that has jumped from a non-human to a human . Avian and pig influenza viruses can, on rare occasions, transmit to humans and cause zoonotic influenza virus infections; these infections are usually confined to people who have been in close contact with infected animals or material such as infected feces and meat, they do not spread to other humans. Symptoms of these infections in humans vary greatly; some are in asymptomatic or mild while others can cause severe disease, leading to severe pneumonia and death. A wide range of Influenza A virus subtypes have been found to cause zoonotic disease. Zoonotic infections can be prevented by good hygiene, by preventing farmed animals from coming into contact with wild animals, and by using appropriate personal protective equipment. As of June 2024, there
10098-453: Was closely related to human influenza. In 1972, it became evident that many subtypes of avian flu were endemic 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
10200-465: Was first discovered in 2009 in little yellow-shouldered bats ( Sturnira lilium ) in Guatemala . In 2012 a second bat influenza A virus IAV(H18N11) was discovered in flat-faced fruit-eating bats ( Artibeus planirostris ) from Peru . Bat influenza viruses have been found to be poorly adapted to non-bat species. Influenza research includes efforts to understand how influenza viruses enter hosts,
10302-534: 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 placed-based interventions, would fail without looking for multifaceted solutions. 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
10404-560: Was of North American origin and is unrelated to the Asian lineage H7N9 which is associated with human infections in China. Several domestic species have been infected with and shown symptoms of H5N1 viral infection, including cats, dogs, ferrets, pigs, and birds. 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
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