Influenza A virus subtype H7N9

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Influenza A virus subtype H7N9
Virus classification OOjs UI icon edit-ltr.svg
(unranked): Virus
Realm: Riboviria
Kingdom: Orthornavirae
Phylum: Negarnaviricota
Class: Insthoviricetes
Order: Articulavirales
Family: Orthomyxoviridae
Genus: Alphainfluenzavirus
Species:
Influenza A virus
Serotype:
Influenza A virus subtype H7N9

Influenza A virus subtype H7N9 (A/H7N9) is a subtype of the influenza A virus, which causes influenza (flu), predominantly in birds. It is enzootic (maintained in the population) in many bird populations. [1] The virus can spread rapidly through poultry flocks and among wild birds; it can also infect humans that have been exposed to infected birds. [2]

Contents

A/H7N9 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. [2]

Symptoms of A/H7N9 influenza vary according to both the strain of virus underlying the infection and on the species of bird or mammal affected. [3] [4] Classification 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 the severity of symptoms in humans. [5] Chickens infected with LPAI A/H7N9 virus display mild symptoms or are asymptomatic, whereas HPAI A/H7N9 causes serious breathing difficulties, a significant drop in egg production, and sudden death. [6]

In mammals, including humans, A/H7N9 influenza (whether LPAI or HPAI) is rare; it can usually be traced to close contact with infected poultry or contaminated material such as feces. [7] Symptoms of infection vary from mild to severe, including fever, diarrhoea, and cough; the disease can often be fatal. [8] [9]

The A/H7N9 virus is considered to be enzootic (continually present) in wild aquatic birds, which may carry the virus over large distances during their migration. [10] The first known case of A/H7N9 influenza infecting humans was reported in March 2013, in China. [11] Cases continued to be recorded in poultry and humans in China over the course of the next 5 years. Between February 2013 and February 2019 there were 1,568 confirmed human cases and 616 deaths associated with the outbreak in China. [12] Initially the virus was low pathogenic to poultry, however around 2017 a highly pathogenic strain developed which became dominant. The outbreak in China has been partially contained by a program of poultry vaccination which commenced in 2017. [13]

Bird-adapted A/H7N9 transmits relatively easily from poultry to humans, although human to human transmission is rare. Its ability to cross the species barrier renders it a potential pandemic threat, especially if it should acquire genetic material from a human-adapted strain. [14] [15]

Virology

The N in H7N9 stands for "Neuraminidase", the protein depicted in this ribbon diagram. Neuraminidase Ribbon Diagram.jpg
The N in H7N9 stands for "Neuraminidase", the protein depicted in this ribbon diagram.

H7N9 is a subtype of Influenza A virus. Like all subtypes it is an enveloped negative-sense RNA virus, with a segmented genome. [16] Influenza viruses have a relatively high mutation rate that is characteristic of RNA viruses. [17] The segmentation of its genome facilitates genetic recombination by reassortment in hosts infected with two different strains of influenza viruses at the same time. [18] [19] 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. [20] [21]

Highly pathogenic avian influenza

This section is an excerpt from Avian influenza § Highly pathogenic avian influenza.[ edit ]

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). [22] This classification system has since been modified to take into account the structure of the virus' haemagglutinin protein. [23] 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. [22] 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. [22] [24]

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. [25]

Epidemiology

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. [26] Symptoms of avian influenza vary according to both 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. [27]

Humans and other mammals can only become infected with avian influenza, including A/H7N9, after prolonged close contact with infected birds or contaminated environments. [28] In mammals including humans, infection with avian influenza (whether LPAI or HPAI) is rare. Symptoms of infection vary from mild to severe, including fever, diarrhoea, and cough. [29]

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. [30] All subtypes of avian Influenza A have potential to cross the species barrier into humans, with H5N1 and H7N9 considered the biggest threats. [31] [32]

History

Outbreak in China, 2013-2019

Live poultry market in Xining, China, 2008. Chicken market in Xining, Qinghai province, China.jpg
Live poultry market in Xining, China, 2008.

Prior to 2013, A/H7N9 had previously been isolated only in birds, with outbreaks reported in the Netherlands, Japan, and the United States. Until the 2013 outbreak in China, no human infections with A/H7N9 had been reported. [7] [33]

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. [14] 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. [34] 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.

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. [13] [35] 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. [13] 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. [35]

As of May 2022, a total of 1568 confirmed A(H7N9) human infections with 616 deaths have been reported, with a case fatality ratio (CFR) of 39%. [36]

Genetic characterisation of the "Asian lineage" strain of avian influenza A/H7N9 shows that it was not related to A/H7N9 strains previously identified in Europe and North America. This new strain resulted from the recombination of genes between several parent viruses noted in poultry and wild birds in Asia. The H7 gene is most closely related to sequences found in samples from ducks in Zhejiang province in 2011.The N9 gene was closely related to isolated wild ducks in South Korea in 2011. Other genes resembled samples collected in Beijing and Shanghai in 2012. The genes would have been carried along the East Asian flyway by wild birds during their annual migration. [37] [38] [39]

The genetic characteristics of Asian lineage A/H7N9 virus are of particular concern because of their pandemic potential. The virus is able to infect humans relatively easily, but does not sustain human-to-human transmission. If the virus were to acquire the ability to transmit easily between humans (either through mutation or genetic reassortment) there is potential for a severe epidemic or pandemic. [14] [13]

Other occurrences

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. [40]

In May 2024, an HPAI A/H7N9 was detected on a poultry farm with 160,000 birds in Terang, Australia. There were 14,000 clinically affected birds. It is presumed that migratory wild birds were the source of the outbreak. [41]

Vaccine

The continuing evolution and antigenic drift of the A/H7N9 virus makes further development difficult as any significant outbreak could involve a completely new strain. A number of human vaccines targeting A/H7N9 have been trialled in relatively small groups of subjects; in the event of any outbreak, a candidate vaccine virus would be distributed to manufacturers [42] [43] [44]

See also

Related Research Articles

<i>Influenza A virus</i> Species of 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.

<span class="mw-page-title-main">Avian influenza</span> Influenza caused by viruses adapted to birds

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 in many bird populations.

<span class="mw-page-title-main">Influenza A virus subtype H5N1</span> Subtype of influenza A virus

Influenza A virus subtype H5N1 (A/H5N1) is a subtype of the influenza A virus, which causes influenza (flu), predominantly in birds. It is enzootic in many bird populations, and also panzootic. A/H5N1 virus can also infect mammals that have been exposed to infected birds; in these cases, symptoms are frequently severe or fatal.

<span class="mw-page-title-main">Influenza pandemic</span> Pandemic involving influenza

An influenza pandemic is an epidemic of an influenza virus that spreads across a large region and infects a large proportion of the population. There have been six major influenza epidemics in the last 140 years, with the 1918 flu pandemic being the most severe; this is estimated to have been responsible for the deaths of 50–100 million people. The 2009 swine flu pandemic resulted in under 300,000 deaths and is considered relatively mild. These pandemics occur irregularly.

<span class="mw-page-title-main">Influenza A virus subtype H3N2</span> Virus subtype

Influenza A virus subtype H3N2 (A/H3N2) is a subtype of influenza A virus (IAV). Some human-adapted strains of A/H3N2 are endemic in humans and are one cause of seasonal influenza (flu). Other strains of H1N1 are endemic in pigs and in birds. 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 a type-1 hemagglutinin (H) protein and a type-1 neuraminidase (N) protein.

<span class="mw-page-title-main">Global spread of H5N1</span> Spread of bird flu

The global spread of H5N1 influenza in birds is considered a significant pandemic threat. While other H5N1 influenza strains are known, they are significantly different on a genetic level from a highly pathogenic, emergent strain of H5N1, which was able to achieve hitherto unprecedented global spread in 2008. The H5N1 strain is a fast-mutating, highly pathogenic avian influenza virus (HPAI) found in multiple bird species. It is both epizootic and panzootic. Unless otherwise indicated, "H5N1" in this timeline refers to the 2008 highly pathogenic strain of H5N1.

<span class="mw-page-title-main">Transmission and infection of H5N1</span> Spread of an influenza virus

Transmission and infection of H5N1 from infected avian sources to humans has been a concern since the first documented case of human infection in 1997, due to the global spread of H5N1 that constitutes a pandemic threat.

<span class="mw-page-title-main">Influenza A virus subtype H5N2</span> Virus subtype

H5 N2 is a subtype of the species Influenzavirus A. The subtype infects a wide variety of birds, including chickens, ducks, turkeys, falcons, and ostriches. Affected birds usually do not appear ill, and the disease is often mild as avian influenza viral subtypes go. Some variants of the subtype are much more pathogenic than others, and outbreaks of "high-path" H5N2 result in the culling of thousands of birds in poultry farms from time to time. It appears that people who work with birds can be infected by the virus, but suffer hardly any noticeable health effects. Even people exposed to the highly pathogenic H5N2 variety that killed ostrich chicks in South Africa only seem to have developed conjunctivitis, or a perhaps a mild respiratory illness. There is no evidence of human-to-human spread of H5N2. On November 12, 2005 it was reported that a falcon was found to have H5N2. On June 5, 2024, the first confirmed human case of H5N2 was reported in Mexico.

<span class="mw-page-title-main">Influenza A virus subtype H7N7</span> Virus subtype

Influenza A virus subtype H7N7 (A/H7N7) is a subtype of Influenza A virus, a genus of Orthomyxovirus, the viruses responsible for influenza. Highly pathogenic strains (HPAI) and low pathogenic strains (LPAI) exist. H7N7 can infect humans, birds, pigs, seals, and horses in the wild; and has infected mice in laboratory studies. This unusual zoonotic potential represents a pandemic threat.

<span class="mw-page-title-main">Influenza A virus subtype H7N2</span> Virus subtype

Influenza A virus subtype H7N2 (A/H7N2) is a subtype of the species Influenza A virus. This subtype is one of several sometimes called bird flu virus. H7N2 is considered a low pathogenicity avian influenza (LPAI) virus. With this in mind, H5 & H7 influenza viruses can re-assort into the Highly Pathogenic variant if conditions are favorable.

<span class="mw-page-title-main">H5N1 genetic structure</span> Genetic structure of Influenza A virus

The genetic structure of H5N1, a highly pathogenic avian influenza virus, is characterized by a segmented RNA genome consisting of eight gene segments that encode for various viral proteins essential for replication, host adaptation, and immune evasion.

<span class="mw-page-title-main">Global spread of H5N1 in 2006</span> 2006 worldwide disease outbreak

The global spread of H5N1 in birds is considered a significant pandemic threat.

<span class="mw-page-title-main">Global spread of H5N1 in 2005</span> Pandemic threat

The global spread of H5N1 in birds is considered a significant pandemic threat.

<span class="mw-page-title-main">Global spread of H5N1 in 2004</span>

The global spread of H5N1 in birds is considered a significant pandemic threat.

<span class="mw-page-title-main">Fujian flu</span> Strains of influenza

Fujian flu refers to flu caused by either a Fujian human flu strain of the H3N2 subtype of the Influenza A virus or a Fujian bird flu strain of the H5N1 subtype of the Influenza A virus. These strains are named after Fujian, a coastal province in Southeast China.

<span class="mw-page-title-main">Goose Guangdong virus</span> Strain of H5N1 influenza virus

The Goose Guangdong virus refers to the strain A/Goose/Guangdong/1/96 (Gs/Gd)-like H5N1 HPAI viruses. It is a strain of the Influenzavirus A subtype H5N1 virus that was first detected in a goose in Guangdong in 1996. It is an HPAI virus, meaning that it can kill a very high percentage of chickens in a flock in mere days. It is believed to be the immediate precursor of the current dominant strain of HPAI A(H5N1) that evolved from 1999 to 2002 creating the Z genotype that is spreading globally and is epizootic and panzootic, killing tens of millions of birds and spurring the culling of hundreds of millions of others to stem its spread.

<span class="mw-page-title-main">Human mortality from H5N1</span>

H5N1 influenza virus is a type of influenza A virus which mostly infects birds. H5N1 flu is a concern due to the its global spread that may constitute a pandemic threat. The yardstick for human mortality from H5N1 is the case-fatality rate (CFR); the ratio of the number of confirmed human deaths resulting from infection of H5N1 to the number of those confirmed cases of infection with the virus. For example, if there are 100 confirmed cases of a disease and 50 die as a consequence, then the CFR is 50%. The case fatality rate does not take into account cases of a disease which are unconfirmed or undiagnosed, perhaps because symptoms were mild and unremarkable or because of a lack of diagnostic facilities. The Infection Fatality Rate (IFR) is adjusted to allow for undiagnosed cases.

<span class="mw-page-title-main">Influenza</span> Infectious disease

Influenza, commonly known as "the flu" or just "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 symptoms begin one to four days after exposure to the virus and last for about two to eight days. Diarrhea and vomiting can occur, particularly in children. Influenza may progress to pneumonia from the virus or a subsequent bacterial infection. Other complications include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease.

Since 2020, outbreaks of avian influenza subtype H5N1 have been occurring, with cases reported from every continent as of May 2024. Some species of wild aquatic birds act as natural asymptomatic carriers of a large variety of influenza A viruses, which can infect poultry, other bird species, mammals and humans if they come into close contact with infected feces or contaminated material, or by eating infected birds. In late 2023, H5N1 was discovered in the Antarctic for the first time, raising fears of imminent spread throughout the region, potentially leading to a "catastrophic breeding failure" among animals that had not previously been exposed to avian influenza viruses. The main virus involved in the global outbreak is classified as H5N1 clade 2.3.4.4b, however genetic diversification with other clades such as 2.3.2.1c has seen the virus evolve in ability to cause significant outbreaks in a broader range of species including mammals.

References

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