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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. [1] The H5N1 strain is a fast-mutating, highly pathogenic avian influenza virus (HPAI) found in multiple bird species. It is both epizootic (an epidemic in non-humans) and panzootic (a disease affecting animals of many species especially over a wide area). Unless otherwise indicated, "H5N1" in this timeline refers to the 2008 highly pathogenic strain of H5N1.
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Tens of millions of birds have died of H5N1 influenza and hundreds of millions of birds have been slaughtered and disposed of, to limit the spread of H5N1. Countries that have reported one or more major highly pathogenic H5N1 outbreaks in birds (causing at least thousands but in some cases millions of dead birds) are (in order of first outbreak occurrence): South Korea, Vietnam, Japan, Thailand, Cambodia, Laos, Indonesia, China, Malaysia, Russia, Kazakhstan, Mongolia, Turkey, Romania, Croatia, Ukraine, Cyprus, Iraq, Nigeria, Egypt, India, France, Niger, Bosnia, Azerbaijan, Albania, Cameroon, Myanmar, Afghanistan, Israel, Pakistan, Jordan, Burkina Faso, Germany, Sudan, Ivory Coast, Djibouti, Hungary, United Kingdom, Kuwait, Bangladesh, Saudi Arabia, Ghana, Czech Republic, Togo, Nepal, Bhutan, the Philippines, and Chile. [2]
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Highly pathogenic H5N1 has been found in birds in the wild in numerous other countries: Austria, Bulgaria, Denmark, Greece, Iran, Italy, Poland, Serbia and Montenegro, Slovakia, Slovenia, Spain, Sweden, Switzerland, and Uruguay. [3] Surveillance of H5N1 in humans, poultry, wild birds, cats and other animals remains very weak in many parts of Asia and Africa. Much remains unknown about the exact extent of its spread.[ citation needed ]
H5N1 has low pathogenic varieties endemic in birds in North America. H5N1 has a highly pathogenic variety that is endemic in dozens of species of birds throughout south Asia and parts of Africa. So far, it is very difficult for humans to become infected with H5N1. The presence of highly pathogenic (deadly) H5N1 around the world in both birds in the wild (swans, magpies, ducks, geese, pigeons, eagles, etc.) and in chickens and turkeys on farms has been demonstrated in millions of cases with the virus isolate actually sequenced in hundreds of cases yielding definitive proof of the evolution of this strain of this subtype of the species Influenzavirus A (bird flu virus).[ citation needed ]
According to Robert G. Webster:
The epicenters of both the Asian influenza pandemic of 1957 and the Hong Kong influenza pandemic of 1968 were in Southeast Asia, and it is in this region that multiple clades of H5N1 influenza virus have already emerged. The Asian H5N1 virus was first detected in Guangdong Province, China, in 1996, when it killed some geese, but it received little attention until it spread through live-poultry markets in Hong Kong to humans in May 1997, killing 6 of 18 infected people. [...] From 1997 to May 2005, H5N1 viruses were largely confined to Southeast Asia, but after they had infected wild birds in Qinghai Lake, China, they rapidly spread westward. [...] The intermittent spread to humans will continue, and the virus will continue to evolve.Map [4]
On July 25, 2008, the Food and Agriculture Organization (FAO) released an Avian Influenza Disease Emergency Situation Update, reporting that H5N1 pathogenicity was continuing to gradually rise in endemic areas but the avian influenza disease situation in farmed birds was being held in check by vaccination. Eleven outbreaks of H5N1 were reported worldwide in June 2008 in five countries (China, Egypt, Indonesia, Pakistan and Vietnam) compared to 65 outbreaks in June 2006 and 55 in June 2007. [5]
Between 2003 and May 2024, the World Health Organization has recorded 892 cases of confirmed H5N1 influenza, leading to 463 deaths. [6] The true fatality rate may be lower because some cases with mild symptoms may not have been identified as H5N1. [7]
2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | Total | ||||||||||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR | cases | deaths | CFR |
1 | 0 | 0% | 1 | 0 | 0% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
8 | 5 | 62.5% | 8 | 5 | 62.5% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 0 | 0% | 2 | 0 | 0% | 3 | 0 | 0% | 1 | 1 | 100% | 1 | 0 | 0% | 8 | 1 | 12.5% | |||||||||||||||||||||||||||||||||||||||||||||||||||
4 | 4 | 100% | 2 | 2 | 100% | 1 | 1 | 100% | 1 | 0 | 0% | 1 | 0 | 0% | 1 | 1 | 100% | 8 | 8 | 100% | 3 | 3 | 100% | 26 | 14 | 53.8% | 9 | 4 | 44.4% | 6 | 4 | 66.7% | 7 | 1 | 14.0% | 69 | 42 | 60.9% | ||||||||||||||||||||||||||||||
1 | 1 | 100% | 1 | 1 | 100% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 0 | 0% | 1 | 0 | 0% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 1 | 100% | 8 | 5 | 62.5% | 13 | 8 | 61.5% | 5 | 3 | 60.0% | 4 | 4 | 100% | 7 | 4 | 57.1% | 2 | 1 | 50.0% | 1 | 1 | 100% | 2 | 1 | 50.0% | 2 | 2 | 100% | 2 | 0 | 0% | 6 | 1 | 16.7% | 1 | 1 | 100% | 1 | 0 | 0% | 55 | 32 | 58.2% | ||||||||||||||||||||||||
1 | 0 | 0% | 1 | 0 | 0% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 0 | 0% | 1 | 0 | 0% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
18 | 10 | 55.6% | 25 | 9 | 36.0% | 8 | 4 | 50.0% | 39 | 4 | 10.3% | 29 | 13 | 44.8% | 39 | 15 | 38.5% | 11 | 5 | 45.5% | 4 | 3 | 75.0% | 37 | 14 | 37.8% | 136 | 39 | 28.7% | 10 | 3 | 30.0% | 3 | 1 | 33.3% | 359 | 120 | 33.4% | ||||||||||||||||||||||||||||||
1 | 1 | 100% | 1 | 1 | 100% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
20 | 13 | 65.0% | 55 | 45 | 81.8% | 42 | 37 | 88.1% | 24 | 20 | 83.3% | 21 | 19 | 90.5% | 9 | 7 | 77.8% | 12 | 10 | 83.3% | 9 | 9 | 100% | 3 | 3 | 100% | 2 | 2 | 100% | 2 | 2 | 100% | 1 | 1 | 100% | 200 | 168 | 84.0% | ||||||||||||||||||||||||||||||
3 | 2 | 66.6% | 3 | 2 | 66.6% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | 2 | 100% | 1 | 0 | 0% | 3 | 2 | 66.7% | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 0 | 0% | 1 | 0 | 0% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 1 | 100% | 1 | 1 | 100% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 1 | 100% | 1 | 1 | 100% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | 1 | 33.3% | 3 | 1 | 33.3% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 | 0 | 0% | 2 | 0 | 0% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
17 | 12 | 70.6% | 5 | 2 | 40.0% | 3 | 3 | 100% | 25 | 17 | 68.0% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
12 | 4 | 33.3% | 12 | 4 | 33.3% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 0 | 0% | 4 | 0 | 0% | 5 | 0 | 0% | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 0 | 0% | 7 | 0 | 0% | 8 | 0 | 0% | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3 | 3 | 100% | 29 | 20 | 69.0% | 61 | 19 | 31.1% | 8 | 5 | 62.5% | 6 | 5 | 83.3% | 5 | 5 | 100% | 7 | 2 | 28.6% | 4 | 2 | 50.0% | 2 | 1 | 50.0% | 2 | 2 | 100% | 1 | 0 | 0% | 1 | 1 | 100% | 129 | 65 | 50.0% | ||||||||||||||||||||||||||||||
4 | 4 | 100% | 46 | 32 | 69.6% | 98 | 43 | 43.9% | 115 | 79 | 68.7% | 88 | 59 | 67.0% | 44 | 33 | 75.0% | 73 | 32 | 43.8% | 48 | 24 | 50.0% | 62 | 34 | 54.8% | 32 | 20 | 62.5% | 39 | 25 | 64.1% | 52 | 22 | 42.3% | 145 | 42 | 29.0% | 10 | 3 | 30.0% | 4 | 2 | 50.0% | 0 | 0 | 0% | 1 | 1 | 100% | 1 | 0 | 0% | 2 | 1 | 50.0% | 6 | 1 | 16.7% | 12 | 4 | 33.3% | 16 | 2 | 12.5% | 898 | 463 | 51.6% |
In January 2004 a major new outbreak of H5N1 surfaced in Vietnam and Thailand's poultry industry, and within weeks spread to ten countries and regions in Asia, including Indonesia, South Korea, Japan and China. In October 2004 researchers discovered H5N1 is far more dangerous than previously believed because waterfowl, especially ducks, were directly spreading the highly pathogenic strain of H5N1 to chickens, crows, pigeons, and other birds and that it was increasing its ability to infect mammals as well. From this point on, avian influenza experts increasingly refer to containment as a strategy that can delay but not prevent a future avian flu pandemic.[ citation needed ]
Variants have been found in a number of domestic cats, leopards, and tigers in Thailand, with high lethality. [18] "The Thailand Zoo tiger outbreak killed more than 140 tigers, causing health officials to make the decision to cull all the sick tigers in an effort to stop the zoo from becoming a reservoir for H5N1 influenza. [19] [20] A study of domestic cats showed H5N1 virus infection by ingestion of infected poultry and also by contact with other infected cats (Kuiken et al., 2004)." [21] The initial OIE report reads: "the clinical manifestations began on 11 October 2004 with weakness, lethargy, respiratory distress, and high fever (about 41-42 degrees Celsius). There was no response to any antibiotic treatment. Death occurred within three days following the onset of clinical signs with severe pulmonary lesions." [22]
In January 2005 an outbreak of avian influenza affected thirty three out of sixty four cities and provinces in Vietnam, leading to the forced killing of nearly 1.2 million poultry. Up to 140 million birds are believed to have died or been killed because of the outbreak. In April 2005 there begins an unprecedented die-off of over 6,000 migratory birds at Qinghai Lake in central China over three months. This strain of H5N1 is the same strain as is spread west by migratory birds over at least the next ten months. In August 2005 H5N1 spread to Kazakhstan, Mongolia and Russia. On September 29, 2005, David Nabarro, the newly appointed Senior United Nations System Coordinator for Avian and Human Influenza, warned the world that an outbreak of avian influenza could kill 5 to 150 million people. David Nabarro later stated that as the virus had spread to migratory birds, an outbreak could start in Africa or the Middle East. Later in 2005 H5N1 spread to Turkey, Romania, Croatia and Kuwait.[ citation needed ]
In 2005, it was discovered that H5N1 "could be infecting up to half of the pig population in some areas of Indonesia, but without causing symptoms [...] Chairul Nidom, a virologist at Airlangga University's tropical disease center in Surabaya, Java, was conducting independent research earlier this year. He tested the blood of 10 apparently healthy pigs housed near poultry farms in western Java where avian flu had broken out, Nature reported. Five of the pig samples contained the H5N1 virus. The Indonesian government has since found similar results in the same region, Nature reported. Additional tests of 150 pigs outside the area were negative." [23]
In January, Japan, Hungary, Russia, and the United Kingdom joined the list of nations seeing a resurgence of bird deaths due to H5N1. In February, Pakistan, Turkey, Afghanistan, and Myanmar joined the list and Kuwait saw its first major outbreak of H5N1 avian influenza.
In March Bangladesh and Saudi Arabia each saw their first major outbreak of H5N1 avian influenza and Ghana in May.
As H5N1 continued killing many birds and a few people throughout the spring in countries where it is now endemic, in June Malaysia and Germany saw a resurgence of bird deaths due to H5N1, while the Czech Republic and Togo experienced their first major outbreak of H5N1 avian influenza.
In July France and India also saw a resurgence of bird deaths due to H5N1.[ citation needed ]
"Chairul Anwar Nidom of Airlangga University in Surabaya, Indonesia, told journalists last week that he had taken blood samples from 500 stray cats near poultry markets in four areas of Java, including the capital, Jakarta, and one area in Sumatra, all of which have recently had outbreaks of H5N1 in poultry and people. Of these cats, 20% carried antibodies to H5N1. This does not mean that they were still carrying the virus, only that they had been infected - probably through eating birds that had H5N1. Many other cats that were infected are likely to have died from the resulting illness, so many more than 20% of the original cat populations may have acquired H5N1." [29]
As of the July 25, 2008 FAO Avian Influenza Disease Emergency Situation Update, H5N1 pathogenicity is continuing to gradually rise in wild birds in endemic areas but the avian influenza disease situation in farmed birds is being held in check by vaccination. Eleven outbreaks of H5N1 were reported worldwide in June 2008 in five countries (China, Egypt, Indonesia, Pakistan and Vietnam) compared to 65 outbreaks in June 2006 and 55 in June 2007. The "global HPAI situation can be said to have improved markedly in the first half of 2008 [but] cases of HPAI are still underestimated and underreported in many countries because of limitations in country disease surveillance systems". [5]
H5N1 has been transmitted in laboratories to many species including mice and ferrets to study its effects. A purposely mutated strain [51] in ferrets [48] has engendered a notable international policy debate regarding the openness of scientific research, the mandates of public health, and the potential for bioterrorism. [66]
H5N1 was transmitted in the wild to three civet cats in Vietnam in August 2005 and a stone marten in Germany in March 2006. [67]
The BBC reported that a stray dog in Azerbaijan died from the disease on March 15, 2006. [68]
People living in areas where the A(H5N1) virus has infected birds are advised to keep their cats indoors. "Cats can be infected through the respiratory tract. Cats can also be infected when they ingest the virus, which is a novel route for influenza transmission in mammals. But cats excrete only one-thousandth the amount of virus that chickens do [...] The concern is that if large numbers of felines and other carnivores become infected, the virus might mutate in a series of events that could lead to an epidemic among humans. Dogs, foxes, seals, and other carnivores may be vulnerable to A(H5N1) virus infection, Dr. Osterhaus said. Tests in Thailand have shown that the virus has infected dogs without causing apparent symptoms." [69]
In early April 2024, H5N1 was reported to have spread amongst dairy cow herds in multiple states of the USA, indicating cow-to-cow spread, possibly occurring while the animals were being milked. [70] A dairy worker in Texas also became infected, with conjunctivitis being the main symptom. [71]
Avian influenza virus H3N2 is endemic in pigs ("swine flu") in China and has been detected in pigs in Vietnam, increasing fears of the emergence of new variant strains. Health experts say pigs can carry human influenza viruses, which can combine (i.e. exchange homologous genome sub-units by genetic reassortment) with H5N1, passing genes and mutating into a form which can pass easily among humans. H3N2 evolved from H2N2 by antigenic shift and caused the Hong Kong Flu pandemic of 1968 and 1969 that killed up to 750,000 humans. The dominant strain of annual flu in humans in January 2006 is H3N2. Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 in humans has increased to 91% in 2005. A combination of these two subtypes of the species known as the avian influenza virus in a country like China is a worst-case scenario. In August 2004, researchers in China found H5N1 in pigs. [10]
In Bangkok, Thailand, all the cats in one household are known to have died of H5N1 in 2004. Tigers and leopards in Thai zoos also died, while in 2007 two cats near an outbreak in poultry and people in Iraq were confirmed to have died of H5N1, as were three German cats that ate wild birds. In Austria, cats were infected but remained healthy. Cats in Indonesia were also found to have been infected with H5N1. [29]
The spread to species and populations of birds, and the ability of felids (cats) to catch H5N1 from eating this natural prey, means the creation of a reservoir for H5N1 in cats where the virus can adapt to mammals is one of the many possible pathways to a pandemic.[ citation needed ]
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 endemic in many bird populations.
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.
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.
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.
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.
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.
H5N8 is a subtype of the influenza A virus and is highly lethal to wild birds and poultry. H5N8 is typically not associated with humans. However, seven people in Russia were found to be infected in 2021, becoming the first documented human cases.
The social impact of H5N1 is the effect or influence of H5N1 in human society.
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.
The global spread of H5N1 in birds is considered a significant pandemic threat.
The global spread of H5N1 in birds is considered a significant pandemic threat.
The global spread of H5N1 in birds is considered a significant pandemic threat.
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.
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.
The global spread of H5N1 in birds is considered a significant pandemic threat.
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 in many bird populations. A/H7N9 virus can also infect humans that have been exposed to infected birds; in these cases, symptoms are frequently severe or fatal. A/H7N9 virus is shed in the saliva, mucous, and feces of infected birds. The virus can spread rapidly through poultry flocks and among wild birds.
Cats with avian influenza exhibit symptoms that can result in death. The viruses cats may get include H5N1 or H7N2, which are subtypes of avian influenza. In order to get the virus, cats need to be in contact with waterfowl, poultry, or uncooked poultry that are infected. Two of the main organs that the virus affects are the lungs and liver.
In the early 2020s, an ongoing outbreak of avian influenza subtype H5N8 has been occurring at poultry farms and among wild bird populations in several countries and continents, leading to the subsequent cullings of millions of birds to prevent a pandemic similar to that of the H5N1 outbreak in 2008. The first case of human transmission of avian flu, also known as bird flu, was reported by Russian authorities in February 2021, as several poultry farm workers tested positive for the virus.
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.
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