Influenza A virus subtype H5N1 facts for kids

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Quick facts for kids Influenza A virus subtype H5N1

Colorized transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold) grown in MDCK cells (seen in green).
Virus classification
(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 H5N1
Notable strains
Goose Guangdong virus A/Fujian (H5N1)

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 (maintained in the population) in many bird populations, and also panzootic (affecting animals of many species over a wide area). A/H5N1 virus can also infect mammals (including humans) that have been exposed to infected birds; in these cases, symptoms are frequently severe or fatal.

A/H5N1 virus is shed in the saliva, mucous, and feces of infected birds; other infected animals may shed bird flu viruses in respiratory secretions and other body fluids (such as milk). The virus can spread rapidly through poultry flocks and among wild birds. An estimated half a billion farmed birds have been slaughtered in efforts to contain the virus.

Symptoms of A/H5N1 influenza vary according to both the strain of virus underlying the infection and on the species of bird or mammal affected. 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 other species. Chickens infected with LPAI A/H5N1 virus display mild symptoms or are asymptomatic, whereas HPAI A/H5N1 causes serious breathing difficulties, a significant drop in egg production, and sudden death.

In mammals, including humans, A/H5N1 influenza (whether LPAI or HPAI) is rare. Symptoms of infection vary from mild to severe, including fever, diarrhoea, and cough. Human infections with A/H5N1 virus have been reported in 23 countries since 1997, resulting in severe pneumonia and death in about 50% of cases. As of May 2024, 889 human cases had been identified worldwide, with 463 fatalities, giving a case fatality rate of around 50%; however, it is likely that this may be an overestimate given that mild infections can go undetected and under-reported.

A/H5N1 influenza virus was first identified in farmed birds in southern China in 1996. Between 1996 and 2018, A/H5N1 coexisted in bird populations with other subtypes of the virus, but since then, the highly pathogenic subtype HPAI A(H5N1) has become the dominant strain in bird populations worldwide. Some strains of A/H5N1 which are highly pathogenic to chickens have adapted to cause mild symptoms in ducks and geese, and are able to spread rapidly through bird migration. Mammal species that have been recorded with H5N1 infection include cows, seals, goats, and skunks.

Due to the high lethality and virulence of HPAI A(H5N1), its worldwide presence, its increasingly diverse host reservoir, and its significant ongoing mutations, the H5N1 virus is regarded as the world's largest pandemic threat. Domestic poultry may potentially be protected from specific strains of the virus by vaccination. In the event of a serious outbreak of H5N1 flu among humans, health agencies have prepared "candidate" vaccines that may be used to prevent infection and control the outbreak; however, it could take several months to ramp up mass production.

Signs and symptoms
- Birds
- Humans
Prevention
- Vaccine
- Public health
Treatment
Society and culture
See also

Signs and symptoms

Birds

Clinical signs of H5N1 in birds range from mild—decrease in egg production, nasal discharge, coughing and sneezing—to severe, including loss of coordination, energy, and appetite; soft-shelled or misshapen eggs; purple discoloration of the wattles, head, eyelids, combs, and hocks; and diarrhea. Sometimes the first noticeable sign is sudden death.

Humans

In general, humans who catch a humanized influenza A virus (a human flu virus of type A) usually have symptoms that include fever, cough, sore throat, muscle aches, conjunctivitis, and, in severe cases, breathing problems and pneumonia that may be fatal. The severity of the infection depends in large part on the state of the infected persons' immune systems and whether they had been exposed to the strain before (in which case they would be partially immune). No one knows if these or other symptoms will be the symptoms of a humanized H5N1 flu.

The avian influenza hemagglutinin prefers to bind to alpha-2,3 sialic acid receptors, while the human influenza hemagglutinin prefers to bind to alpha-2,6 sialic acid receptors. This means that when the H5N1 strain infects humans, it will replicate in the lower respiratory tract (where alpha-2,3 sialic acid receptors are more plentiful in humans) and consequently cause viral pneumonia. As of April 2024, there is no human-adapted form of H5N1 influenza, so all humans who have caught it so far have caught avian H5N1.

Prevention

Vaccine

There are several H5N1 vaccines for several of the avian H5N1 varieties, but the continual mutation of H5N1 renders them of limited use to date: while vaccines can sometimes provide cross-protection against related flu strains, the best protection would be from a vaccine specifically produced for any future pandemic flu virus strain. Daniel R. Lucey, co-director of the Biohazardous Threats and Emerging Diseases graduate program at Georgetown University has made this point, "There is no H5N1 pandemic so there can be no pandemic vaccine". However, "pre-pandemic vaccines" have been created; are being refined and tested; and do have some promise both in furthering research and preparedness for the next pandemic. Vaccine manufacturing companies are being encouraged to increase capacity so that if a pandemic vaccine is needed, facilities will be available for rapid production of large amounts of a vaccine specific to a new pandemic strain.

Public health

"The United States is collaborating closely with eight international organizations, including the World Health Organization (WHO), the Food and Agriculture Organization of the United Nations (FAO), the World Organization for Animal Health (OIE), and 88 foreign governments to address the situation through planning, greater monitoring, and full transparency in reporting and investigating avian influenza occurrences. The United States and these international partners have led global efforts to encourage countries to heighten surveillance for outbreaks in poultry and significant numbers of deaths in migratory birds and to rapidly introduce containment measures. The U.S. Agency for International Development (USAID) and the U.S. Department of State, the U.S. Department of Health and Human Services (HHS), and Agriculture (USDA) are coordinating future international response measures on behalf of the White House with departments and agencies across the federal government".

Together steps are being taken to "minimize the risk of further spread in animal populations", "reduce the risk of human infections", and "further support pandemic planning and preparedness".

Ongoing detailed mutually coordinated onsite surveillance and analysis of human and animal H5N1 avian flu outbreaks are being conducted and reported by the USGS National Wildlife Health Center, the Centers for Disease Control and Prevention, the World Health Organization, the European Commission, and others.

Treatment

There is no highly effective treatment for H5N1 flu, but oseltamivir (commercially marketed by Roche as Tamiflu) can sometimes inhibit the influenza virus from spreading inside the user's body. This drug has become a focus for some governments and organizations trying to prepare for a possible H5N1 pandemic. On April 20, 2006, Roche AG announced that a stockpile of three million treatment courses of Tamiflu are waiting at the disposal of the World Health Organization to be used in case of a flu pandemic; separately Roche donated two million courses to the WHO for use in developing nations that may be affected by such a pandemic but lack the ability to purchase large quantities of the drug.

Animal and lab studies suggest that Relenza (zanamivir), which is in the same class of drugs as Tamiflu, may also be effective against H5N1. In a study performed on mice in 2000, "zanamivir was shown to be efficacious in treating avian influenza viruses H9N2, H6N1, and H5N1 transmissible to mammals". In addition, mice studies suggest the combination of zanamivir, celecoxib and mesalazine looks promising producing a 50% survival rate compared to no survival in the placebo arm. While no one knows if zanamivir will be useful or not on a yet to exist pandemic strain of H5N1, it might be useful to stockpile zanamivir as well as oseltamivir in the event of an H5N1 influenza pandemic. Neither oseltamivir nor zanamivir can be manufactured in quantities that would be meaningful once efficient human transmission starts. In September, 2006, a WHO scientist announced that studies had confirmed cases of H5N1 strains resistant to Tamiflu and Amantadine. Tamiflu-resistant strains have also appeared in the EU, which remain sensitive to Relenza.

Society and culture

H5N1 has had a significant effect on human society, especially the financial, political, social, and personal responses to both actual and predicted deaths in birds, humans, and other animals. Billions of dollars are being raised and spent to research H5N1 and prepare for a potential avian influenza pandemic. Over $10 billion have been spent and over 200 million birds have been killed to try to contain H5N1.

People have reacted by buying less chicken, causing poultry sales and prices to fall. Many individuals have stockpiled supplies for a possible flu pandemic. International health officials and other experts have pointed out that many unknown questions still hover around the disease.

Dr. David Nabarro, Chief Avian Flu Coordinator for the United Nations, and former Chief of Crisis Response for the World Health Organization has described himself as "quite scared" about H5N1's potential impact on humans. Nabarro has been accused of being alarmist before, and on his first day in his role for the United Nations, he proclaimed the avian flu could kill 150 million people. In an interview with the International Herald Tribune, Nabarro compares avian flu to AIDS in Africa, warning that underestimations led to inappropriate focus for research and intervention.