Bird Flu Types and Subtypes: H5N1, H5N5, H7N9 and H9N2

Author: Ian C. Langtree - View Credentials
Published: 28 Dec 2024 - Updated: 1 Nov 2026
Publication Type: Informative

Table of Contents:
Synopsis - Definition - Overview - FAQs - Insights, Updates - Related Content

Synopsis

This information explains how bird flu viruses are classified by their hemagglutinin and neuraminidase surface proteins, why only some H5 and H7 strains are considered highly pathogenic, and how H5N1 clade 2.3.4.4b became the driving force behind the current global outbreak after reaching North America in late 2021. Drawing on World Health Organization and CDC data, it reports that 1,000 human H5N1 cases and 479 deaths were recorded in 25 countries between 2003 and July 2026, a fatality rate of about 48 percent that likely overstates the true risk because mild infections often go unreported. It also covers 72 U.S. human cases since 2024, including two deaths - the Louisiana H5N1 death in January 2025 and the world's first human H5N5 case in Washington State later that year. The subtype-by-subtype breakdown, built on figures from public health agencies rather than headlines, is useful for seniors, people with disabilities, caregivers and backyard flock owners who want a clear picture of which strains matter and why.

At a Glance

• 1 - Highly pathogenic H5N2 was identified on two poultry farms in the Fraser Valley of British Columbia in December 2014.
• 2 - Seven Russian poultry workers became the first known human H5N8 cases in 2021, and all of them stayed free of symptoms.
• 3 - Ducks can carry highly pathogenic bird flu without looking sick, while the same viruses can kill chickens at very high rates, often within two days.
• 4 - H9N2 usually causes mild illness in people, with 173 cases and 2 deaths since 2015, but it supplies internal genes to other strains that have infected humans, including H7N9.

Topic Definition

Avian Influenza Subtype

An avian influenza subtype is a category of influenza A virus defined by the particular combination of two proteins on its surface - hemagglutinin (H) and neuraminidase (N). Birds carry 16 hemagglutinin and 9 neuraminidase types, and each pairing, such as H5N1, H7N9 or H9N2, is treated as a separate subtype. Within a subtype, viruses are further divided into clades and genotypes based on their genetic makeup, and laboratory testing in chickens determines whether a given strain is low pathogenic or highly pathogenic. Subtype names help scientists and public health agencies track which viruses are spreading, which animals they infect and how much of a threat they pose to people.

Overview

Bird flu, also known as avian influenza, is a viral infection that mainly affects wild and domestic birds but can occasionally spread to humans and other mammals. Influenza A viruses are classified by two surface proteins: hemagglutinin (HA) and neuraminidase (NA). Scientists have identified 18 HA and 11 NA subtypes in all, and 16 HA subtypes (H1 to H16) and 9 NA subtypes (N1 to N9) are found in birds [Centers for Disease Control and Prevention, n.d.-a]. Each virus is named for its pairing of the two proteins, which is why the news refers to strains such as H5N1, H7N9 and H9N2.

Human Bird Flu Cases and Fatality Rates

From 2003 to July 7, 2026, the World Health Organization recorded 1,000 confirmed human cases of H5N1 and 479 deaths in 25 countries, a case fatality rate of 47.9 percent [World Health Organization Regional Office for the Western Pacific, 2026]. That figure is often quoted as proof of extreme lethality, but experts caution that it probably overstates the real risk. Severe cases are far more likely to be tested and reported than mild ones, so many milder infections never make it into the count.

In the United States, the CDC has reported 72 human cases of H5 bird flu since 2024, mostly among dairy and poultry workers [Centers for Disease Control and Prevention, 2026]. Two of those people have died. The first U.S. H5 death was reported in Louisiana in January 2025 [Centers for Disease Control and Prevention, 2025a], and in November 2025 a Washington State resident with underlying medical conditions died of H5N5, the first human case of that subtype ever reported anywhere [World Health Organization, 2025].

The avian influenza subtypes that have caused the most concern for human health include:

• H5N1 - first identified in geese in southern China in 1996, with the first human cases in Hong Kong in 1997 (18 cases and 6 deaths) [Centers for Disease Control and Prevention, n.d.-b]
• H7N9 - first reported in humans in China in 2013
• H5N6 - human cases reported since 2014
• H5N8 - emerged through gene swapping between 2014 and 2016, with the first human infections detected in 2021
• H5N5 - first human case reported in 2025

Low Pathogenic and Highly Pathogenic Avian Influenza

Low Pathogenic Avian Influenza (LPAI)

Most avian influenza A viruses are low pathogenic. LPAI viruses cause either no signs of disease or only mild illness in chickens and other poultry, such as ruffled feathers or a drop in egg production, and infected wild birds often look perfectly healthy. The concern is that some low pathogenic viruses circulating in poultry can mutate into highly pathogenic strains [Centers for Disease Control and Prevention, n.d.-a].

Highly Pathogenic Avian Influenza (HPAI)

HPAI viruses cause severe disease and high death rates in poultry. Only some H5 and H7 viruses are classified as highly pathogenic - most H5 and H7 viruses found in birds are actually low pathogenic. HPAI infections can affect multiple internal organs and kill chickens at very high rates, often within two days, while ducks can be infected without showing any illness [Centers for Disease Control and Prevention, n.d.-a].

The labels low and highly pathogenic describe a virus's molecular features and how it behaves in chickens under laboratory testing. They do not predict how sick a person would become if infected [Centers for Disease Control and Prevention, n.d.-a].

HPAI viruses can also spill back from poultry into wild birds, which then carry them along migration routes. Some wild species carry these viruses without symptoms while others die in large numbers. This cycle between farms and wild flocks is why surveillance and control measures need to cover both domestic and wild bird populations.

H5N1 Clade 2.3.4.4b and the Global Outbreak

The virus driving the current global outbreak is H5N1 clade 2.3.4.4b. Between 2014 and 2016, gene swapping among H5 viruses produced H5N6 and H5N8, and the H5 hemagglutinin gene diversified into clade 2.3.4.4. From 2018 to 2020, clade 2.3.4.4b became predominant in Asia, Africa, Europe and the Middle East. A new clade 2.3.4.4b H5N1 virus was then detected in wild birds in Canada and the United States in late 2021, and outbreaks in U.S. commercial and backyard poultry began in February 2022 [Centers for Disease Control and Prevention, n.d.-b]. The first U.S. detection was in a wild American wigeon sampled in South Carolina on December 30, 2021, and the virus was of Eurasian origin [Bevins et al., 2022].

Clade 2.3.4.4b viruses stand apart from earlier H5N1 strains because of their rapid geographic spread, frequent reassortment with local influenza viruses and unusually wide host range. Different genotypes now circulate at the same time. Genotype B3.13 has spread in U.S. dairy cows, while genotype D1.1 is found in wild birds and poultry and has been linked to severe human cases [Centers for Disease Control and Prevention, 2024a].

Infected birds shed the virus in their saliva, mucus and feces. People are usually infected through unprotected contact with infected birds or animals, or with contaminated surfaces and environments.

Avian Influenza Subtypes

H5N1

H5N1 remains the predominant subtype in both domestic and wild birds. It is highly pathogenic, has caused outbreaks on every inhabited continent, and has shown an ability to infect a wide range of species, including birds, mammals and humans.

H5N2

Highly pathogenic H5N2 was identified on December 1, 2014, at two poultry premises in the Fraser Valley region of British Columbia, and further commercial farms and small flocks were later found infected [Canadian Food Inspection Agency, n.d.]. Sharing the H5 gene with H5N1 does not on its own make a virus highly pathogenic - each strain has to be tested to determine its pathogenicity.

H5N5

H5N5 had been found mainly in birds and some wild mammals until November 2025, when Washington State reported the first human H5N5 infection worldwide. The patient, an adult with underlying medical conditions who kept backyard poultry and domestic birds, died on November 21, 2025. The virus belonged to clade 2.3.4.4b, contact tracing found no further cases, and there was no evidence of human-to-human transmission [World Health Organization, 2025].

H5N6

H5N6 arose through gene swapping between 2014 and 2016 and, together with H5N8, became predominant globally from 2018 to 2020 [Centers for Disease Control and Prevention, n.d.-b]. Human infections have been reported only in the Western Pacific Region, with 93 cases and 58 deaths since 2014 - a case fatality rate of 62.4 percent [World Health Organization Regional Office for the Western Pacific, 2026].

H5N8

H5N8 is highly lethal to wild birds and poultry and has caused significant poultry outbreaks in Europe and Asia, with spillover into wild birds. It is also known to reassort with other strains, potentially creating new variants.

On February 18, 2021, Russia notified the World Health Organization of the first known human H5N8 infections. Seven poultry farm workers and outbreak responders at a farm in Astrakhan Oblast in southern Russia tested positive. All seven remained without symptoms, no illness was found among their close contacts, and the WHO judged the risk of human-to-human transmission to be low [World Health Organization, 2021].

H7N3

Between May 22 and June 24, 2024, highly pathogenic avian influenza was confirmed on eight commercial poultry properties in Victoria, Australia - H7N3 on seven properties near Meredith and H7N9 on one property near Terang. Agriculture Victoria lifted quarantine on the last affected property in January 2025, ending the outbreak [Agriculture Victoria, 2025]. Strains of this kind are separate from the H5N1 clade 2.3.4.4b virus causing the global outbreak.

H7N9

H7N9 emerged in 2013 and has infected both birds and humans, mainly in China. The WHO has recorded 1,568 human cases and 616 deaths since early 2013, a case fatality rate of 39.3 percent [World Health Organization Regional Office for the Western Pacific, 2026].

H9N2

H9N2 avian influenza virus infections in humans are usually mild. Since 2015, 173 human cases and 2 deaths have been recorded, all in the Western Pacific Region [World Health Organization Regional Office for the Western Pacific, 2026].

H9N2's bigger significance is as a gene donor. It has supplied internal genes to other avian influenza viruses that have infected humans, including H7N9 and H10N8, and it frequently reassorts with other subtypes. H9N2 is constantly evolving, and this genetic flexibility helps it jump the species barrier, potentially leading to zoonotic infections. Its ability to pass genes to other influenza viruses has raised concerns about its possible role in a future pandemic.

Genetic Evolution and Reassortment

Bird flu viruses have shown a remarkable capacity for genetic change. As clade 2.3.4.4b H5N1 has spread around the world, it has continued to swap gene segments with local avian influenza viruses, producing new genotypes in different regions. Laboratory studies of some Eurasian and North American reassortant H5N1 strains have reported changes linked to better replication in mammalian cells and greater virulence in mice, which is why scientists track these genetic shifts so closely.

Recent Severe Human Cases and Genetic Changes

In November 2024, a teenager in British Columbia became critically ill with H5N1 genotype D1.1, the same strain circulating in the province's wild birds and poultry. The source of infection was never identified, although the virus most closely matched wild birds found in the Fraser Valley in October 2024. No other human cases were found and there was no evidence of human-to-human spread [British Columbia Ministry of Health, 2024].

On December 18, 2024, the CDC announced the first severe case of H5N1 in the United States - a Louisiana patient hospitalized after contact with sick and dead birds in backyard flocks, the first U.S. case linked to a backyard flock [Centers for Disease Control and Prevention, 2024b]. The patient's virus was genotype D1.1. Genetic analysis found low-level changes in the hemagglutinin gene that were not present in birds on the patient's property, which the CDC said most likely developed as the virus replicated in the patient. One of these changes was also found in the British Columbia case [Centers for Disease Control and Prevention, 2024a]. The Louisiana patient later died, and the CDC continued to assess the risk to the general public as low, with no person-to-person spread identified [Centers for Disease Control and Prevention, 2025a].

Spread to Mammals and New Regions

There have been increasing reports of infections in mammals, including dairy cows, goats, cats and alpacas, raising concerns about mammal-to-mammal transmission. Since March 2024, H5N1 has been confirmed in dairy cattle in 19 U.S. states. Washington became the latest in October 2026, when two dairy herds tested positive for clade 2.3.4.4b even though they had tested negative in earlier routine surveillance [U.S. Department of Agriculture, Animal and Plant Health Inspection Service, 2026]. Federal officials continue to state that the milk supply remains safe.

The virus has also reached seabird colonies in and around Antarctica, raising fears about its effect on wildlife populations that have never been exposed to it before.

Faster Testing in Hospitals

On January 16, 2025, the CDC issued a Health Alert Network advisory in response to the H5N1 panzootic affecting wild birds, poultry and dairy cows. The CDC recommended that all influenza A-positive specimens from hospitalized patients, especially those in intensive care, be subtyped as quickly as possible, ideally within 24 hours of admission. Faster subtyping helps identify H5 infections that might otherwise be missed during a busy seasonal flu period, so patients get appropriate care, infection control steps begin promptly and public health investigations can start without delay [Centers for Disease Control and Prevention, 2025b].

Prevention, Surveillance and Vaccines

• Surveillance: Continuous monitoring of wild and domestic birds, including testing live and dead birds and milk from dairy herds, helps detect and contain outbreaks early.
• Biosecurity: Strong biosecurity on poultry and dairy farms - controlling the movement of animals, people and equipment and maintaining good hygiene - limits the introduction and spread of the virus.
• Personal protection: People should avoid direct contact with sick or dead birds and animals, wear protective equipment when handling poultry or working with infected livestock, and cook poultry and eggs thoroughly.

Vaccines against H5N1 do exist. The U.S. Food and Drug Administration has licensed three H5N1 vaccines, including AUDENZ and a vaccine made for the U.S. National Stockpile [U.S. Food and Drug Administration, 2024]. These are pandemic preparedness vaccines and are not offered to the general public as part of routine flu vaccination.

Older adults, people with chronic health conditions and people with disabilities may face a greater risk of complications from any influenza infection. Caregivers, backyard flock owners and farm workers in these households should be especially careful to avoid contact with sick animals, and anyone who develops flu-like symptoms or eye redness after contact with birds or livestock should contact a healthcare provider promptly and mention the exposure.

Frequently Asked Questions

What are the symptoms of bird flu in people?

Reported symptoms include eye redness and irritation, fever or feeling feverish, cough, sore throat, runny or stuffy nose, muscle aches, headaches and fatigue. Some people also have diarrhea, nausea or vomiting, and fever is not always present.

How soon after exposure do bird flu symptoms start?

Respiratory symptoms usually begin about 3 days after infection, within a range of about 2 to 7 days. Eye symptoms such as redness can appear sooner, within 1 to 2 days of exposure.

Is there a treatment for bird flu in humans?

Yes. The CDC recommends the antiviral oseltamivir, sold as Tamiflu and as a generic, for confirmed or suspected cases. It works best when started within two days of symptoms beginning, but later treatment can still help people who are very sick or at high risk.

Can people who were exposed to bird flu but feel well take antiviral medicine?

The CDC says people exposed to infected animals, raw milk, contaminated environments or an infected person without proper protective equipment may be offered oseltamivir twice daily for 5 days to help prevent illness. A healthcare provider makes that decision.

Is it safe to eat chicken and eggs during a bird flu outbreak?

Yes, when they are handled and cooked properly. Cooking poultry and eggs to an internal temperature of 165 degrees Fahrenheit destroys avian influenza viruses, and raw poultry should be kept away from cooked or ready-to-eat foods.

Can you get bird flu from drinking raw milk?

The CDC says it is not yet known whether bird flu can spread to people through raw milk or raw milk cheese from infected cows. Pasteurization kills avian influenza viruses, so choosing pasteurized milk and dairy products is the safer option.

What complications can severe bird flu cause?

Severe infections can lead to pneumonia, respiratory failure, acute respiratory distress syndrome, kidney injury, sepsis, inflammation of the brain and multi-organ failure. Warning signs include high fever, shortness of breath, confusion and seizures.

Have most recent human bird flu cases in the United States been severe?

No. Most U.S. human cases since 2024 have been mild, and eye redness has been the most common symptom. Serious illness and deaths have occurred but remain uncommon.

References

• Agriculture Victoria. (2025). H7 avian influenza eradicated from poultry in Victoria.
• Bevins, S. N., Dilione, K. E., Cumbee, J. C., Jr., Douglass, K. E., Killian, M. L., Lenoch, J. B., Shriner, S. A., and Torchetti, M. K. (2022). Intercontinental movement of highly pathogenic avian influenza A(H5N1) clade 2.3.4.4 virus to the United States, 2021. Emerging Infectious Diseases, 28(5).
• British Columbia Ministry of Health. (2024). Final update on human avian influenza case in B.C.
• Canadian Food Inspection Agency. (n.d.). Canadian Food Inspection Agency investigation: 2014 avian influenza outbreak in British Columbia.
• Centers for Disease Control and Prevention. (n.d.-a). Avian influenza type A.
• Centers for Disease Control and Prevention. (n.d.-b). Emergence and evolution of H5N1 bird flu.
• Centers for Disease Control and Prevention. (2024a). Genetic sequences of highly pathogenic avian influenza A(H5N1) viruses identified in a person in Louisiana.
• Centers for Disease Control and Prevention. (2024b). CDC confirms first severe case of H5N1 bird flu in the United States.
• Centers for Disease Control and Prevention. (2025a). First H5 bird flu death reported in United States.
• Centers for Disease Control and Prevention. (2025b). Accelerated subtyping of influenza A in hospitalized patients (Health Alert Network Health Advisory CDCHAN-00520).
• Centers for Disease Control and Prevention. (2026). A(H5) bird flu: Current situation.
• U.S. Department of Agriculture, Animal and Plant Health Inspection Service. (2026). USDA confirms highly pathogenic avian influenza in two dairy herds in Washington.
• U.S. Food and Drug Administration. (2024). Influenza virus vaccine, H5N1 (for National Stockpile).
• World Health Organization. (2021). Avian influenza A(H5N8) - Russian Federation (Disease Outbreak News).
• World Health Organization. (2025). Avian influenza A(H5N5) - United States of America (Disease Outbreak News).
• World Health Organization Regional Office for the Western Pacific. (2026). Avian influenza weekly update number 1053.

More on this topic: Avian Influenza (Bird Flu) Publications - Influenza and Colds Publications - Respiratory Disorders Publications

Insights, Analysis, and Developments

Editorial Note: Bird flu is no longer a story about one virus. H5N1 has now been confirmed in dairy cattle in 19 U.S. states, with Washington herds added in October 2026, and genotypes such as D1.1 have caused severe illness in people even when the source of infection could not be traced. H7N9 has caused 1,568 human cases and 616 deaths since 2013, and H5N6 has killed more than half of the people known to be infected. The CDC has asked hospitals to subtype influenza A samples within a day of admission so cases are not missed, and the FDA has already licensed three H5N1 vaccines for pandemic preparedness. For older adults, people with chronic conditions or disabilities and the caregivers who support them, the practical lessons stay the same - avoid sick or dead animals, choose pasteurized dairy, cook poultry and eggs thoroughly, and tell a doctor about any animal exposure if flu-like symptoms or eye redness appear.

Author Credentials: Ian C. Langtree is a writer, editor, and the founder and Editor-in-Chief of Disabled World, which he established in 2004 as one of the longest-running independent resources for disability news and information. Over more than two decades, he has written and edited thousands of articles on disability rights, health, accessibility, and assistive technology. Drawing on a global perspective shaped by extensive travel and lived experience, Ian is a committed advocate of the Social Model of Disability, the framework developed by disabled activists in the 1970s that focuses on removing societal barriers rather than on individual impairments. His writing is grounded in that framework and in a lasting commitment to disability rights, accessibility, and social inclusion. To learn more about Ian's background, expertise, and accomplishments, visit his full biography.