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Public Health

Vetted resources and information on current public health events.

What is "Bird Flu"?

Animal influenza viruses are distinct from human seasonal influenza viruses and do not easily transmit between humans. However, zoonotic influenza viruses - animal influenza viruses that may occasionally infect humans through direct or indirect contact - can cause disease in humans ranging from a mild illness to death.


Avian influenza or "bird flu" refers to the disease caused by infection with avian (bird) influenza (flu) Type A viruses. These viruses naturally spread among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species. Avian influenza A viruses have been isolated from more than 100 different species of wild birds around the world. Bird flu viruses do not normally infect humans. However, sporadic human infections with  bird flu viruses have occurred.

Avian Influenza Reservoirs

Wild aquatic birds (waterfowl such as ducks, geese, swans, gulls, terns; shorebirds such as storks, plovers, sandpipers) can be infected with avian influenza A viruses in their intestines and respiratory tract, and some species, such as ducks, may not get sick. These birds (especially dabbling ducks) are considered reservoirs, or hosts, for avian influenza A viruses, since they can become infected and transmit the infection while not becoming sick.

Domesticated Birds

Infected birds can shed avian influenza A viruses in their saliva, nasal secretions, and feces. Susceptible domesticated birds (including farmed poultry) may become infected with avian influenza A viruses through direct contact with infected waterfowl or other infected poultry, or through contact with surfaces that have been contaminated with the viruses. Because avian influenza A viruses are extremely contagious among birds, these viruses can end up sickening and killing certain domesticated (and farmed) bird species, such as chickens and turkeys.

When avian influenza A(H5) or A(H7) virus outbreaks occur in poultry, depopulation (or culling, also called “stamping out”) of infected flocks is usually carried out. In addition, surveillance of flocks that are nearby or linked to the infected flock(s) and quarantine of exposed flocks with culling if disease is detected, are the preferred control and eradication methods.

Avian Influenza A Virus Classification
Avian influenza A viruses are  classified into the following two categories:
Low Pathogenic Avian Influenza (LPAI)

Low pathogenic avian influenza viruses cause either no signs of disease or mild disease in chickens/poultry (such as ruffled feathers and a drop in egg production). 

Most avian influenza A viruses are low pathogenic and cause few signs of disease in infected wild birds. In poultry, some low-pathogenic viruses can mutate into highly pathogenic avian influenza viruses.

Highly Pathogenic Avian Influenza (HPAI)

Highly pathogenic avian influenza viruses cause severe disease and high mortality in infected poultry. 

Only some avian influenza A(H5) and A(H7) viruses are classified as HPAI A viruses, while most A(H5) and A(H7) viruses circulating among birds are LPAI A viruses.

Avian Influenza A Subtypes

Influenza A Viruses Naming Convention

Influenza A viruses are divided into subtypes on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA).

There are 18 known HA subtypes and 11 known NA subtypes.

Many different combinations of HA and NA proteins are possible. For example, an “A(H7N2) virus” designates an influenza A virus subtype that has an HA 7 protein and an NA 2 protein. Similarly, an “A(H5N1)” virus has an HA 5 protein and an NA 1 protein.

Avian A(H5) Viruses

There are nine (9) known subtypes of A(H5) viruses: 

A(H5N1) A(H5N4) A(H7N7)
A(H5N2) A(H5N5) A(H8N8)
A(H5N3) A(H6N6) A(H9N9)

Most A(H5) viruses identified worldwide in wild birds and poultry are LPAI, but occasionally HPAI A(H5) viruses have been detected, including the ongoing HPAI H5N1 global outbreak.

HPAI A(H5N1) Outbreak 2022-2024

HPAI A(H5N1) viruses that have been currently associated with poultry outbreaks and wild bird detections in many countries. In recent months there also have been numerous non-avian species found infected with HPAI A(H5N1), including many mammals such as red foxes, seals, bears, etc.

Human HPAI A(H5) Infections

Human infections with HPAI A(H5N1) virus have been reported in 19 countries since 2003, resulting in severe pneumonia and death in more than 50% of cases. Human infections with HPAI A(H5N6) virus have been reported since 2014 from two countries with death occurring in more than 40% of cases, and human infections with HPAI A(H5N8) virus were reported from one country in 2021.

Avian A(H6) Viruses
LPAI A(H6) virus outbreaks in birds are not internationally reportable, therefore, its true prevalence is unknown. However, LPAI A(H6) viruses have been identified in various species of wild waterfowl and domestic poultry in Eurasia and the Americas.  Known subtypes of A(H6) viruses include LPAI A(H6N1) and A(H6N2). In 2013, Taiwan reported the first known human infection with LPAI A(H6N1) virus.
Avian A(H7) Viruses

There are nine (9) known subtypes of A(H7) viruses:

A(H7N1) A(H7N4) A(H7N7)
A(H7N2) A(H7N5) A(H7N8)
A(H7N3) A(H7N6) A(H7N9)

Most A(H7) viruses identified worldwide in wild birds and poultry are LPAI viruses.

Human A(H7) Infections

Avian influenza A(H7) virus infection of humans have occurred sporadically. The most frequently identified A(H7) viruses associated with human infections are avian influenza A(H7N9) viruses, which were first detected in China in 2013. While human infections with A(H7N9) viruses are uncommon, they have resulted in severe respiratory illness and death in approximately 40% of reported cases.

In addition to A(H7N9) viruses, human infections with A(H7N2), A(H7N3), A(H7N4), and A(H7N7) viruses have been reported and have primarily caused mild to moderate illness with symptoms that included conjunctivitis and/or upper respiratory tract symptoms.

Avian A(H9) Viruses
There are nine (9) known subtypes of A(H9) viruses:
A(H9N1) A(H9N4) A(H9N7)
A(H9N2) A(H9N5) A(H9N8)
A(H9N3) A(H9N6) A(H9N9)

All A(H9) viruses identified worldwide in wild birds and poultry are LPAI viruses.

Human A(H9) Infections

A(H9N2) virus has been detected in bird populations in Asia, Europe, the Middle East and Africa.  Sporadic A(H9N2) virus infections have been reported in people with mild upper respiratory tract illness, although some infections have resulted in death.

Avian A(H10) Viruses
There are six (6) identified subtypes of A(H10) virus:
A(H10N3) A(H10N5) A(H10N7)
A(H10N4) A(H10N6) A(H10N8)
Non-Avian A(H10) Infections

A(H10N4) was found in a mink in 1984 and A(H10N5) was found in swine (pigs) in 2008.

Human A(H10) Infections

The A(H10) virus subtypes known to have caused human infections include A(H10N3), A(H10N7), and A(H10N8).

  • Egypt reported the first human infections with A(H10N7) virus in 2004
  • Australia reported human infections with A(H10N7) virus in March 2010
  • The first human infections with A(H10N8) virus were reported in China in December 2013
  • The first human A(H10N3) virus infection was reported in China in June 2021

Most A(H10) virus infections in people have resulted from exposure to infected poultry.

Species Susceptible to Influenza A Viruses

Species Susceptible in Influenza A Viruses

Influenza A viruses have been detected and are known to circulate in seven different animal species or groups, including:

  • humans
  • wild water birds
  • domestic poultry
  • swine
  • horses
  • dogs
  • bats

In many other animal species, avian influenza A viruses have been reported to cause occasional infections, but do not regularly spread among them (e.g., cats and seals). Equine (horse) influenza A(H3N8) virus routinely circulates and can cause illness in horses, and canine (dog) influenza A(H3N2) virus routinely circulates and can cause illness in dogs.

Highly Pathogenic Avian Influenza (HPAI)

Highly Pathogenic Avian Influenza

Highly pathogenic avian influenza (HPAI) viruses cause severe disease and high mortality in infected poultry. While most strains of A(H5) and A(H7) are low pathogenic (LPAI) viruses, there are several strains of A(H5) and A(H7) that are considered highly pathogenic (HPAI).

These HPAI A(H5) or A(H7) virus infections can cause disease that affects multiple internal organs with mortality up to 90% to 100% in chickens, often within 48 hours. However, ducks can be infected without any signs of illness.

HPAI A(H5) and A(H7) virus infections in poultry also can spill back into wild birds, resulting in further geographic spread of the virus as those birds migrate. 

While some wild bird species can be infected with some HPAI A(H5) or A(H7) virus subtypes without appearing sick, other HPAI A(H5) and A(H7) virus subtypes can cause severe disease and mortality in some infected wild birds as well as in infected poultry.

Avian Influenza in Humans

Avian Influenza A Subtypes

Five subtypes of avian influenza A viruses are known to have caused human infections:

  • H5
  • H6
  • H7
  • H9
  • H10

The most frequently identified subtypes of avian influenza causing human infections are H5, H7, and H7. The majority of avian influenza A virus infections reported in people are:

  • A(H5N1)
  • A(H7N9)
  • HPAI A(H5N6)
  • LPAI A(H9N2)

Human infections with other subtypes, such as A(H6N1), A(H10N3), A(H10N7), and A(H10N8), have been detected in small numbers of people. In the United States, no HPAI A(H7) virus infections have ever been reported in people; however, there have been four laboratory-confirmed cases of LPAI A(H7N2) virus infection in people.