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Avian Influenza

Avian Influenza 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.

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 the past year numerous non-avian species found infected with HPAI A(H5N1) including many wild mammals, including foxes, raccoons, bears, seals, etc.  In recent months there has been an outgoing multi-state outbreak in U.S. dairy cattle, as well as domestic cats, and house mice. 

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.

In June 2024, the first ever laboratory confirmed human infection HPAI A(H5N2) was reported in Mexico.

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.

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.

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.

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.