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Avian Influenza

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 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.

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.

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.