Avian Flu Viruses

Avian influenza is caused by influenza A viruses. Influenza A viruses are found in a variety of animals, including birds, pigs, whales, horses, seals, and humans. (In contrast, influenza B viruses only infect humans. Influenza C viruses occur in pigs and humans, but rarely cause illness in humans.)

Avian influenza A was first identified in the 1900s during an outbreak in poultry in Italy. Since then, the disease has reappeared at irregular intervals around the world.

Influenza A viruses come in many different subtypes based on differences in their proteins. Each subtype can have many different strains. New subtypes and strains arise when the virus undergoes genetic mutations.

Most avian influenza A viruses are low pathogenicity viruses that usually cause mild disease in poultry and wild birds. In contrast, high pathogenicity viruses can cause severe illness and high mortality in poultry and in wild birds—including influenza A viruses H5N1, H7N7, and H7N3.

Viruses that are highly pathogenic to birds are not necessarily highly pathogenic to humans. Human infections have ranged from mild (H7N3 and H7N7) to severe and fatal (H7N7, H5N1).

Low pathogenicity viruses have the potential to evolve into high pathogenicity viruses, as has been documented in some poultry flocks.


Mutations are changes in the genetic code of an organism. These changes occur randomly in all living things. The building blocks of the genetic code are called nucleotide bases. There are four kinds of nucleotide bases, and their sequence determines what kinds of proteins an animal produces. A mutation occurs when one nucleotide base is randomly substituted for another. Some of these changes may affect the virus, possibly making it more harmful or less harmful—or it may have no effect at all.

Another type of mutation can occur when viruses exchange information with one another. These mutations can occur when different kinds of viruses come into contact with one another in a single host. Hypothetically, for example, a virus that is easily spread from person to person could exchange genetic information with a highly pathogenic strain of avian flu virus, creating a new strain that can be transmitted easily among humans.

The potential for this type of mutation to occur is greatest when there are many opportunities for the virus to multiply—in large flocks with many infected birds. The virus can spread more quickly in crowded conditions, and birds in high-density flocks may be more susceptible to the disease because the stressful conditions may weaken their immune systems.

Viruses have more opportunities to exchange information where these flocks are in close contact with humans or other domestic animals, raising the potential for a human or a pig, for example, to serve as a host for two flu viruses that can exchange genetic information and become more harmful to humans.

The Spanish Flu and other influenza A pandemics

The Spanish Flu of 1918 was an influenza A bird flu virus that mutated and caused a human pandemic. The new strain was not significantly deadly to birds, but it was fast-spreading and dangerous to humans. In the United States, about 2.5 percent of the people who contracted the disease died. More than half of those deaths were from complications from the flu, especially pneumonia and sinusitis.

There were three generally recognized influenza A pandemics in the 20th century:

1918: H1N1 strain (Spanish Flu) 550,000 deaths in the US, about 50 million deaths worldwide

1957: H2N2 (Asian Flu) 69,800 deaths in the United States, about 1 million deaths worldwide

1968: H3N2 (Hong Kong Flu) 33,800 deaths in the US, about 700,000 deaths worldwide

In comparison, each year about 33,000–36,000 people in the United States die from other kinds of flu (influenza B).