Types of Bird Flu (Avian Flu)

There are many different subtypes of type A influenza viruses. These subtypes differ because of certain proteins on the surface of the virus (hemagglutinin [HA] and neuraminidase [NA] proteins). Each different combination of HA and NA proteins is a different subtype of the virus.

Infection with certain avian influenza A viruses (e.g., some H5 and H7 strains) can cause widespread disease and death among some species of domestic birds. Since 1997, several outbreaks of H5 and H7 influenza have occurred among poultry in the United States.

In some parts of the world, the H5N1 avian influenza virusand H7N9 virus have resulted in bird flu in humans. In people, avian flu caused by the H5N1 virus is very aggressive and poorly understood. Bird flu has a high death rate in humans, possibly because people have little immune system protection against avian influenza viruses.

Bird Flu Transmission

Once infected, birds shed (cast off) the influenza virus in saliva, nasal secretions, and feces. Other birds become infected when they come into contact with contaminated excretions (e.g., feces) or surfaces. The viruses spread when infected birds move from place to place, and also through contaminated shoes and clothing, farm equipment, feed, cages, and vehicles.

In people, most cases of bird flu infection have occurred as a result of contact with infected birds. Infection also may result from contact with contaminated surfaces, and from eating undercooked poultry or eggs from infected birds. Avian influenza infection may be possible through contact with a person who has it, but the spread of bird flu from one person to another is rare.

Researchers look at a number of factors—including the genetic markers of each virus—to help determine the risk for human infection. Changes (mutations) in viruses, making person-to-person transmission easier, could result in a worldwide outbreak (pandemic) of bird flu.

The U.S. Centers for Disease Control and Prevention (CDC) recommends that the general public avoids contact with wild birds and with chickens or turkeys that appear sick or have died.

Updated by Remedy Health Media

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 01 Aug 2007

Last Modified: 01 Sep 2015