Infectious agents, such as viruses, bacteria and fungi, can spread disease from one living being to another. Bird flu, or avian influenza, is an infectious respiratory disease caused by a virus that occurs naturally among birds, according to the Centers for Disease Control and Prevention. Various strains of this virus are categorized according to the pathogenicity, or ability to produce disease. There are at least 15 different types of bird flu, according to Health Canada.
Wild Birds
The avian influenza virus is common among wild birds. These are low pathogen strains, meaning they cause very little disease to the wild birds that carry the virus in their intestines. Even though it produces few symptoms in wild birds, avian flu is very contagious among birds and a strain of this infectious agent can pass from wild birds to domesticated birds.
Domesticated Birds
The virus is passed out of the body through a bird's nasal secretions, saliva and stool. The high pathogen strains of avian flu make domesticated birds extremely sick. Birds who are kept indoors, as in a commercial egg farm, are not as likely to contract avian influenza as are birds who are allowed outdoors, such as free-range chickens.
Bird to Human Transmission
Another avian influenza virus, Influenza A H7, has been known to infect people in direct contact with infected birds . Influenza A H5 infections among humans are possible; this virus can cause severe illness or even death.
Human to Human Transmission
In 2004, bird flu jumped from birds to humans in a strain of the infectious agent scientists dubbed H5N1. The group in which H5N1 is part of can be high pathogenic or low pathogenic, causing mild or severe illness. H5N1 is not easily passed from human to human, but viruses can mutate and change into a form that is easily transmitted from one person to another.
Subtypes
There are many different subtypes possible with influenza A. These different subtypes differ because of, and are named for, two proteins found on the surface of the virus: hemagglutinin and neuraminidase. Specifically, the H in H1N1 stands for hamagglutinin and the N in H1N1 indicates it is the first subtype of neuraminidase. There are currently 16 known hemagglutinin subtypes of influenza A virus, and nine subtypes of neuraminidase influenza A. All known subtypes of influenza A are found in birds, but these subtypes can cross over and infect humans, and have done so several times throughout history. There are only three subtypes of influenza A virus currently among humans, including H1N1, H1N2 and H3N2, according to the Centers for Disease Control and Prevention.
References
- Centers for Disease Control and Prevention: Key Facts About Avian Influenza (Bird Flu) and Avian Influenza A (H5N1) Virus
- University of Missouri Extension: Avian Influenza Biosecurity for Small Flock Owners
- Health Canada: It's Your Health: Avian Influenza (Bird Flu)
- Centers for Disease Control and Prevention: Avian Influenza A Viruses
