Influenza causes

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Taxonomy

• There are three types of influenza viruses: A, B and C.
• Human influenza A and B viruses are responsible for causing seasonal epidemics of the disease almost every winter in the United States.
• The emergence of a new and very different influenza virus to infect people can cause an influenza pandemic.
• Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.
• The international naming convention for influenza viruses uses the following components to name the virus:

• The antigenic type (e.g., A, B, C)
• The host of origin (e.g., swine, equine, chicken, etc. For human-origin viruses, no host of origin designation is given.)
• Geographical origin (e.g., Denver, Taiwan, etc.)
• Strain number (e.g., 15, 7, etc.)
• Year of isolation (e.g., 57, 2009, etc.)
• For influenza A viruses, the hemagglutinin and neuraminidase antigen description in parentheses (e.g., (H1N1), (H5N1)

Influenza A

• Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes. (H1 through H18 and N1 through N11 respectively.)
• Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus (CDC 2009 H1N1 Flu website) emerged to cause illness in people. This virus was very different from the human influenza A (H1N1) viruses circulating at that time. The new virus caused the first influenza pandemic in more than 40 years. That virus (often called “2009 H1N1”) has now replaced the H1N1 virus that was previously circulating in humans.

Influenza B

• Influenza B viruses are not divided into subtypes, but can be further broken down into lineages and strains.
• Currently circulating influenza B viruses belong to one of two lineages: B/Yamagata and B/Victoria.

Natural Reservoir

Different hemagglutinin and neuraminidase subtypes and the species in which they have been detected

Species	Hemagglutinin Subtypes	Neuraminidase Subtypes
Humans	H1, H2, H3, H5, H6, H7, H9, H10	N1, N2, N6, N7, N8, N9
Poultry	H1, H2, H3, H4, H5, H6, H7, H8, H9, H10, H11, H12, H13, H14, H15, H16	N1, N2, N3, N4, N5, N6, N7, N8, N9
Pigs	H1, H2, H3, H4, H5, H9	N1, N2
Bats	H17, H18	N10, N11
Adapted from CDC