Influenza A: Definition and Etiology

by John Fisher, MD

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    00:01 In our discussion of respiratory tract infections, we turn now to the very important subject of influenza A.

    00:11 Influenza A is a an orthomyxovirus virus and it’s the cause of an acute, febrile, respiratory illness which occurs in outbreaks of varying severity almost every winter in temperate climates and year-round in tropical climates.

    00:28 And there’s a picture of what this culprit looks like.

    00:35 As an orthomyxovirus, it’s a single-stranded RNA virus and there are three different types of influenza virus: A, B, and C.

    00:48 It causes more severe and widespread disease than almost any other viral infection.

    00:56 And the key parts of the influenza virus are those spikes that you see sticking out.

    01:04 One of them, if you notice, is named HA-hemagglutinin.

    01:11 The other is neuraminidase.

    01:15 Those are the two key spikes that allow the virus to get into a respiratory cell and make more copies of itself, and the new copies have to emerge because of the second spike, more about that shortly.

    01:37 The history of influenza is tragically fascinating and first came to world attention in World War I with the so-called Spanish flu of 1918.

    01:54 Twenty to forty percent of the world population became ill with 50 million deaths worldwide, 675,000 in the U.S. alone.

    02:06 The soldiers were absolutely decimated by the Spanish flu.

    02:14 The Asian flu was the next, but shrunk in proportion compared to the Spanish flu.

    02:24 There were one to two million deaths worldwide, more than 3,500 deaths in the U.K., and about 70,000 deaths in the United States.

    02:35 Now, most of these deaths are due to the complications of the flu.

    02:42 And remember that we had no antibiotics in 1918 and we had a relative few antibiotics in 1957.

    02:55 The Hong Kong flu of 1968 accounted for a million deaths worldwide, 33,000 in the United States.

    03:05 And we were all concerned about the return of the Spanish flu, which was a similar virus in 1976 and many people were immunized, but it never caused a pandemic that was expected.

    03:18 There was another pandemic of H1N1 influenza in 2009 with 480,000 cases worldwide with 6,000 deaths, much lower than the others.

    03:36 Let’s talk about the naming of influenza viruses.

    03:41 There’s lots of variability about these spikes, for example, the hemagglutinin spikes there are 16 antigenic types, and the neuraminidase spikes there are 9 antigenic types.

    03:54 And the viruses can change and mix and match these types and some of us have immunity to some types but not to others.

    04:06 The formal name for an influenza virus, first of all, if it’s influenza A that’s listed first, and then the first place that it was isolated and the date it was first isolated.

    04:23 So that would be A/Puerto Rico/8/34.

    04:31 More commonly, we refer to the viruses at just by their antigenic types, like A/H1N1 or A/H3N2.

    About the Lecture

    The lecture Influenza A: Definition and Etiology by John Fisher, MD is from the course Upper Respiratory Infections.

    Included Quiz Questions

    1. Single-stranded RNA virus
    2. Double-stranded RNA virus
    3. Single-stranded DNA virus
    4. Double-stranded DNA virus
    1. HA hemagglutinin and NA neuraminidase surface glycoproteins
    2. The helical nucleocapsid structure
    3. The lipid bilayer membrane
    4. The M2 protein
    5. The intracellular glycoproteins M and N
    1. The various combinations of the 16 different HA hemagglutinin antigenic types and 9 different NA neuraminidase antigenic types.
    2. The various molecular structure of the viral RNA, e.g helical nucleocapsid versus circular plasmid.
    3. The various surface lipoproteins.
    4. The various antigenic types of the M2 protein.
    5. Whether it is a double stranded RNA virus versus and single stranded retrovirus.

    Author of lecture Influenza A: Definition and Etiology

     John Fisher, MD

    John Fisher, MD

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