Influenza A: Definition and Etiology

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.