Influenza: Not as Simple as ABC's and 123's

December 14, 2015

A new flu season is never elementary for primary care. Two questions on flu and its spread that may prove educational.

[[{"type":"media","view_mode":"media_crop","fid":"41143","attributes":{"alt":"©Tatiana53/Shutterstock.com","class":"media-image media-image-right","id":"media_crop_7728138375003","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"4296","media_crop_rotate":"0","media_crop_scale_h":"0","media_crop_scale_w":"0","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","style":"height: 199px; width: 265px; float: right;","title":"©Tatiana53/Shutterstock.com","typeof":"foaf:Image"}}]]The new flu season is in full swing and chances are you are treating the illness every day even as you continue to provide vaccination against it. Most clinicians are aware of the A and B strains of influenza but a C strain can also cause disease in humans, although symptoms are mild at worst. Take a minute from your double-booked day and let’s see how well you know the ABC’s of influenza.

1.  Pandemic flu can be caused by:

A. “A” strains only

B. Usually “A” strains, but also occasionally “B” strains

C. Usually “A” strains, but also rarely “B” strains

D. “A” and “B” stains roughly equally

For answer, discussion, and next question, please click here.

Answer: A. “A” strains only.

Pandemics occur when a novel flu virus arises with surface antigens that most of the population’s immune systems have never “seen,” meaning that most individuals who are exposed will be infected. This happens in one of two ways, but both require an animal reservoir. Influenza A infects a lot of species, with porcine and avian strains being the most relevant for humans. Influenza B does infect seals but I’m not aware of a whole lot of human-seal close contact. One way a pandemic strain emerges is when an animal strain mutates and develops the ability to infect humans and to spread from human to human. The “mother of all flu pandemics” in 1918 unfolded this way.

All subsequent pandemics have arisen through a reassortment of human and animal genes. The combination and rearrangement most likely happens in a porcine host that is simultaneously infected with several different animal or human strains and starts shedding novel combinations of flu virus, the majority of which, fortunately, are noninfectious.

A few other letters also are important in a discussion of influenza A. The 2 main antigens on the surface of influenza A that result in production of neutralizing antibodies are the N (neuraminidase) and H (hemagglutinin) antigens.

2. Which of the following combinations cause the the CDC and WHO the most concern during conversations about our next pandemic flu?

A. H1N1 (an H1N1 caused the 2009 and 1918 pandemics)

B. H3N2 (an H3N2 caused the 1968 “Hong Kong” flu)

C. H2N2 (an H2N2 caused the 1957 “Asian” flu)    

D. H5N1 (a strain of bird flu currently circulating) 

For answer, discussion, and next question, please click here.

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The reason for the suspicion is that the US government has a stockpile of H5N1 vaccine in storage. The H5N1 avian strain identified in Southeast Asia in 2005 has been transmitted from bird-to-human, but not from human-to-human in a sustained manner. Among the identified cases, the mortality rate is in the 50% range. For comparison, smallpox is thought to have about a 30% mortality rate. The 1918 flu pandemic had an estimated 3-5% death rate around the world.  I personally worry a lot more about a terrorist trying to “weaponize” this H5N1 virus (by figuring out how to make it transmissible from human to human) than I do about the other potential biologic weapons of mass destruction like anthrax or smallpox.

If you got both questions right, give yourself an “A.”