Update 4-26-09: I see this page is getting a lot of hits from Google in the wake of the Swine Flu outbreak in in California and Mexico. Please note that I am not suggesting this new Swine Flu is likely to be even remotely as dangerous as the 1918 flu! This post was written before the outbreak and was mainly supposed to be pointing out a curiosity.

If you want to learn more about the present outbreak I highly recommend the Effect Measure blog.

My own blog posts on influenza are mainly focused on the underlying molecular biology and immunology, rather than the epidemiology that’s of special interest now.

You may also be interested in the Google Flu Tracker; but note that it’s probably not accurately reflecting case number at the moment.

Influenza signThis turns out to be an old hypothesis that I had never before run across, not a brand-new insight; but it’s still a holy crap moment for me.

In 1918, an immensely virulent strain of influenza virus swept across the globe, killing tens if not hundreds of millions of people.   (See here for a little more.)  It came from nowhere, and it disappeared as fast as it came, leaving no descendants; we haven’t seen the 1918 influenza since 1918. 1

Or so I thought.

Turns out the 1918 flu never left.  Its great-great-grandchildren are still out there – mutated, more or less harmless to man, but still recognizable.  In the same way as the dinosaurs still roam the Earth, chirping and clucking, the 1918 influenza virus became swine flu.

Early serological studies linked … the swine H1N1 1930 virus isolate to the 1918 pandemic virus. Laidlaw suggested that the swine influenza virus could be the 1918 pandemic influenza virus which became established in pigs. … Interestingly, the 1930 swine influenza virus may still be circulating in swine. 2

This was proposed — I had no idea — as early as 19353, and several lines of observation have subsequently supported the concept.  The most recent paper,2 from which I learned all this fascinating stuff, directly demonstrated that  “the human 1918 influenza virus can infect and replicate in pigs and cause clinical disease and lesions in the infected animals,” though the disease in swine is fairly mild.  Their reconstruction of the story:

… one could speculate that the initial interspecies transmission of influenza virus during the 1918 pandemic occurred from people to pigs and only later appeared to occasionally transmit back to people, … likely contributing at least regionally to the maintenance and spread of the disease. The virus spread throughout the swine population, adapted to the swine host, and subsequently resulted in the current lineage of the classical H1N1 swine influenza viruses. 2

(Apparently genetic evidence suggests that the circulating H1N1 strains of human influenza are also descendants of the 1918 flu, which I also did not know — clearly I really need to learn a lot more about influenza — making this finding even less surprising to those in the know.  But still.  Holy crap.)

  1. Actually, since early 1919, I guess, but you get my point.[↩]
  2. Weingartl, H., Albrecht, R., Lager, K., Babiuk, S., Marszal, P., Neufeld, J., Embury-Hyatt, C., Lekcharoensuk, P., Tumpey, T., Garcia-Sastre, A., & Richt, J. (2009). Experimental Infection of Pigs with the Human 1918 Pandemic Influenza Virus Journal of Virology, 83 (9), 4287-4296 DOI: 10.1128/JVI.02399-08[↩][↩][↩]
  3. Laidlaw, P. P. 1935. Epidemic influenza: a virus disease. Lancet 1:1118-1124[↩]