We now know that the identity of the virus, which was first isolated from a pig in 1930, has changed. The swine influenza virus, a type A virus, is made up of a specific genetic sequence of hemagglutinin (H) and neuraminidase (N) proteins. According to the American Association of Swine Veterinarians, there are several combinations of H and N viruses (in pigs H1N1, H1N2, and H3N2; in humans, H1N1, H1N2, H2N2, and H3N2), though few have been consistent in either swine or human populations. It is the H1N1 subtype that is currently spreading through both communities.
"It’s unfortunate that this flu strain is being called 'swine' flu, because the virus is a combination of viruses including swine, [avian] and human influenzas," explains Dr. Bret Marsh, the Indiana state veterinarian. "The reality is that swine flu hasn’t been found in swine populations in the United States." Instead, the CDC believes the swine flu is being spread among people who had no contact with pigs.
Transmission of animal to human diseases is rare, because the receptor cells (the protein molecules that allow binding of the proteins in viruses) in humans differ just enough from the receptor cells in other mammals and birds that the viruses they carry do not find a hospitable home in humans. But in some cases a virus is able to genetically reassort itself, so that it can spread from one animal group to another, and then continue to spread without further contact with the original host animal.
What appears to have occurred in this instance is a combination of swine, avian, and human influenza viruses coming together within the same host -- in this case, a pig -- where it recombined during the genetic replication process, creating a new and more virulent influenza strain.
Officials are playing catch-up at this point in the race to learn more about this new influenza strain. The precautions being taken may appear to be exaggerated, especially since the level of fatal outcomes has remained low, but there is reason behind the madness. What health officials fear most is a repeat of the Spanish influenza epidemic of 1918-1919, which resulted in the deaths of millions of humans, the most fatal flu epidemic on record. Preventing the same outcome is the key concern.
There are, however, differences between this outbreak and the flu epidemic of 1918. The flu of 1918 could not be tracked to its point of origin, and does not appear to have been a reassortant virus. While every flu since has been related genetically to the Spanish flu, each has been a genetically reassorted variant, changed by the inclusion of an avian flu virus -- which can be tracked to its origin. Health professional do have an origin point at which to begin tracking the current disease, and are in the process of creating a working vaccine to control it.
The biggest difference between 1918 and 2009, and the method by which we will prevent another fatal global pandemic, may be as simple as clean hands and covering your nose and mouth with a tissue when you sneeze or cough (check out the CDC's swine influenza page for more information). What was unknown in 1918 was that invisible bacteria spreads illnesses. That knowledge alone is our biggest defense against this flu strain, and nearly all other zoonotic diseases.
Image: Gleb / via Flickr