Today WHO reports over 8800 cases of influenza A (H1N1) occurring globally, with most of the new cases stemming from continuing transmission in Mexico and a growing epidemic in Japan. WHO also reports 74 deaths; 4 of these occurred in the US, but this is not inclusive of the death of a New York school principal reported today.
“Swine flu” has actually been around for quite some time — H1N1 emerged to cause the 1918 influenza pandemic. Over time, the virus adapted to its host, and now circulates as a less virulent seasonal strain among humans.
Agent-Host interaction
Why would a virus adapt to become less virulent? The selection process is crafty. A virus or other pathogen that is too virulent will kill off its host before it has the chance to be transmitted to enough other hosts to maintain circulation in the population. A virus that becomes less virulent (this can happen through small genetic changes during virus replication) enables itself to reproduce in the host and also allows the host survive and share the pathogen with his or her contacts. Over time, this is a natural selection process that can allow the less virulent strain to dominate.
What’s different about H1N1 now?
Influenza viruses have a segmented genome. As the genome replicates inside a host cell, each virion is packed with the requisite gene segments. Imagine, however, if the same cell in the same host were infected with two completely different influenza viruses. As the viruses replicate themselves, the gene segments can be mixed up, or re-assorted, using the host as a mixing vessel. This results in a dramatically different strain of influenza. It happens that swine are particularly good as influenza mixing vessels. In the 1990’s, surveillance in the US began to see a new influenza virus infecting herds of pigs. This virus was a triple reassortant, combining genes from swine, avian, and human viruses. Sporadic human cases were of the triple reassortant were observed in the US, and you can read about the epidemiology of recent cases in this New England Journal of Medicine article.
The novel H1N1 virus currently circulating around the world, known in the literature as S-OIV (swine-origin influenza A (H1N1) virus), is the essentially a quadruple reassortant – the triple reassortant with an additional Eurasian swine influenza virus reassortment. But it still has it’s roots in the 1918 strain. Read about the current epidemiology of US cases of S-OIV in this article, Emergence of a Novel Swine-Origin Influenza A (H1N1)Virus in Humans, and be sure to catch the daily updates on case tallies on US cases from the CDC at http://www.cdc.gov/h1n1flu/ and global cases from WHO.
