The origin of the Spanish flu Pandemic, and the relationship between the near simultaneous outbreaks in humans and swine, have been controversial. One hypothesis is that the virus strain originated at Fort Riley, Kansas, in viruses in poultry and swine which the fort bred for food; the soldiers were then sent from Fort Riley around the world, where they spread the disease. Similarities between a reconstruction of the virus and avian viruses, combined with the human pandemic preceding the first reports of influenza in swine, led researchers to conclude that the influenza virus jumped directly from birds to humans, and swine caught the disease from humans. Others have disagreed, and more recent research has suggested that the strain may have originated in a non-human mammalian species. An estimated date for its appearance in mammalian hosts has been put at the period 1882–1913. This ancestor virus diverged about 1913–1915 into two clades which gave rise to the classical swine and human H1N1 influenza lineages. The last common ancestor of human strains dates to between February 1917 and April 1918. Because pigs are more readily infected with avian influenza viruses than are humans, it is suggested that they were the original recipient of the virus, passing the virus to humans sometime between 1913 and 1918.
An effort to recreate the 1918 flu strain (a subtype of avian strain H1N1) was a collaboration among the Armed Forces Institute of Pathology, Southeast Poultry Research Laboratory and Mount Sinai School of Medicine in New York City; the effort resulted in the announcement (on October 5, 2005) that the group had successfully determined the virus's genetic sequence, using historic tissue samples recovered by pathologist Johan Hultin from a female flu victim buried in the Alaskan permafrost and samples preserved from American soldiers.
Figure 1. An electron micrograph showing recreated 1918 influenza virions.
On January 18, 2007, Kobasa et al. reported that monkeys (Macaca fascicularis) infected with the recreated strain exhibited classic symptoms of the 1918 pandemic and died from a cytokine storm — an overreaction of the immune system. This may explain why the 1918 flu had its surprising effect on younger, healthier people, as a person with a stronger immune system would potentially have a stronger overreaction.
On September 16, 2008, the body of Yorkshireman Sir Mark Sykes was exhumed to study the RNA of the Spanish flu virus in efforts to understand the genetic structure of modern H5N1 bird flu. Sykes had been buried in 1919 in a lead coffin which scientists hope will have helped preserve the virus. In December 2008, research by Yoshihiro Kawaoka of the University of Wisconsin linked the presence of three specific genes (termed PA, PB1, and PB2) and a nucleoprotein derived from 1918 flu samples to the ability of the flu virus to invade the lungs and cause pneumonia. The combination triggered similar symptoms in animal testing.