The influenza pandemic that killed between 50 million and 100 million people around the world in 1918 was blamed on birds. However, a new study has found that birds were not entirely at fault.
A new study titled, "A synchronized global sweep of the internal genes of modern avian influenza virus," led by evolutionary biologist and professor at the University of Arizona, Michael Worobey, analyzed over 80,000 gene sequences from flu viruses that were isolated from humans, birds, horses, pigs, and bats, and mapped them according to evolutionary relationships between viruses from different host species. The map that emerged showed that the flu in 1918 did originate from an avian host, but they had circulated around the world since the 1870s through horses.
However, what is revolutionary about the study is that in providing the most comprehensive analysis of the relationships of these influenza viruses across different host species over time, it was revealed that in the mid 19th century there was a "global sweep" that replaced six of the virus's eight genes that code for protein, hinting that the ancestor of modern flu viruses first appeared in horses.
The study also showed that the avian virus and the equine virus (or H7N7) that plagued Toronto and North America in 1872 were linked, and the genomic analysis is in agreement with historical facts pertaining to the epidemic and the period. The flu originated from the birds, and were carried around the globe by horses.
The study was not clear how the virus jumped from fowl to horse, and if there was ever a jump back to fowl from horse. However, taking into consideration the proliferation of horses at the time during the era before the combustion engine arrived, the world did not stand a chance against the spread of the virus. It was this virus that contributed most of its genes to the avian flu virus in 1918.
In 2005, a genetic analysis of the 1918 virus that was harvested from the preserved tissue of a victim from the time, pointed to an avian origin. In 2009, a different study revealed that the viral genes circulated in both swine and humans for about 2 to 15 years before the 1918 pandemic struck, and at that time, combined to make the lethal virus that felled millions.
"We now have this idea that the source for a lot of influenza virus we see now worldwide is potentially equine, whereas the dogma has been for so long that its avian," said Oliver Pybus, an evolutionary biologist at the University of Oxford, UK. "It's a fascinating study, and quite a surprise."
"Once you resolve the evolutionary trees for these viruses correctly, everything snaps into place and makes much more sense," said Worobey.