The new H1N1 virus is only distantly related to its nearest relatives, suggesting that its genes may have been circulating undetected in pig populations for some time, according to new research published online by the journal Science. Scientists also confirm the novel status of the virus, noting that it contains a combination of gene segments that has never been reported before.
"This study reinforces the fact that swine are an important reservoir of influenza viruses with the potential to cause significant respiratory outbreaks or even a possible pandemic in humans, and the results of the study show the global need for more systemic surveillance of influenza viruses in pigs," commented Dr Nancy Cox, one of the authors of the paper and Director of the Influenza Division at the Centers for Disease Control and Prevention (CDC) in the US.
The study was partly supported by the EU through the EMPERIE (European management platform for emerging and re-emerging infectious disease entities) project, which is funded under the Seventh Framework Programme (FP7).
The researchers arrived at their conclusions by sequencing the genomes of over 70 samples of the H1N1 virus, most of which were taken from patients in Mexico and the US.
"From our analysis, we have confirmed that the novel H1N1 virus likely originated from pigs," said Dr Cox, adding that the new virus is very different from human H1N1 viruses, which indicates that "seasonal influenza H1 vaccines may not protect people from infection with this novel virus."
With the help of the samples, the researchers were able to piece together the history of the virus. Six of the eight gene segments studied came from a known strain of swine flu that has been circulating in North America and Asia since 1998. The remaining two segments come from Eurasian swine virus strains that have never been detected outside Eurasia until now. This combination of gene segments has never been seen before.
The samples also turned out to be extremely similar to one another, indicating that the virus crossed into humans just once. Alternatively, very similar viruses may have crossed into humans a number of times.
"While our analysis shows that all gene segments are derived from swine influenza viruses, at this time we do not know if the virus entered the human population directly from swine or via an intermediate host, nor do we know for certain the exact host that the viruses might previously have circulated in to obtain its current properties," Dr Cox pointed out.
She added that veterinary researchers around the world are now delving into their freezers to see if they have samples from pigs or other animals that could shed new light on the emergence of the new virus.
According to Dr Cox, the fact that the viruses are similar "makes our job of coming up with a reference candidate vaccine virus much, much easier."
According to the latest figures from the World Health Organization (WHO), a total of 12,954 cases (including 92 deaths) of H1N1 influenza have been officially diagnosed across 46 countries worldwide. Most of these cases are in the US and Mexico, which have so far reported 6,764 and 4,174 cases respectively.
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