In a new, highly detailed study of swine flu virus, H1N1, researchers have found that the pathogen is more virulent than previously believed.
Led by University of Wisconsin-Madison virologist Yoshihiro Kawaoka, the study has found that the H1N1 virus exhibits an ability to infect cells deep in the lungs, where it can cause pneumonia and, in severe cases, death.
Seasonal viruses typically infect only cells in the upper respiratory system.
"There is a misunderstanding about this virus. People think this pathogen may be similar to seasonal influenza. This study shows that is not the case. There is clear evidence the virus is different than seasonal influenza," Nature magazine quoted Kawaoka as saying.
He says that the ability to infect the lungs is a quality frighteningly similar to those of other pandemic viruses, notably the 1918 virus, which killed tens of millions of people at the tail end of World War I.
The study has also found another similarity to the 1918 virus-people born before 1918 harbor antibodies that protect against the new H1N1 virus.
Kawaoka reveals that the virus could become even more pathogenic as the current pandemic runs its course, and the virus evolves to acquire new features.
It is now flu season in the world's southern hemisphere, and the virus is expected to return in force to the northern hemisphere during the fall and winter flu season.
For the study, the researchers infected different groups of mice, ferrets and non-human primates with the pandemic virus and a seasonal flu virus.
They found that the H1N1 virus replicates much more efficiently in the respiratory system than seasonal flu, and causes severe lesions in the lungs similar to those caused by other more virulent types of pandemic flu.
"When we conducted the experiments in ferrets and monkeys, the seasonal virus did not replicate in the lungs. The H1N1 virus replicates significantly better in the lungs," said Kawaoka.
The study also assessed the immune response of different groups to the new virus, and, surprisingly, found that people exposed to the 1918 virus, all of whom are now in advanced old age, have antibodies that neutralize the H1N1 virus.
The study also indicated that existing and experimental antiviral drugs could form an effective first line of defense against the virus and slow its spread.