Swine flu penetrates deeper into the lungs and can inflict more damage than ordinary seasonal flu, scientists have found.
The discovery could explain why the virus is able to cause severe illness in people with no underlying health problems.
Experts also believe swine flu is closely related to strains responsible for the 1918 pandemic which killed up to 40 million people worldwide.
Evidence suggests that people born after 1920 have little natural resistance to the virus.
US and Japanese researchers tested the ability of swine flu strains obtained from infected patients to cause disease in mice, ferrets, and macaque monkeys.
They found that swine-origin influenza viruses (S-OIVs) were about five times more harmful than seasonal versions of the H1N1 flu strain.
Their effect on the lungs was reminiscent of H5N1 bird flu, which is almost always fatal when it infects humans. But in the case of swine flu, most victims recover.
The viruses were also found to infect pigs without causing disease symptoms. This could be why there were no reports of flu outbreaks in pigs before humans started to get ill.
Close inspection showed that the new strains were similar to the 1918 pandemic viruses.
Antibodies collected from patients born before 1920 were able to recognise them, and would presumably offer some immunity. But there was little evidence individuals born after 1920 harboured antibodies that could target swine flu.
A report on the research appeared online today in the journal Nature.
The scientists, led by Dr Yoshihiro Kawaoka from the University of Wisconsin-Madison in the US, wrote: "Most human infections with swine-origin H1N1 influenza viruses (S-OIVs) seem to be mild; however, a substantial number of hospitalised individuals do not have underlying health issues, attesting to the pathogenic potential of S-OIVs..
"Our findings indicate that S-OIVs are more pathogenic in mammalian models than seasonal H1N1 influenza viruses."
The good news from the research was that Tamiflu and other antiviral drugs were effective against swine flu viruses.
However the scientists warned that sustained person-to-person transmission might lead to the emergence of more dangerous, drug resistant strains.
Commenting on the research, Professor Ian Jones, from the University of Reading, said: "This complete analysis of the current H1N1 is what we've been waiting for. It shows that the new virus is about five times more pathogenic than seasonal H1N1 but that, nonetheless, the major outcome to infection is recovery. For the few cases of severe infection the data will help in clinical management of hospitalised patients."
Professor Wendy Barclay, chair in influenza virology at Imperial college London, said: "By comparison with a seasonal human H1N1 virus, it is shown that the sw (swine) origin H1N1 infect cells deeper into the respiratory tract. It must be borne in mind that typical circulating human strains of H1N1 have been associated with rather mild illness in recent years, and that the sw origin H1N1 may be behaving in these animal models more like the type of H3N2 viruses that caused a pandemic in 1968."
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