A team of researchers from the United States and Japan set out to test if swine flu strains obtained from infected patients could cause disease in mice, ferrets and macaque monkeys. Their study led to two key findings. First, they learned that swine flu viruses (S-OIVs) are five times as more harmful as seasonal versions of the H1N1 flu strain, penetrating deeper into the lungs and inflicting more damage than ordinary seasonal flu. Tests in the animals showed that swine flu thrives in greater numbers throughout the respiratory system, including the lungs, and causes lesions, rather than staying in the nose and throat like seasonal flu. On a positive note, however, the research showed that the swine flu virus is vulnerable to both the main antiviral drugs, Tamiflu and Relenza, and to two experimental antiviral agents.

As the scientists, led by Dr. Yoshihiro Kawaoka from the University of Wisconsin-Madison, noted: "Most human infections with swine-origin H1N1 influenza viruses seem to be mild; however, a substantial number of hospitalized 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." Their findings may help explain why the swine flu pandemic has caused serious illness and deaths among people with no underlying health problems. However, other scientists are cautioning that the study exposed animals to much high doses of swine flu as compared to those involved when people typically become infected.

In addition, the scientists found that antibodies collected from patients born before 1920 were able to recognize the swine flu strains, appearing to give those born before the 1918 flu pandemic immunity to the current swine flu - but not to the seasonal flu that occurs each year. "I'm very concerned because clearly the swine flu virus is different from seasonal influenza," says Dr. Kawaoka. "It's a lot more severe, but it is still not as severe as the 1918 influenza," he says.

The research, which broadly replicates those of two studies published recently showing that swine flu causes more severe infections than seasonal flu in ferrets, will be published in the journal Nature.