THURSDAY, July 2 (HealthDay News) -- Scientists have uncovered some intriguing clues about why the new swine flu frequently brings on gastrointestinal distress and vomiting, symptoms not usually associated with seasonal flu.

In experiments with ferrets, research teams in the United States and the Netherlands found that the new H1N1 flu virus replicated more extensively in the respiratory tract, going to the lungs, whereas the seasonal flu virus stayed in the animals' nasal cavity. The U.S. team also found that the new virus, unlike the seasonal one, went into the ferrets' intestinal tract.

Such distinctions, the U.S. researchers said, can make a difference in establishing appropriate public health responses as the pandemic continues around the world, so far sickening more than a million people in the United States alone.

"Findings from the study demonstrate that, in ferrets, the novel 2009 H1N1 influenza virus leads to increased morbidity and increased respiratory disease when compared to contemporary seasonal human influenza viruses," said researcher Terrence M. Tumpey, a senior microbiologist in the influenza branch of the U.S. Centers for Disease Control and Prevention in Atlanta.

"Additionally, virus transmission was less effective in ferrets infected intranasally with novel 2009 H1N1 influenza virus, compared to those infected with contemporary seasonal human influenza viruses," he added.

The reports are published in the July 2 online edition of Science.

When both teams looked at how easily the new H1N1 virus can be transmitted, they came to different conclusions, however.

The Dutch researchers found that the new H1N1 virus and the seasonal flu virus were equally good in infecting the animals.

But Tumpey's team found that the swine flu virus might not be transmitted as easily as the seasonal flu virus. "The novel 2009 H1N1 influenza viruses exhibited less efficient respiratory droplet transmission in ferrets, in comparison to the high-transmissibility of a seasonal H1N1 virus," he said.

Ferrets are used to study influenza because the flu virus affects them in a similar way to humans, the researchers noted.

"One thing we know for sure about influenza viruses is that they are unpredictable," Tumpey added. "The characteristics that the virus is displaying today might not hold true in the upcoming months."

It is important to remember, he said, that this is a new influenza virus never seen in humans before April 2009.

"The virus does not appear to be fully adapted to its new human host," Tumpey said. "How the virus may adapt further as it circulates among people is not known. However, this uncertainty makes it imperative that the virus and the epidemiology of the outbreak be closely monitored."

Dr. Pascal James Imperato, dean of the School of Public Health at SUNY Downstate Medical Center in New York City, found the new research added key information to what was already known.

"In this study, it was found that the 2009 H1N1 virus was less efficiently transmitted by droplet infection in ferrets compared to the seasonal human H1N1 virus," Imperato said. "This is a significant finding as it indicates that the 2009 swine flu virus might not be as easily transmitted between humans as its seasonal counterpart."

On the other hand, he added, the findings also "collectively demonstrate that it has the potential to cause serious clinical illness that also results in gastrointestinal symptoms, which were, in fact, observed in a number of patients."

On June 11, the World Health Organization declared the first flu pandemic since 1968, triggered by the rapid spread of the H1N1 swine flu virus across North America, Australia, South America, Europe and regions beyond. Two weeks ago, U.S. health officials said they were considering a swine flu immunization campaign that could involve an unprecedented 600 million doses of vaccine. That would dwarf the 115 million vaccine doses given annually for seasonal flu.

More information

The U.S. Centers for Disease Control and Prevention has more on swine flu.

SOURCES: Terrence M. Tumpey, Ph.D., senior microbiologist, influenza branch, U.S. Centers for Disease Control and Prevention, Atlanta; Pascal James Imperato, M.D., M.P.H., dean, and distinguished service professor, School of Public Health, SUNY Downstate Medical Center, New York City; July 2, 2009, Science

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