Scientists in Hong Kong say they may have helped explain why the H5N1 bird flu virus kills so many healthy young adults -- it apparently causes a "storm" of immune system chemicals that overwhelms the patient.
The H5N1 virus caused proteins known as cytokines to rush to infected lung tissue -- evidence of a so-called cytokine storm, an immune system overreaction that can be fatal.
The study, published in the online medical journal Respiratory Research, might suggest that if H5N1 does cause a pandemic, it could disproportionately affect the young and healthy as compared with seasonal flu, which kills many elderly people but few young adults.
It also raises questions about how effective drugs will be in controlling such a pandemic, experts said.
"We have to see if it is true and if we can do anything about it," cautioned Dr. Anthony Fauci, head of the U.S. National Institute of Allergy and Infectious Diseases, who was not involved in the research.
But if the experiment does accurately show what happens in people, it may mean patients with H5N1 infections will need drugs that depress the immune response in addition to antivirals, Fauci said.
The H5N1 flu has swept through flocks of poultry but has so far infected only 124 people in four countries and killed 64 since it re-emerged in 2003.
It does not easily infect people, but when it does, it kills about half of them.
"The reasons for this unusual severity of human disease have remained unclear," Michael Chan and Malik Peiris of the University of Hong Kong and colleagues wrote in their report.
They took samples of H5N1 from a patient who died of the infection in a 1997 outbreak, from two patients infected in Vietnam in 2004, and a sample of a Hong Kong patient with ordinary H1N1 seasonal flu.
They used the virus to infect lung tissue samples taken from other, non-flu patients.
The H5N1 viruses brought in a storm of cytokines -- the immune system´s inflammatory chemicals -- including IP-10, interferon beta, RANTES and interleukin-6. The H1N1 virus caused a much smaller effect.
And the later, Vietnamese strains caused a bigger cascade than the 1997 H5N1 strain.
This could be because of continued mutations, the researchers said. "The H5N1 viruses have continued to reassort, acquiring different internal genes from other influenza viruses of avian origin," the researchers wrote.
The study, published on the Internet at http://respiratory-research.com/, may explain the severe respiratory distress suffered by H5N1 patients, who often say they struggle to breathe.
Michael Osterholm, an infectious disease expert at the University of Minnesota who has been advising the U.S. government on the risks of a flu pandemic, said the study supports predictions that any possible H5N1 pandemic would be especially severe.
It means being young and healthy could actually work against people who become infected.
"Anyone could experience this very severe, life-threatening illness," Osterholm said in a telephone interview.
"This is looking more and more like an H1N1 1918."
The worst recorded influenza epidemic was in 1918, when an H1N1 strain swept the globe in a few months, killing anywhere between 20 million and 100 million people, depending on the estimate. In comparison, a pandemic in 1957 killed 2 million and one caused by an H3N2 virus in 1968 killed 1 million.
"In 1918, even among the very young and the very old, there was a ten-fold increase in deaths," Osterholm said. "There was a 1,000-fold increase in young adults."
Fauci said researchers are now testing various drugs that may affect the immune system to see if they would help patients better survive H5N1.