Bird Flu Causes Immune "Storm", Study Finds
Date: 11-Nov-05
Country: USA
Author: Maggie Fox, Health and Science Correspondent
They compared samples taken from patients infected with H5N1 to a sample from a patient with ordinary, seasonal H1N1 flu.
The H5N1 virus caused immune system chemicals 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.
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.
"While human-to-human transmission of the H5N1 subtype influenza virus appears to be inefficient so far, the disease has exceptional severity in those affected with reported mortality rates ranging from 33 percent in Hong Kong in 1997 to 55 percent in Thailand and Vietnam in 2004," Michael Chan and Malik Peiris of the University of Hong Kong and colleagues wrote in their report.
"The reasons for this unusual severity of human disease have remained unclear."
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 flu.
They used the virus to infect lung tissue samples taken from other, non-flu patients.
CHEMICAL STORM
The H5N1 viruses brought in a storm of cytokines -- the immune system's signalling chemicals -- including IP-10, interferon beta, RANTES and interleukin-6.
And the later, Vietnamese strains caused a bigger cascade than the 1997 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/, explains 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 US 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."
This, in turn, could be bad news in trying to treat an pandemic, Osterholm said. Antiviral drugs such as Tamiflu and Relenza can help if given within a day or so of infection.
"The idea that we may have 24 to 48 hours is based on the H3N2 model," Osterholm said. But if patients develop respiratory distress from a cytokine storm, even that may be too late, he said.
