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Scientists in Europe and US have discovered why H5N1 Avian Flu virus strain hasn't spread easily among people
By Finfacts Team
Mar 22, 2006, 21:32

Scientists in the US and the Netherlands have discovered why the H5N1 strain of the avian flu virus hasn't spread easily among people.

An Emergency Hospital for US Influenza Patients - The effect of the 1918/19 influenza epidemic was so severe that the average life span in the US was depressed by 10 years. The influenza virus had a profound virulence, with a mortality rate at 2.5% compared to the previous influenza epidemics, which were less than 0.1%. The death rate for 15 to 34-year-olds of influenza and pneumonia were 20 times higher in 1918 than in previous years (Taubenberger). People were struck with illness on the street and died rapid deaths. One anectode shared of 1918 was of four women playing bridge together late into the night. Overnight, three of the women died from influenza (Hoagg). Others told stories of people on their way to work suddenly developing the flu and dying within hours (Henig). One physician writes that patients with seemingly ordinary influenza would rapidly "develop the most viscous type of pneumonia that has ever been seen" and later when cyanosis appeared in the patients, "it is simply a struggle for air until they suffocate," (Grist, 1979). Another physician recalls that the influenza patients "died struggling to clear their airways of a blood-tinged froth that sometimes gushed from their nose and mouth," (Starr, 1976). The physicians of the time were helpless against this powerful agent of influenza. Source: www.stanford.edu/group/virus/uda/

According to research published this week in the journals Science and Nature, the bird virus primarily infects cells deep in the human lung, possibly making it difficult for the germ to spread.

The H5N1 avian-flu virus was first detected in Hong Kong in 1997, and has since killed millions of birds and has also killed almost 200 humans. The virus has spread across the globe, and has been detected in birds in both Europe and Africa, but not yet in the US.

The findings, help explain why the deaths in eight countries since 2003, mostly from close contact with infected poultry, has not resulted in the spread of the  virus from an initial human host to other people.

"Our findings provide a rational explanation for why H5N1 viruses rarely infect and spread from human to human, although they can replicate efficiently in the lungs," said Yoshihiro Kawaoka, a virologist at the University of Wisconsin-Madison who led the research published in Nature. He is also attached to the University of Tokyo.

Humans generally have very limited immunity to such avian viruses, and health experts say that if the bird-flu virus begins to spread between people, it could result in a global pandemic of the disease. The fact that a pandemic hasn't happened may be explained by the the new findings.

Kawaoka's team found that only cells deep within the respiratory system have the surface molecule, or receptor, the flu virus needs to enter a cell. Flu viruses, like most viruses, need to enter and take over cells within their host to replicate and spread to still more cells.

In the second study, researchers at the Erasmus Medical Center in Rotterdam examined tissue from human corpses to check which cells the bird virus would become attached to. According to research leader Thijs Kuiken, who is a veterinary pathologist, H5N1 attached to cells deep in the lung, but not cells in the throat where human flu viruses multiply. The study appears in the journal Science.

Dr. Kuiken said the findings provide a possible explanation for why the avian virus doesn't pass easily between people. "It must reach the lower respiratory tract to replicate, and it's harder to spread by coughing and sneezing," he said.

The studies indicate that the bird-flu strains moving across the world through migratory fowl still have to undergo key genetic changes to set off a feared human pandemic like the so-called Spanish Flu in 1918/19 that killed an estimated 100 million people.

There is therefore more time to prepare for an eventual shift, if it happens.

"No one knows whether the virus will evolve into a pandemic strain, but flu viruses constantly change," Kawaoka said. "Certainly, multiple mutations need to be accumulated for H5N1 to become a pandemic strain."