posted by admin on 24/03/06
Scientists have a new clue why the feared H5N1 strain of the avian-influenza virus hasn't spread easily among people.
According to research published this week by teams in the U.S. and the Netherlands, 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 claimed millions of birds and more than 100 human lives.
The virus has continued to spread across the globe and has been detected in birds in both Europe and Africa, although not yet in the U.S.
Humans have very limited immunity to such avian viruses, and health authorities say that if the bird-flu virus begins to spread between people, it could spark a global outbreak of deadly respiratory disease.
That hasn't happened, and the new findings may help explain why.
Using tissues from human cadavers, researchers at the Erasmus Medical Center in Rotterdam studied which cells the bird virus would become attached to. According to Thijs Kuiken, a veterinary pathologist who directed the research, 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 called the finding a possible explanation for why the avian virus doesn't jump easily between people. "It must reach the lower respiratory tract to replicate, and it's harder to spread by coughing and sneezing," he said.
Other doctors questioned that conclusion, saying that patients with such infections were likely to cough more heavily. "I don't think it directly affects the transmissibility just because it's in the lower respiratory tract," said Nikki Shindo, an influenza expert at the World Health Organization who has studied human cases, most recently in Turkey.
Dr. Shindo said that if the virus has to reach deep into the lung before causing an infection, that could make it harder to catch.
In a separate study released yesterday by the journal Nature, a research group headed by Yoshihiro Kawaoka of the University of Wisconsin-Madison reached similar conclusions after analyzing the specific molecules on human cells to which influenza viruses attach.
They found that molecules preferred by H5N1 are expressed mostly by cells deep in the lung. To spread between humans, the virus would need to adapt in a way that enabled it to attach to a different type of molecule present in cells higher in the respiratory tract.
"This restriction may contribute to the inefficient human-to-human transmission of H5N1 viruses seen to date," Dr. Kawaoka wrote in his report.