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An international team of researchers, led by University of Wisconsin-Madison flu expert Yoshihiro Kawaoka, has identified the biological roadblock that prevents the avian influenza virus, H5N1, from transmitting easily between people.

Researchers familiar with the study, published today in the journal Nature, say the findings are "comforting" because they indicate that it may be difficult for the disease to become the deadly human pandemic many have feared.

"This paper gets better every time I read it," said William Schaffner, a flu expert and head of the preventive medicine department at Vanderbilt University Medical Center in Nashville, Tenn., who was not involved in the study. "They combined modern immunochemistry with old-fashioned anatomical reasons for explaining why certain diseases spread and why some don't."

Since 2003, more than 100 people worldwide have died from H5N1. Most of these people have had close contact with infected poultry. However, the disease has not been able to jump easily between people. And although there are a handful of purported human-to- human cases, these have been among family members and caregivers who were in close contact with the sick.

To understand why it's been so difficult for H5N1 to spread between people, Kawaoka and his colleagues asked the question: What are the molecular barriers that limit this transmission?

To answer that, they examined cells on tissue samples taken from the respiratory tracts of eight people. They were looking for specific receptors or surface molecules that are known to bind to H5N1 influenza viruses.

They discovered that only cells located in the deep, dark recesses of the human lower respiratory tract could bind to avian flu. Those in the upper respiratory tract, where human flus are carried, could not.

Human flus, which can be contracted through the air, generally move between people by catching a ride on the currents of sneezes and coughs. But because the avian flu is lodged so deeply in the lungs, once it's in, the virus has a difficult time climbing back out.

The finding may also explain why the disease manifests itself as a deadly pneumonia, Schaffner said.

He said that while the vast majority of human flus begin as bronchitis, with pneumonia occurring only as a result of "the bacterial wreckage" caused by those upper infections, the avian flu goes directly to the lungs, causing a pneumonia that is viral in origin.

# posted by Medical Information : 3:16 AM