Now, a study of cells in the human respiratory tract reveals a simple anatomical difference in the cells of the system that makes it difficult for the virus to jump from human to human.
The finding, reported today (March 23, 2006) in the journal Nature, is important because it demonstrates a requisite characteristic for the virus to equip itself to easily infect humans, the key development required for the virus to assume pandemic proportions.
The new report, by a research group led by University of Wisconsin-Madison virologist Yoshihiro Kawaoka, describes experiments using tissue from humans that showed that only cells deep within the respiratory system have the surface molecule or receptor that is the key that permits the avian flu virus to enter a cell.
Flu viruses, like many other types of viruses, require access to the cells of their hosts to effectively reproduce. If they cannot enter a cell, they are unable to make infectious particles that infect other cells -- or other hosts.
"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," the authors of the study write in the Nature report.
By looking at human tissues, Kawaoka's group noted that the cells in the upper portions of the respiratory system lacked the surface receptors that enable avian H5N1 virus to dock with the cell. Receptors are molecules on the surface of cells that act like a lock. A virus with a complementary binding molecule -- the key -- can use the surface receptor to gain access to the cell. Once inside, it can multiply and infect other cells.