Bird flu is a killer when it infects human beings, but so far it has not mutated into a virus that people can catch from each other. Now scientists are learning why. Scientists are learning why the avian flu virus H5N1 has not yet mutated into a strain that can be transmitted between humans. It turns out that the avian strain can’t bind to human cells. But only one major mutation is needed for the flu to become transmittable between people. Is this mutation likely to occur?

A study of cells in the human respiratory tract reveals a simple anatomical difference between humans and birds that makes it difficult for the virus to jump from human to human. But viruses are opportunistic, and H5N1 may discover a way to overcome this in the future. Virologist Yoshihiro Kawaoka says, “No one knows whether the virus will evolve into a pandemic strain, but flu viruses constantly change. Certainly, multiple mutations need to be accumulated for the H5N1 virus to become a pandemic strain.”

Kawaoka discovered 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.

He says, “Deep in the respiratory system, (cell) receptors for avian viruses, including avian H5N1 viruses, are present?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?But these receptors are rare in the upper portion of the respiratory system. For the viruses to be transmitted efficiently, they have to multiply in the upper portion of the respiratory system so that they can be transmitted by coughing and sneezing.”

Existing strains of bird flu must undergo key genetic changes to become the type of flu virus that could cause another flu pandemic like the one that struck in 1918 that killed between 30 million and 50 million people worldwide. While over 200 people have contracted bird flue virus and about a hundred people have died from it?mostly in Asia?they all got it directly from poultry. None of them caught the virus the way humans usually do, from other people.

In, Bjorn Carey interviews Netherlands virologist Thijs Juiken, who says that while few people have actually caught the disease, those who for whom it was fatal died from pneumonia. Pneumonia is a disease that can be bacterial or viral and it is common among patients who have been in the hospital for a long time. However, viral pneumonia is usually not dangerous to healthy adults.

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