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Why are some flu viruses more dangerous than others?

By Tim Sandle     Nov 23, 2014 in Science
Certain types of avian influenza viruses have the potential to cause more severe disease in humans compared with others. This has come from new research which warns such viruses must be monitored carefully.
Virologists have discovered that flu viruses expressing the low pathogenicity avian H1, H6, H7, H10 or H15 hemagglutinins (these are genes that encode the surface protein for the virus) led to fatal infections in mice. Furthermore, these viruses resulted in significant cell damage in human lung cells. This was compared with avian influenza viruses with other subtypes.
Hemagglutinin is a substance that causes red blood cells to agglutinate. Hemagglutinin is seen as important in terms of virulence. For example, with the infamous 1918 flu pandemic, as specific hemagglutinin was seen as key to the spread of the infection. Hemagglutinin is seen to induce a marked inflammatory response.
To show varying levels of virulence and the link to hemagglutinin, researchers developed a series of viruses that were similar to the subtypes that have a low pathogenicity. Each of the avian influenza virus tested was almost genetically identical to each other; the only difference being that they expressed different hemagglutinin subtypes. Once the viruses had been grown in culture, the scientists transferred the viruses into mice. This method allowed for a direct comparison of different hemagglutinins in action.
Avian influenza refers to "influenza caused by viruses adapted to birds. All known viruses that cause influenza in birds belong to the species influenza A virus. Each virus has a series of subtypes that differ only by the tiniest detail.
The research showed that viruses expressing the H1, H6, H7, H10 and H15 subtypes all caused rapid weight loss and fatal pneumonia infections in the mice. However, the H2, H3, H5, H9, H11, H13, H14 and H16-expressing viruses caused only mild weight loss and no significant disease.
The findings demonstrate the significant role that hemagglutinins play in mammalian cell death. The study suggests that these factors need careful study.
The study was carried out by the U.S. National Institute of Allergy and Infectious Diseases and the findings have been published in the journal mBio. The study is called “Contemporary Avian Influenza A Virus Subtype H1, H6, H7, H10, and H15 Hemagglutinin Genes Encode a Mammalian Virulence Factor Similar to the 1918 Pandemic Virus H1 Hemagglutinin.”
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