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How Flu Becomes Deadly

Kathleen Blanchard's picture

Researchers from the Children's Hospital of Philadelphia, Pennsylvania have discovered how flu becomes deadly. Symptoms of the same strain of flu can cause mild symptoms in some individuals, yet lead to pneumonia in others; even causing death, as seen with pneumonia deaths related to swine H1N1 flu.

The scientists have found that flu becomes deadly because it paralyzes the immune system of people who are otherwise in good health. Furthermore, the effect is lasting. Bacterial infections that develop on top of flu symptoms lead to severe illness. When the immune system becomes paralyzed, bacterial infections develop, causing flu to become more deadly.

Lead researcher for the study, Kathleen Sullivan, M.D., Ph.D., Chief of the Division of Allergy and Immunology at the Children's Hospital of Philadelphia says, "We have a very limited understanding of why some people who get influenza simply have a bad cold and other people become very sick and even die. The results of this study give us a much better sense of the mechanisms underlying bacterial infections arising on top of the viral infection."

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The researchers studied pediatric patients diagnosed with severe flu. They measured cytokine levels, markers of inflammation that initiate a cascade of events to help the body fight disease by signaling proteins within the cells, called toll-like receptors. The researchers found decreased toll-like receptor response in the children with severe flu. The researchers believe it may be diminished toll-like receptor response paralyzing the immune system, making flu deadly, and leading to secondary infection.

The study compared patients who were healthy, had mild flu, and who had RSV (respiratory syncytial virus), finding that flu becomes deadly specifically as the result of the flu virus. The findings explain why bacterial infections often occur on top of flu virus.

The study is published online in the Journal of Leukocyte Biology. John Wherry, Ph.D., Deputy Editor of the journal says, "Despite major medical advances since the devastating flu outbreak of 1918 and 1919, influenza virus infection remains a very serious threat, and the current swine flu outbreak is a grim reminder of this fact”. The study can help scientists develop individualized treatment for flu by understanding how flu becomes deadly for some, and not for others.