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Explained: Why influenza makes people vulnerable to bacterial infection

Researchers at Sweden’s Karolinska Institutet have described findings leading to so-called “superinfections”. It can also contribute to research on Covid-19, the scientists suggest.

By: Express News Service | New Delhi | Updated: December 1, 2020 11:21:09 am
A box of flu shots at Bristol. The deaths of most influenza patients are caused by bacterial infections rather than the flu virus. (David Crigger/Bristol Herald Courier via AP)

Influenza is caused by a virus, but the most common cause of death in influenza patients is secondary pneumonia caused by bacteria, rather than the influenza virus itself. While this is well known, what is largely unknown is why influenza infections lead to an increased risk of bacterial pneumonia.

Now, researchers at Sweden’s Karolinska Institutet have described findings leading to so-called “superinfections”. The study is published in the journal PNAS. It can also contribute to research on Covid-19, the scientists suggest.

The researchers cite the example of the Spanish flu, which was an influenza pandemic that swept across the world in 1918–20. Unlike many other pandemics, the Spanish flu disproportionately hit young healthy adults. And one important reason for this was “superinfections” caused by bacteria, in particular pneumococci.

Pneumococcal infections are the most common cause of community-acquired pneumonia and a leading global cause of death. A prior influenza virus infection is often followed by a pneumococcal infection. In the new study, researchers looked at mechanisms behind this increased susceptibility: influenza induces changes in the lower airways that affect the growth of pneumococci in the lungs.

The mechanism

The researchers used an animal model for their studies. They found that different nutrients and antioxidants, such as vitamin C, leak from the blood. This creates an environment in the lungs that favours growth of the bacteria. The bacteria adapt to the inflammatory environment by increasing the production of an enzyme called HtrA. The presence of HtrA weakens the immune system and promotes bacterial growth in the influenza-infected airways.

In a statement, principal investigator Birgitta Henriques Normark, microbiologist at Karolinska Institutet, said: “The ability of pneumococcus to grow in the lower airways during an influenza infection seems to depend on the nutrient-rich environment with its higher levels of antioxidants that occurs during a viral infection, as well as on the bacteria’s ability to adapt to the environment and protect itself from being eradicated by the immune system. 📣 Follow Express Explained on Telegram

Possible treatments

The researchers suggest the results could be used to find new therapies for double infections between the influenza virus and pneumococcal bacteria. “A possible strategy can therefore be use of protease inhibitors to prevent pneumococcal growth in the lungs,” lead author Vicky Sender said in the statement. The researchers note that it is still not known if Covid-19 patients are also sensitive to such secondary bacterial infections.

Source: Karolinska Institute


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