Because the flu can be fatal

Normally it is one of the diseases that occupies us all winter, but Covid-19 has decided otherwise. But it shouldn’t be forgotten too quickly. Seasonal flu, which usually strikes in winter, is also responsible for other infections, because it carries, without knowing why, a risk of bacterial pneumonia. Researchers from the Karolinska Institute (Sweden) have discovered why secondary infections, especially those related to influenza, claim so many lives around the world every year. The study published in the journal PNAS on October 28, 2020, it could also contribute to Covid-19 research.

To understand the deadly potential of the flu, let’s go back in time. The Spanish flu is a flu pandemic that hit the world between 1918 and 1919 and which, unlike many other pandemics, disproportionately affected healthy young adults. Among the many reasons behind this massacre (between 20 and 50 million deaths), one concerns secondary infections caused by bacteria, in particular pneumococci.

Less nutrients, more infections

The flu is caused by a virus, but the virus is not directly responsible for death. The most common cause of death is bacterial pneumonia rather than the flu virus itself. Pneumococcal infections are the most common cause of pneumonia and a leading cause of death worldwide.

Previous influenza virus infection increases susceptibility to pneumococcal infections, but the mechanisms behind it are not fully understood. This is the mechanism the researchers identified, including influenza-induced changes in the lower respiratory tract that affect the growth of pneumococci in the lungs.

By running tests on animals, the researchers found that several nutrients and antioxidants, such as vitamin C and other substances that normally protect cells, escape from the blood, creating an environment in the lungs that promotes growth. bacteria. Bacteria adapt to the inflammatory environment by increasing the production of the bacterial enzyme HtrA.

When present, this enzyme weakens the immune system and promotes bacterial growth in the respiratory tract infected with the flu. The absence of HtrA stops bacterial growth.

“The ability of the pneumococcus to grow in the lower respiratory tract during flu infection appears to depend on the nutrient-rich environment with its higher levels of antioxidants occurring during viral infection, as well as the ability of bacteria to adapt to the environment. and protect yourself from eradication by the immune system“Explains Birgitta Henriques Normark, professor in the department of microbiology, cancer and cell biology at the Karolinska Institute.

Surely a similar superinfection mechanism for Covid-19

These findings provide valuable insight into how bacteria integrate with their environment in the lungs. They could be used to find new therapies for double infections between the influenza virus and pneumococcal bacteria.

“HtrA is an enzyme that helps weaken the immune system and allows pneumococcal bacteria to enter the protective cell layer within the airways.explains the first author of the article, Vicky Sender, a researcher in the same department as her colleague. One possible strategy could therefore be the use of protease inhibitors to prevent the growth of pneumococci in the lungs.. “

If this mechanism works for the flu, it is currently impossible to know if people with Covid-19 are also sensitive to the mechanism of bacterial superinfection. However, the researchers believe similar mechanisms could be found in severe patients with Covid-19.

“Acute inflammation of the lungs, regardless of the cause, is likely to lead to the leakage of nutrients and antioxidants and an environment that promotes bacterial growth. “, emphasizes Birgitta Henriques Normark.