How dry air increases susceptibility to influenza

Influenza viruses cause seasonal outbreaks in temperate regions, with an increase in disease and mortality in the winter months. Dry air combined with cold temperature is known to enable viral transmission, but the impact of ambient humidity on host response to influenza virus infection and disease outcome remains unclear.

Researchers found that exposure to low humidity increases the susceptibility of mice infected with the influenza virus to severe disease by impairing tissue repair, mucociliary clearance, and innate antiviral defenses.

The authors housed mice at low (10–20%) or high (50%) relative humidity for 4–5 days prior to respiratory challenge with a highly virulent strain of the influenza A virus. Mice housed at low humidity suffered a worse disease course due to caspase-1/11 signaling, with more rapid weight loss, a drop in body temperature, and shortened survival, compared with mice housed at high humidity.

Exposure to low humidity also impaired airway tissue repair, reduced tracheal mucociliary clearance—an innate defense mechanism that removes pathogens, allergens, and debris—and decreased the expression of interferon-stimulated genes, which restrict the spread of the influenza virus.

According to the authors, the results suggest that controlling relative humidity may be important for preventing influenza infection and disease symptoms during winter. 

https://www.pnas.org/content/early/2019/05/07/1902840116

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