Avian Flu Diary: PNAS: Low Ambient Humidity Impairs Barrier Function & Innate Resistance Against Influenza Infection

Photo Credit PHIL (Public Health Image Library)

#14,075

Although flu circulates at low levels year-round in the tropics, for most of the world influenza is primarily a seasonal threat, which appears to peak during times of of lower temperatures and humidity.

While the reasons are only partially understood, we've seen research indicating that influenza viruses survive longer in the environment when temperature and humidity fall within certain ranges.

In 2008 researchers Jeffrey Shaman and Melvin Kohn established a correlation between the AH (Absolute Humidity) and the survival, and transmission of the influenza virus (see It's Not So Much The Heat, It's The Humidity).

A 2012 study (see Influenza Virus Survival At Opposite Ends Of The Humidity Spectrum) found both extremely low and extremely high humidity were conducive to flu transmission – at least when it resides in mucus and respiratory fluids like those found in your nose, throat, or lungs.

In 2013, in NIH Study: Climate & Influenza Transmission, we looked at a PLoS One study called High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs that concluded that `. . . maintaining indoor relative humidity at levels greater than 40% can significantly reduce the infectious capacity of aerosolized flu virus.'

The irony here is that hospitals are normally kept cool and dry in order to curb the growth of bacteria, but may be unintentionally providing an environment conducive to the spread of respiratory viruses like influenza, SARS & MERS.

Although largely anecdotal, there is growing evidence suggesting that raising the humidity inside homes, offices, and health care facilities might be a reasonably effective NPI (Non-pharmaceutical Intervention) during times of heightened flu activity (see PLoS One Humidity as a non-pharmaceutical intervention for influenza A).

This is not exactly a new idea, as the Chinese have boiled vinegar for centuries in their homes to `disinfect the air' during epidemics (including SARS). While vinegar is unproven to add any beneficial effect, vinegar is 95% water, and boiling it undoubtedly raises the humidity inside their homes.

Unfortunately many Chinese cook indoors on coal stoves, and carbon monoxide poisoning using this method is not uncommon (see SCMP Two dead after boiling white vinegar).

Most of the research on this topic has centered on the viability of the flu virus at lower humidity levels, but the impact of lower ambient humidity on the host's ability to fight off infection may also play a role.

With the caveat that the following study was done using mice - which may or may not fairly represent a human response - researchers have found that lower ambient humidity can impair `. . . mucociliary clearance, innate antiviral defense, and tissue repair function'.

Low ambient humidity impairs barrier function and innate resistance against influenza infection

Eriko Kudo, Eric Song, Laura J. Yockey, Tasfia Rakib, Patrick W. Wong, Robert J. Homer, and Akiko Iwasaki
PNAS first published May 13, 2019 https://doi.org/10.1073/pnas.1902840116

Contributed by Akiko Iwasaki, April 4, 2019 (sent for review February 19, 2019; reviewed by Gabriel Núñez and Peter Palese)

Significance

Influenza virus causes 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. In this study, we asked whether humidity impacts the host response to influenza virus infections.

Exposure of mice to low humidity conditions rendered them more susceptible to influenza disease. Mice housed in dry air had impaired mucociliary clearance, innate antiviral defense, and tissue repair function. Moreover, mice exposed to dry air were more susceptible to disease mediated by inflammasome caspases.

Our study provides mechanistic insights for the seasonality of the influenza virus epidemics, whereby inhalation of dry air compromises the host’s ability to restrict influenza virus infection.

Abstract

In the temperate regions, seasonal influenza virus outbreaks correlate closely with decreases in humidity. While low ambient humidity is known to enhance viral transmission, its impact on host response to influenza virus infection and disease outcome remains unclear.

Here, we showed that housing Mx1 congenic mice in low relative humidity makes mice more susceptible to severe disease following respiratory challenge with influenza A virus. We find that inhalation of dry air impairs mucociliary clearance, innate antiviral defense, and tissue repair. Moreover, disease exacerbated by low relative humidity was ameliorated in caspase-1/11–deficient Mx1 mice, independent of viral burden. Single-cell RNA sequencing revealed that induction of IFN-stimulated genes in response to viral infection was diminished in multiple cell types in the lung of mice housed in low humidity condition.

These results indicate that exposure to dry air impairs host defense against influenza infection, reduces tissue repair, and inflicts caspase-dependent disease pathology.

(Continue . . . )

This is a lengthy, and highly detailed study and so you'll want to follow the link to read it in its entirety. There are obviously a lot of unanswered questions, including the issue of influenza transmission in the tropics, or in the summer.

As with last year's Humidity as a non-pharmaceutical intervention for influenza A), the authors (in the discussion section) touch upon the potential practical application of increasing ambient humidity.

Our study suggests that increasing ambient humidity may be a viable strategy to reduce disease symptoms and to promote more rapid recovery in influenza-infected individuals.

During the opening months of the next pandemic pharmaceutical interventions (vaccines & antivirals) may be unavailable, ineffective, or plagued by manufacturing delays, and so the immediate goal will be to slow the spread of the virus and to limit its impact through the use of NPIs (see Community Pandemic Mitigation's Primary Goal : Flattening The Curve).

While it is not entirely clear how valuable raising the humidity in your home or office would be during a pandemic - or even a bad flu season - it might be worth considering as an experimental adjunct to the other NPI's (hand washing, covering coughs, staying home when sick, avoiding crowds, etc.) that are likely to be promoted.

For more on environmental factors that may affect virus viability, you may wish to revisit: