# 1185
One of the enduring mysteries about influenza is why it is predominantly a winter phenomenon. Outbreaks occur, in the northern hemisphere, generally between November and March, while the Southern Hemisphere see's flu outbreaks between May and September.
In the tropics, near the equator, there is no well defined flu season and cases can occur year round.
Why is influenza seasonal?
In what may be at least a partial answer to that question we have a study released yesterday in PLoS Pathogens entitled Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature by Anice C. Lowen, Samira Mubareka, John Steel, and Peter Palese.
Some excerpts, reparagraphed for readability:
Using the guinea pig as a model host, we show that aerosol spread of influenza virus is dependent upon both ambient relative humidity and temperature.
Twenty experiments performed at relative humidities from 20% to 80% and 5 °C, 20 °C, or 30 °C indicated that both cold and dry conditions favor transmission.
The relationship between transmission via aerosols and relative humidity at 20 °C is similar to that previously reported for the stability of influenza viruses (except at high relative humidity, 80%), implying that the effects of humidity act largely at the level of the virus particle. For infected guinea pigs housed at 5 °C, the duration of peak shedding was approximately 40 h longer than that of animals housed at 20 °C; this increased shedding likely accounts for the enhanced transmission seen at 5 °C.
To investigate the mechanism permitting prolonged viral growth, expression levels in the upper respiratory tract of several innate immune mediators were determined. Innate responses proved to be comparable between animals housed at 5 °C and 20 °C, suggesting that cold temperature (5 °C) does not impair the innate immune response in this system.
Although the seasonal epidemiology of influenza is well characterized, the underlying reasons for predominant wintertime spread are not clear. We provide direct, experimental evidence to support the role of weather conditions in the dynamics of influenza and thereby address a long-standing question fundamental to the understanding of influenza epidemiology and evolution.
Author Summary
In temperate regions influenza epidemics recur with marked seasonality: in the northern hemisphere the influenza season spans November to March, while in the southern hemisphere epidemics last from May until September.
Although seasonality is one of the most familiar features of influenza, it is also one of the least understood. Indoor crowding during cold weather, seasonal fluctuations in host immune responses, and environmental factors, including relative humidity, temperature, and UV radiation have all been suggested to account for this phenomenon, but none of these hypotheses has been tested directly.
Using the guinea pig model, we have evaluated the effects of temperature and relative humidity on influenza virus spread. By housing infected and naïve guinea pigs together in an environmental chamber, we carried out transmission experiments under conditions of controlled temperature and humidity.
We found that low relative humidities of 20%–35% were most favorable, while transmission was completely blocked at a high relative humidity of 80%. Furthermore, when guinea pigs were kept at 5 °C, transmission occurred with greater frequency than at 20 °C, while at 30 °C, no transmission was detected.
Our data implicate low relative humidities produced by indoor heating and cold temperatures as features of winter that favor influenza virus spread.
Low relative humidity (RH) and low temperatures appear to enhance the transmissibility of the influenza virus on multiple levels, possibly affecting the shedding of virus particles from those infected, the virus particles themselves, and the susceptibility of new victims.
While this study indicates that low RH and Temps promote the spread of influenza, it doesn't pinpoint exactly why. There are theories, however.
- It is thought that breathing dry air could lend to the desiccation of the nasal mucosa, which could render the host more susceptible to infection.
- It is believed that cold air temperatures reduce the frequency of cilia beats (cilia are whip-like appendages of many living cells found in the lining of the lungs and trachea that are used to move fluid upward and out of the lungs) which could slow mucociliary clearance and increase the odds of infection.
- Recent research has shown that the influenza virus is maximally stable at low RH (20%–40%), minimal at intermediate RH (50%), and high at elevated RH (60%–80%).
- At low relative humidity, evaporation of water from exhaled large droplets would occur rapidly, leading to the formation of lighter, smaller, and more persistent droplet nuclei. When the relative humidity is high, small respiratory droplets would take on water, increase in size and fall to the ground more quickly.
There may be other factors at work here as well, and not all of them may be due to low temperatures or humidity. Still, this is important work that expands our knowledge about how influenza is spread. More research is underway, as there are still many unanswered questions.
Why, for instance, does influenza spread in the tropics at all, given that this experiment indicates that transmission is severely hampered at tropical temperatures and humidity?
This study was based on airborne transmission of the virus.
It may well turn out that most tropical transmission of influenza occurs from fomites (contaminated inanimate objects like shopping carts or telephones), or handshakes, or intimate contact.
These researchers are now delving into that question.
We've still got a long way to go, and many unanswered (or, at best, partially answered) questions.
But we are making progress.
