Influenza virus transmission is dependent on humidity and temperature

Posted by ajcann on October 20, 2007

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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, these authors evaluated the effects of temperature and relative humidity on influenza virus spread. By housing infected and nave guinea pigs together in an environmental chamber, they carried out transmission experiments under conditions of controlled temperature and humidity. They 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. The data implicate low relative humidities produced by indoor heating and cold temperatures as features of winter that favor influenza virus spread.

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. This provides 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.