Background

Influenza A viruses (IAVs) are one of the major causes of seasonal respiratory infections in the world, resulting in half million deaths annually (Johnson et al., 2014). IAV outbreaks occur during the winter season in temperate regions, peaking between November and March in the Northern Hemisphere and between May and September in the Southern Hemisphere (Tamerius et al., 2013; Alonso et al., 2015). Virological research in guinea pigs shows that low temperature and humidity enables better aerosol transmission of influenza virus (Lowen et al., 2007). In addition, epidemiological studies demonstrate that a drop in absolute humidity correlates closely with the rise in influenza-related deaths in humans (Shaman et al., 2010). It is worth noting that in tropical and subtropical climate regions, which are wet and warm, the virus can thrive on surfaces of objects and cause fomite transmission (Shek and Lee, 2003, Moura et al., 2009).

Laboratory mice are generally maintained at macroenvironmental temperature and relative humidity ranges of 64 to 79 °F (17.8 to 26.1 °C) and 30% to 70%, respectively (Clark et al., 1997). However, these environmental conditions do not reflect our home, office or school in the winter season. Moreover, intranasal inoculation of virus using pipette is the most standard route used to study influenza infection in mice, which significantly affects the effective humidity in the upper respiratory system, as a liquid solution is being delivered. On the contrary, influenza infections between humans occur by either aerosol droplets or through contaminated surface contact (Lakdawala and Subbarao, 2012). Therefore, to better mimic human infection, we utilized a protocol to infect mice with influenza virus through aerosol exposure after housing the animals in dry air conditions, similar to house found in the winter months. In this protocol, we describe the use of an environmental chamber to mimic indoor conditions found during the winter seasons, namely, 20 °C and 10-20% RH, along with the use of a nebulization system.