Figure 1. Challenge by oropharyngeal aspiration.

Mice were lightly anesthetized as described in the materials and methods and manually restrained in an upright position (A). Small curved forceps were applied to gently open the mouth and secure the tongue to the lower jaw (A, inset). (B) 30 µl of inoculum was administered to the back of the mouth using a pipette and sterile tip. The nares were blocked by the second investigator (C) to prevent obligate nasal breathing and compel inhalation of the inoculums.