Effect of prior influenza virus infections on susceptibility of AKR/J mice to respiratory challenge with Legionella pneumophila.

Influenza virus administered intranasally to AKR/J mice, followed 3 days later by Legionella pneumophila inoculated intranasally, caused significantly greater mortality than did either of the two agents administered alone. Viable concentrations of both bacteria and viruses dropped in sequentially infected animals, despite the ultimate fatal outcome. Viral concentrations, however, did not decrease as rapidly in sequentially infected as in singly infected mice. Histopathologic lesions were consistent with viral replication aided by elaboration of a bacterial toxin. This observation contrasts with the more commonly observed sequence in which the bacterium proliferates after the virus interferes with host defense. Cell-free preparations were found to have toxic activity.