Lethal synergism induced in mice by influenza type A virus and type Ia group B streptococci.

Intranasal inoculation of CD-1 or BALB/c mice with low doses of influenza A/PR8/34 (HON1) virus followed 48 h later by intranasal inoculation of low doses of type Ia group B streptococci effected a lethal synergism. At a constant input dose of virus, a direct relationship between input dose of bacteria and percent mortality was observed; the converse was also true. An inverse relationship between input dose of group B streptococci, but not input dose of virus, and mean time to death was observed in CD-1 but not in BALB/c mice. The kinetics of influenza A/PR8/34 virus and group B streptococcal replication in singly and dually infected BALB/c mice was determined by assaying samples from the lungs, liver, spleen, and blood for viable group B streptococci and infectious influenza A/PR8/34 virus. No significant difference in virus replication in the lung was observed between singly and dually infected mice. Extrapulmonary dissemination of virus was not observed. Concurrent virus infection effected a 10,000- to 100,000-fold increase in the levels of type Ia group B streptococci in the lung. Potentiation of group B streptococcal infection of the lung was not associated with bacteremia or infection of the liver or spleen, a finding contrary to previous observations of fulminant septicemia after intranasal inoculation of mice with input doses of group B streptococci less than one-tenth of the pulmonary levels observed in the present study.