Aerosol infection of animals with strains of Legionella pneumophila of different virulence: comparison with intraperitoneal and intranasal routes of infection

Infection of guinea-pigs by intranasal (i.n.) instillation of 109 viable organisms of two newly isolated strains of Legionella pneumophila (74/81, serogroup 1; 166/81, serogroup 3) did not induce disease, but 104 organisms administered as a small particle aerosol (< 5 μm diameter) produced a fatal widespread bronchopneumonia within 3 days. Milder illness and less extensive bronchopneumonia were also produced in rhesus monkeys and marmosets by one of these two strains (74/81). Mice were resistant to induction of disease by aerosols of both these two strains, though organisms did persist in the lungs for at least 4 days. Both of these L. pneumophila strains were pathogenic for guinea-pigs by aerosol infection over a wide range of doses but the serogroup 1 type strain (NCTC 11192) was not. There was no mortality after infection of guinea-pigs by intranasal instillation of any of these strains but all proved to be fatal after intraperitoneal (i.p.) injection of large doses. Guinea-pigs, rhesus monkeys and marmosets exposed to aerosol infection with L. pneumophila provide relevant models for studying the pathogenesis of Legionnaires' disease.