The course of Legionella pneumonia in guinea pigs after inhalation of various quantities of L. pneumophila

The course of legionella pneumonia in guinea pigs after infection with various quantities of virulent L. pneumophila serogroup 1 organisms by aerosol exposure was investigated. The clinical course, histopathological characteristics, manifestations in the lungs and clearance of the legionella organisms from the lungs and spleen were followed. Four groups were exposed to 4.3 X 10(4), 4.7 X 10(5), 5.0 X 10(6) and 1.0 X 10(8) aerosolized legionellae, respectively. The most striking clinical symptoms were fever and weight loss, which were found in 67-94% and 33-100% of the animals, depending on the dose of L. pneumophila organisms administered. Spontaneous death occurred only in animals receiving the highest dose and always within 10 days. All animals exhibited exponential growth of legionella organisms in the lungs. Maximal growth occurred 5 to 7 days after exposure and varied from 9.3 X 10(7) to 7.4 X 10(8) organisms/both lungs. Twenty-two days after exposure, legionellae could still be cultured from lung tissue. Between 2 and 7 days after exposure, the spleen cultures were positive for legionellae in 41% of the animals receiving the lowest dose and in 83% of all other animals; legionellae could no longer be cultured from spleen tissue after day 7. Depending on the dose, peripherally localized areas of bronchopneumonia increased in size with time, tending to become confluent lobar pneumonia. The microscopic changes were not related to the number of inhaled organisms. In the cellular infiltrate, PMN predominated until day 7 and macrophages thereafter. Seroconversion was found in all animals that survived greater than 7 days. The present animal model closely mimics the course of events in human legionella pneumonia, thus enabling us to further study the factors involved in host resistance against legionella as well as the efficacy of various antimicrobial agents in normal and immunosuppressed animals.