A self-limited febrile illness produced in guinea pigs associated with oral administration of Legionella pneumophila.

The presumed route of human infection by Legionella pneumophila is inhalation. We investigated possible oral transmission of legionellosis in guinea pigs. Fifty-six guinea pigs (group 1) were given virulent L. pneumophila, serogroup 1, in drinking water. Fifty-nine guinea pigs (group 2) were inoculated with L. pneumophila via gastric intubation. Nineteen guinea pigs (group 3) were given heat-killed L. pneumophila in drinking water. Twenty-four guinea pigs (group 4, positive control) were inoculated intraperitoneally with L. pneumophila. Twenty-seven guinea pigs (group 5, negative control) were either intubated gastrically with phosphate-buffered saline or given drinking water without L. pneumophila. Sixty-six of 115 (57%) of the guinea pigs orally inoculated with viable L. pneumophila (groups 1 and 2) had a temperature greater than or equal to 103 degrees F and 8 of 115 (7%) had diarrhea, compared with 0 of 19 (0%) and 0 of 19 (0%), respectively, in group 3 and 1 of 27 (4%) and 0 of 27 (0%), respectively, in group 5. There were no fatalities in groups 1, 2, 3, and 5 compared with 15 of 24 (63%) in group 4. Groups 1, 2, and 4 consistently showed pneumonitis and splenitis. The pneumonitis of groups 1 and 2 was mild, predominantly interstitial, and mainly composed of macrophages; neither gross nor microscopic evidence of aspiration was seen. In group 1, 4 of 29 (14%) guinea pigs tested seroconverted to L. pneumophila compared with 0 of 7 (0%) in group 3 and 0 of 10 (0%) in group 5. In groups 1 and 2 combined, L. pneumophila was isolated from the lung of 5 of 57 (11%) guinea pigs and spleen of 5 of 47 (11%) guinea pigs compared with 0 of 14 guinea pigs in group 5. We conclude that viable L. pneumophila administered orally produces a self-limited febrile illness in guinea pigs.