Efficacy of azithromycin in the treatment of guinea pigs infected with Legionella pneumophila by aerosol.

Azithromycin and erythromycin were compared for efficacy in guinea pigs infected with an aerosol containing Legionella pneumophila. When administered intraperitoneally, azithromycin was very effective in the treatment of experimental Legionnaires' disease. Even at the low dose of 3.6 mg/kg/day it gave 100% survival and eliminated lung infectivity two days following infection. In contrast, erythromycin at a much higher dose (96 mg/kg/day) gave only 83.3% survival and failed to eliminate organisms from the lung six days after infection. The histological findings confirmed the superiority of azithromycin. A single dose of azithromycin given intraperitoneally at 3.6 or 14.4 mg/kg gave survival rates of 83.3 and 100%, respectively. Azithromycin was also found to be superior to erythromycin in eliminating lung infectivity and reducing mortality, when administered orally. However, oral administration of azithromycin was not as effective as intraperitoneal when assessed by lung histopathology, although it was still superior to oral erythromycin treatment.