Anaerobic bacterial pneumonia with septicemia in two racehorses.

Anaerobic bacterial pneumonia with septicemia was diagnosed in 2 Thoroughbred racehorses referred with respiratory tract disease that had failed to respond to initial treatment with various antibiotics including penicillin and trimethoprim-sulfamethoxazole. Multiple anaerobic organisms, including Bacteroides spp and Fusobacterium spp, were isolated from blood and transtracheal aspirates obtained from both horses and from aspirates of cutaneous nodules obtained from 1 horse. The latter horse responded to metronidazole treatment followed by procaine penicillin G administration and regained its health over the following 6 months. The other horse did not respond as favorably to a similar antibiotic regimen and died following an acute episode of pulmonary hemorrhage after remaining intermittently febrile for 7 weeks. Although in vitro antimicrobial susceptibility tests indicated that all anaerobic organisms isolated from both horses were susceptible to penicillin, the infection in these horses responded poorly to initial treatments with this drug. We speculated that adequate penicillin concentration was not attained in the deep foci of infection in the lungs. Animals with anaerobic bacterial infections that fail to respond to penicillin or from which penicillin-resistant anaerobes are isolated may benefit from treatment with metronidazole.