Pharmacokinetics of Penicillin-G in Serum and Nasal Washings of Pasteurella Multocida Free and Infected Rabbits

Treatment of infections due to Pasteurella multocida in rabbits usually consists of a single intramuscular injection of penicillin G and results in variable cure rates. We report here the levels of penicillin G in serum and nasal washings in P. multocida free and P. multocida infected rabbits following a single intramuscular injection over a 24 hours period. These levels were compared to the minimal inhibitory concentrations (MICS) of 50 clinical isolates of P. multocida of rabbit origin. Infected rabbits demonstrated higher serum levels of penicillin G over P. multocida free rabbits at 1, 3, and 5 hours after the injection. At 16 and 24 hours the penicillin G serum levels dropped to 0.14 and 0.07 microgram/ml, respectively, from 1.90 micrograms/ml at 8 hours. At these levels of penicillin G only 20% of the isolates tested in vitro for susceptibilities (MICs) would be inhibited. Levels of penicillin G in nasal washings at 4 hours after injection were 0.063 microgram/ml, or one-fourth the MIC of 80% of the P. multocida isolates tested (0.25 microgram/ml). In contrast, serum levels of approximately 5 micrograms/ml were seen at this time. Our results suggest that therapeutic blood levels could be achieved if rabbits are given injections of procaine penicillin G at 8 hour intervals.