Lung Abscess Due to  -Lactamase-Producing Pasteurella multocida

Pasteurella multocida is a commensal of the upper respiratory tract of various animals. The spectrum of human diseases caused by P. multocida varies from soft-tissue infections following bites and scratches to systemic infections, including respiratory tract infections resulting from airborne contamination and/or chronic carriage. Antimicrobial resistance of Pasteurella strains originating from animals has been reported for many years. Human P. multocida isolates are usually susceptible to penicillins. We report a case in which a strain of β-lactamase-producing P. multocida was isolated from a lung abscess specimen and review related cases. A 75-year-old woman with chronic bronchitis was hospitalized with fever (temperature, 40°C), weight loss, increased productive cough, and dyspnea. A chest x-ray was unremarkable, whereas the WBC count was 13,900/mm3 (89% neutrophils) and the one-hour erythrocyte sedimentation rate was 63 mm/h. The patient was treated for 8 days with intravenous amoxicillin (1.5 g/d). Because of persistent signs of infection, a new chest radiograph was obtained, which showed a left lung abscess; the abscess was confirmed by CT. Culture of specimens obtained by bronchoalveolar lavage yielded P. multocida subspecies multocida (105 cfu/mL) and Haemophilus parainfluenzae (103 cfu/mL). Production of β-lactamase by both isolates was detected by a chromogenic test using nitrocefin-impregnated disk and the synergic effect between clavulanate and amoxicillin by disk diffusion testing. Minimum inhibitory concentrations of amoxicillin and amoxicillin/clavulanate for the P. multocida strain were of 8 and 0.25 µg/mL, respectively. Consequently, amoxicillin was changed to intravenous amoxicillin/clavulanic acid (3 g/d). A favorable outcome was achieved, and the patient was discharged from the hospital; medication at the time of discharge was oral amoxicillin/clavulanic acid for 2 months. Our patient had no known exposure to animals other than her cat. Two different strains of P. multocida (subspecies multocida and septica) were isolated from the cat's saliva, but none of them was found to produce β-lactamase. A MEDLINE search of the literature revealed only 4 welldocumented cases of human infections due to penicillin-resistant P. multocida [1–4]. All 4 cases were respiratory tract infections. The principal features of these cases along with our case are presented in Table 1. The respiratory tract is the most common site of pasteurella infections after soft tissues [5]. Previous studies have shown that impaired pulmonary defenses, predominantly associated with bronchiectasis, chronic bronchitis, and bronchogenic carcinoma, may predispose people to P. multocida respiratory tract infections, which occur mostly in the elderly [6]. The spectrum of disease includes pneumonia, tracheobronchitis, lung abscess, and empyema. Finally, the clinical features of respiratory tract infections due to penicillinresistant P. multocida are indistinguishable from those of infections due to penicillin-susceptible strains. Summary of data on respiratory tract infections caused by penicillin-resistant Pasteurella multocida. In animals, it has been shown that the penicillin resistance of Pasteurella strains may be associated with an ROB-1 blactamase-encoding plasmid, very similar to that found in some Haemophilus species [7]. The presence of this plasmid has been reported only by Rosenau et al. [8] in 2 clinical isolates (those recovered in cases 2 and 3). In cases 1 and 4, it is likely that β-lactamase production may be implicated, since no other mechanism has yet been shown to induce penicillin resistance of Pasteurella species.Moreover, clinical outcomes confirmthis hypothesis (the death of patient 1 after a 15-day course of therapy with ampicillin and then penicillin, and the recovery of patient 4 after amoxicillin/clavulanate therapy). Our strain of P. multocida also produced β-lactamase and originated from the respiratory tract. As far as we know, penicillin- resistant P. multocida has not yet been isolated fromother sources, including soft-tissue infections following animal bites or scratches [9, 10]. The fact that penicillin-resistant P.multocida strains so far have been found only in respiratory tract infections supports the hypothesis that resistance is transmitted endogenously, directly from oropharyngeal flora present in humans. Indeed, in our case, it is conceivable that a penicillin-susceptible strain originating from the cat could have acquired resistance by plasmid transmission from the patient's H. parainfluenzae strain. The possible emergence of penicillinresistant P. multocida in humans should be considered.