Rapidly Progressive Pasteurella multocida Soft-Tissue Infection After a Cat Bite: Diagnosis by Culture and MALDI-TOF Mass Spectrometry: A Case Report

Abstract

Background: Pasteurella multocida is a Gram-negative coccobacillus that lives in the mouth and upper airway of cats and dogs. It causes soft-tissue infection after bites and scratches. The infection can progress within hours. The organism can also be missed in the laboratory because it is small and pleomorphic.

Case presentation: A 57-year-old man with hypertension and coronary artery disease was bitten on the left forearm by a cat. He received tetanus toxoid and rabies post-exposure prophylaxis and was discharged. He returned within 24 hours with throbbing pain and intermittent fever. Examination showed a 3-cm fluctuant swelling over the posterior forearm with warm, tender, red skin. He had no known immune deficiency disorders and was hemodynamically stable. No regional lymphadenopathy was found. Incision and drainage under local anesthesia released about 3 mL of pus. Oral amoxicillin-clavulanate and linezolid were started empirically. A Gram-stained smear of the pus showed small, pleomorphic Gram-negative coccobacilli. Culture produced convex, greyish, non-hemolytic colonies on blood and chocolate agar and no growth on MacConkey agar. MALDI-TOF mass spectrometry identified the isolate as P. multocida. Disk diffusion testing showed susceptibility to penicillin, ampicillin, amoxicillin-clavulanate, chloramphenicol, trimethoprim-sulfamethoxazole and levofloxacin. Linezolid was stopped and amoxicillin-clavulanate was continued alone. At follow-up, the cellulitis had resolved and the wound showed healthy granulation tissue.

Conclusion: P. multocida should be suspected in any rapidly progressive soft-tissue infection after a cat bite. Culture followed by MALDI-TOF MS enabled species-level identification after overnight incubation. Susceptibility results then allow the empirical regimen to be narrowed to a single agent. A history of animal contact should always be sought.

Keywords: Antimicrobial Stewardship, Cat Bite, MALDI-TOF MS, Pasteurella Multocida, Soft-tissue Infection, Zoonosis

Introduction

Animal bites are a steady cause of emergency department visits. Dogs cause most bites, and cats cause a smaller share. Cat bites are more likely to become infected than dog bites. A cat's teeth are long and thin. They leave deep, narrow puncture wounds that close quickly and trap saliva and skin bacteria in the tissue. The wound may look minor at first, so patients often delay care until infection is established. The bacteria that grow from infected bite wounds usually reflect the mouth flora of the biting animal. Most infected wounds contain a mixture of aerobic and anaerobic species.[1]

Pasteurella multocida is the organism most often linked to infection after a cat bite. It is a small, non-motile, Gram-negative coccobacillus and a facultative anaerobe. It is part of the normal flora of the mouth and upper airway in many healthy cats and dogs. The species has several subspecies. It carries a polysaccharide capsule and other virulence factors that help it resist host defenses. Humans acquire it through bites, scratches, licks over broken skin and contact with animal secretions.[2]

The clinical picture in humans varies. Most patients have a localized soft-tissue infection, such as cellulitis or an abscess at the site of the bite. Some develop infection of the tendon sheath, joint or bone. Others develop bacteremia, pneumonia, meningitis or endocarditis. Invasive disease is reported more often in older adults and in people with liver disease or weakened immunity. It also occurs in healthy people. A history of a bite is not always present, and contact with a pet may be the only clue. Even local infection can cause marked pain and loss of function, and a collection of pus needs drainage.[3]

Early knowledge of the disease came from clinical series. A review of 34 cases published in 1984 described the range of presentations, from wound infection after a bite to respiratory and systemic illness. It linked most infections to contact with animals.[4]

The timing of illness is a useful clue. In many patients, the infection begins within 24 hours of the bite or scratch. Pain, swelling and redness develop quickly, and pus may drain from the wound. This onset is faster than that of most other bacterial skin infections and should raise suspicion of Pasteurella.[5]

Each patient with an animal bite needs more than treatment of the wound. Tetanus immunization must be checked. In regions where rabies is present, every bite must also be assessed for rabies exposure. These measures protect against a toxin and a virus. They do not prevent bacterial infection. Empirical antimicrobial therapy for infected animal bites should provide coverage for Pasteurella species, streptococci, staphylococci, and anaerobic organisms. Antibiotic prophylaxis is usually considered for puncture wounds, wounds over the hand or a joint, and bites in patients with other illnesses. Patients treated in this way still need review within a day or two, because infection can appear despite early care.

Laboratory identification can be difficult. The organism is small and pleomorphic on Gram stain. It can be mistaken for Haemophilus species or other coccobacilli. It grows on blood and chocolate agar but does not grow on MacConkey agar. These features are useful, but they only suggest the diagnosis. Biochemical identification takes at least a day after colonies appear. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) identifies a bacterium from a single colony within minutes. It is now available in many clinical laboratories. For fastidious organisms such as Pasteurella species, accuracy depends on the reference database that each laboratory holds.

Treatment combines drainage of any collection with an effective antibiotic. Most isolates are susceptible to penicillin and other beta-lactam drugs. Amoxicillin-clavulanate is a standard oral choice for bite wounds because it also covers other organisms from the animal's mouth. Empirical regimens are often broad, because the cause is not known when the patient first presents. Some include a drug that acts against Gram-positive organisms such as Staphylococcus aureus, which also causes abscesses. Once the organism is identified, the regimen can be narrowed. Narrowing lowers drug exposure, adverse effects and cost.

Reports that follow a single patient from the bite through each laboratory step to the final antibiotic choice are useful for clinicians and laboratory staff alike. They show how the clinical and laboratory findings fit together. They also show how a result changes treatment at the bedside. They help laboratories decide when a small Gram-negative coccobacillus in a bite wound should be taken to species level.

We report a man in his fifties who developed a forearm abscess within a day of a cat bite. We describe the clinical findings, the culture and MALDI-TOF MS results and the change in antibiotic therapy after identification. The case demonstrates how infection progresses rapidly in an innocuous looking small wound.

Case presentation

A 57-year-old man presented to the emergency department after a cat bite to the left forearm (Figure 1). He had primary hypertension and coronary artery disease. He was not known to be immunocompromised. He received tetanus toxoid and rabies post-exposure prophylaxis and was discharged with advice for outpatient follow-up.

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He returned less than 24 hours later with worsening symptoms. He had throbbing pain and intermittent fever. A 3-cm fluctuant swelling had formed over the posterior aspect of the forearm. The surrounding skin was red, warm and tender. His pulse was 83 beats/min and his blood pressure was 138/90 mmHg. He remained hemodynamically stable. No regional lymphadenopathy was found.

Incision and drainage was done under local anesthesia and released about 3 mL of frank pus (Figure 2). The pus was sent at once to the microbiology laboratory for examination and culture. Oral amoxicillin-clavulanate (1 g twice daily) and linezolid (600 mg twice daily) were started empirically, with routine analgesia.

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A direct Gram-stained smear of the pus showed occasional inflammatory cells and small, pleomorphic Gram-negative coccobacilli against a proteinaceous background. The specimen was inoculated onto 5% sheep blood agar, chocolate agar and MacConkey agar. After 24 hours of aerobic incubation at 37 °C, convex, greyish, non-hemolytic colonies grew on blood and chocolate agar (Figure 3A). There was no growth on MacConkey agar. A Gram stain from a colony showed Gram-negative coccobacilli (Figure 3B). The isolate was identified as P. multocida by MALDI-TOF MS (VITEK MS, bioMérieux, Marcy-l'Étoile, France).

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Susceptibility testing was done by Kirby-Bauer disk diffusion and read according to CLSI M45 guidelines (Figure 4). The isolate was susceptible to penicillin, ampicillin, amoxicillin-clavulanate, chloramphenicol, trimethoprim-sulfamethoxazole and levofloxacin.

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After identification and susceptibility results, linezolid was stopped. Treatment continued with amoxicillin-clavulanate alone. At follow-up, the cellulitis had resolved. The drainage site showed healthy granulation tissue with minimal serous discharge, and wound care was continued.

Discussion

This patient developed a forearm abscess within 24 hours of a cat bite, and P. multocida was identified from the pus. The finding fits the known microbiology of cat bites. Talan et al. studied 107 patients with infected dog and cat bites. Pasteurella species grew from 75% of the infected cat bites.[6] Holst et al. identified 159 Pasteurella strains from 146 infected people in Sweden. Of these, 95 were P. multocida subsp. multocida, 28 were P. canis and 21 were P. multocida subsp. septica.[7] Cat exposure and a short interval to symptoms should therefore prompt the clinician and the laboratory to consider this organism.

The laboratory path in our case had several steps. The direct smear showed small Gram-negative coccobacilli, but this finding is not specific. Growth on blood and chocolate agar with no growth on MacConkey agar narrowed the possibilities. Species-level identification then relied on MALDI-TOF MS. Zangenah et al. compared four methods on 66 Pasteurella isolates. The Bruker and VITEK MS systems each identified 59 isolates (89%) correctly, while VITEK 2 identified 32 (48.5%). Biochemical tests identified 62 isolates (94%). The mean time to identification was about 10 minutes for MALDI-TOF MS and 20 hours for biochemical tests,[8] suggesting that MALDI-TOF is not superior to meticulous biochemical testing for bacterial identification. Its advantage is its rapid turnaround time, saving hours as compared to biochemical testing, and this allowed us to report the species as soon as colonies were available. The smear is also valuable on its own. Gram-negative coccobacilli in pus from a bite wound should lead the clinician to confirm that the antibiotic covers Pasteurella while culture is pending. Our empirical regimen already included amoxicillin-clavulanate, which acts against this organism.

Our patient had hypertension and coronary artery disease but no known immune deficiency. He had a local infection only. He was stable, had no regional lymphadenopathy and recovered with drainage and oral therapy. Oehler et al. reviewed bite-related and septic syndromes caused by cats and dogs. They described P. multocida and Capnocytophaga canimorsus as causes of septic shock, meningitis and other severe disease after animal exposure.[9] We cannot say whether early drainage and antibiotics prevented such spread in our patient. The review is a reminder that a bite that looks minor can still carry risk, and that patients need early review.

The 2014 guideline of the Infectious Diseases Society of America covers skin and soft-tissue infections, including animal bites. Stevens et al. advise that abscesses be drained and that material be sent for culture. They list amoxicillin-clavulanate among the oral drugs for bite wounds.[10] Our management followed this approach. The empirical regimen also included linezolid, which has activity against Gram-positive organisms but not against P. multocida. Once culture and susceptibility results were available, linezolid was stopped. The isolate was susceptible to penicillin, ampicillin and amoxicillin-clavulanate, and the patient improved on amoxicillin-clavulanate alone. This shows how a prompt microbiological result can shorten exposure to an unneeded drug. Susceptibility results are worth obtaining even when penicillin is expected to work. They confirm the choice and guide a change if the response is poor.

Case reports describe wider disease from this organism, including in people with no clear immune defect. Yadav described pneumonia in an immunocompetent man who worked as a zookeeper and kept a pet dog.[11] Diaz et al. reported septic shock from P. multocida in an immunocompetent 76-year-old man from a rural area.[12] Kuwahara and Imanaka described bacteremia and septic arthritis in an older patient who had no animal bite. The joint was not drained, and the patient recovered.[13] Lagadinou et al. reported infection of a hip prosthesis in a 67-year-old woman. She was treated with joint washout and intravenous antibiotics.[14] Mor et al. described a 56-year-old woman with a spinal epidural abscess and paraplegia. Her blood cultures were first misidentified as Haemophilus influenzae. She improved after targeted antibiotics and surgery.[15]

These reports share two points with our case. A history of animal contact was the key to the diagnosis in several of them, and it should be asked for in every patient with a compatible illness. Correct species identification also affected management, as the report by Mor et al. shows. In our patient, an accurate result allowed treatment to be narrowed to one oral agent. Providing a clear history of cat-bite exposure on the laboratory request form may assist the microbiology laboratory in considering Pasteurella species and selecting appropriate culture methods.

This report has limits. It describes one patient. Identification relied on MALDI-TOF MS, and no molecular confirmation was done. Follow-up was short, and we cannot comment on recurrence or late complications. A larger series would be needed to compare outcomes by antibiotic regimen.

Conclusion

P. multocida should be considered in any patient with a rapidly progressive soft-tissue infection after a cat bite or other animal exposure. In this patient, Gram stain, growth on blood and chocolate agar but not MacConkey agar, and MALDI-TOF MS enabled rapid species-level identification after overnight culture. Susceptibility results allowed treatment to be narrowed to amoxicillin-clavulanate alone. Early microbiological identification supports targeted therapy and sensible use of antibiotics. The diagnosis also depends on a clear history of animal contact and on close work between clinician and laboratory.

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