Microbiology Case Study: A Middle-Aged Woman with Forearm Pain

Case History

A middle-aged female was evaluated for left forearm pain and erythema following a cat bite one-day prior, and was prescribed trimethoprim-sulfamethoxazole for management in the outpatient setting.  She subsequently presented for follow-up where she was noted to have a 3 x 4 cm raised, red, indurated lesion of left arm without any discharge (Image 1).  MRI demonstrated a 6.5 x 2.3 x 2.3 cm abscess within the distal ulnar soft tissues with surrounding cellulitis.  As her pain and erythema were progressively worsening, she was admitted for surgical management.   

Upon admission, a bedside incision found purulent drainage which grew mixed anaerobic gram negative rods.  Blood cultures collected at this same time were negative and remained so for the duration of her hospital course.  Empiric antibiotic therapy was initiated with piperacillin-tazobactam, and the patient underwent formal surgical incision and drainage.  Intraoperative findings were notable for abscess, diffuse and severe tendinopathy, and a thick inflammatory rind surrounding the associated neurovascular bundle.  Intraoperative cultures were obtained and sent to the microbiology laboratory.  The patient’s postoperative course was uneventful, and she was discharged with plans to complete a two week course of amoxicillin-clavulanate.  Follow-up clinic visits demonstrated successful recovery, with a well-healed incision and normal grip strength and range of motion.

Laboratory Identification

Bacterial culture of abscess material collected intraoperatively grew smooth, mucoid colonies on chocolate and blood agars with less than 24 hours of incubation at 35°C in CO2 (Image 2, bacterial isolate). Growth was notably absent on MacConkey agar. Gram stain of the colony revealed tiny, gram negative coccobacilli (Image 2). Biochemical testing determined this organism to be indole, oxidase, and catalase positive.  The organism was definitively identified as Pasteurella multocida by MALDI-TOF MS.

Image 1. Arm lesion prior to incision and drainage.
Image 2. P. multocida growth on Blood and Chocolate agars. Gram stain from a colony revealed small, gram negative coccobacilli (far right).

Discussion

Members of the genus Pasteurella are small, Gram-negative coccobacilli which are able to readily grow on Sheep’s blood agar and chocolate agar, but will typically not grow on MacConkey media.  Infection with these organisms is usually considered to be a zoonosis, with both wild and domestic animals serving as reservoirs.  In animal hosts they can be part of the endogenous flora or pathogens.  P. multocida is the most common member of the genus associated with human infections, which has now been divided into multiple taxonomic subspecies through the use of more discriminatory molecular methods.  Biochemically, P. multocida is positive for catalase, oxidase, indole, and nitrate reduction.

Animal bite wounds are often polymicrobial and contain mixtures of both aerobic and anaerobic organisms.  These organisms can be reflective of the oral flora of the biting animal, or of endogenous skin flora of the bite victim.1  While 80-90% of bites per year can be attributed to dogs, an estimated 400,000 cat bites (5-10% of the total) occur in the United States annually.2  While dog bites often manifest as localized crush injuries with contusions and/or lacerations, a majority of such wounds are accessible to irrigation and cleaning which leads to a relatively low infection rate (5-10%).  By contrast, cat bites are often deep, localized puncture wounds which provide excellent environments for the growth of both aerobic and anaerobic bacteria.  While feline bite wounds may appear less severe after cursory inspection, these wounds can be considerably more difficult to clean, resulting in overall infection rates up to 50%.3 

Management of bite wounds includes cleansing, irrigation and debridement.  Importantly, antimicrobial therapy should include coverage for both aerobic and anaerobic bacteria.4  In this case, amoxicillin-clavulanate was utilized with good results, and provides coverage for the most common oral aerobes and anaerobes encountered in animal bite wounds.  Amoxicillin-clavulanate also has activity against beta-lactamase producing bacteria such as Prevotella sp. and Porphyromonas sp. which are oral anaerobes of dogs, cats, and humans.  The use of macrolides should be avoided due to variable activity against Pasteurella multocida.4  As in this case, bite wounds most frequently are encountered on the upper extremities, and Pasteurella sp. is one of the most common isolates recovered from bites from both cats and dogs (50% of dog bites, and 75% of cat bites).2

References

1. Abrahamian FM, Goldstein EJC. 2011. Microbiology of Animal Bite Wound Infections. Clinical Microbiology Reviews 24:231.

2. Bula-Rudas FJ, Olcott JL. 2018. Human and Animal Bites. Pediatrics in Review 39:490.

3. Kannikeswaran N, Kamat D. 2008. Mammalian Bites. Clinical Pediatrics 48:145-148.

4. Stevens DL, Bisno AL, Chambers HF, Dellinger EP, Goldstein EJC, Gorbach SL, Hirschmann JV, Kaplan SL, Montoya JG, Wade JC. 2014. Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections: 2014 Update by the Infectious Diseases Society of America. Clinical Infectious Diseases 59:e10-e52.

 -Francesca Lee, MD, is an associate professor in the Departments of Pathology and Internal Medicine (Infectious Diseases) at UT Southwestern Medical Center.

-Huy Dao, MLS(ASCP)CM graduated from the University of Minnesota and has worked for eight years as medical technologist for eight years.  He is interested in clinical mycology and bacteriology.

-Andrew Clark, PhD, D(ABMM) is an Assistant Professor at UT Southwestern Medical Center in the Department of Pathology, and Associate Director of the Clements University Hospital microbiology laboratory. He completed a CPEP-accredited postdoctoral fellowship in Medical and Public Health Microbiology at National Institutes of Health, and is interested in antimicrobial susceptibility and anaerobe pathophysiology.

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