Unique presentation of an intramuscular abscess caused by Bacteroides pyogenes in the setting of a cat bite: A case report

Abstract

Cat bites are a relatively common emergency department presentation. The most common species isolated from cat bite wounds is Pasteurella multocida. In this case report, we discuss a unique case of an intramuscular abscess caused by Bacteroides pyogenes. Antibiotic resistance in Bacteroides species is a growing concern and thus should be treated with caution. We present an 81-year-old female who presented to the emergency department with left calf pain that began 3 weeks prior to the emergency department presentation. She was initially seen by her primary care provider and was prescribed oral Trimethoprim/Sulfamethoxazole and Amoxicillin/Clavulanate. Despite oral antibiotics, she began to have worsening swelling, erythema, and fevers. In the emergency department, the patient’s left calf was noticeably erythematous and tender to palpation. Lab work showed a C-reactive protein of 12.4 mg/dl, erythrocyte sedimentation rate of 38 mm/h, and white blood cell count of 15.1. Computed tomography imaging showed an irregularly shaped fluid collection at the junction of the calf musculature and upper aspect of the Achilles tendon. The decision was made to perform an operative incision and debridement on the evening of her presentation. On postoperative day 2, the wound cultures grew positive for both Pasturella multocida and Bacteroides pyogenes. She was subsequently discharged with oral Amoxicillin/Clavulanate 875 mg twice daily for 4 weeks. Further studies can be performed to identify the most efficient ways to isolate and treat this species. This case report emphasizes the importance of ordering both aerobic and anaerobic cultures and aiming antimicrobial therapy against both Pasteurella and anaerobic bacteria.

Introduction

Animal bites account for 1% of all emergency department visits each year, of which cat bites make up approximately 5%–20%. ¹ The nuances of managing cat bites are derived from the fact that these injuries occur as a small puncture wound, causing many patients to avoid seeking immediate medical care. However, cat bites can lead to local wound complications or more severe systemic manifestations. Even so, many patients who suffer cat bites only seek medical attention once the bite wound has progressed beyond a simple puncture wound. ¹

Cat bites that progress to more severe infections are the result of polymicrobial. ² A study by Talan et al. ² found that 75% of cultures from cat bite wounds grew Pasteurella multocida, the most common species isolated from the feline oropharynx. This study also showed that cat bites can also be infected with aerobic bacteria such as streptococci, staphylococci, Moraxella, and Neisseria, or anaerobic pathogens such as fusobacterium, Bacteroides, Porphyromonas, and Prevotella. ² Isolation of the correct pathogen and appropriate antimicrobial therapy is vital to help minimize the progression of bite wounds. Bite wounds can cause local infections such as cellulitis, abscess formation, and tenosynovitis. If not appropriately treated, local infections can lead to more severe manifestations such as bacteremia, osteomyelitis, meningitis, and endocarditis. ¹

In this case report, we discuss a unique case of an intramuscular abscess caused by Bacteroides pyogenes. While B. pyogenes has been shown to be part of the polymicrobial flora in the animal oropharynx, the first report of B. pyogenes causing infection from a cat bite was relatively recent in 2011. ³ Since then, the orthopedic literature has shown to be lacking in reporting an intramuscular abscess formation secondary to animal bites with this bacteria. We present a case of an intramuscular calf abscess caused by a cat bite with B. pyogenes positive cultures to highlight the importance of maintaining a broad differential for bacterial possibilities, appropriate culture techniques, and treatment approach.

Case presentation

An 81-year-old female with a medical history of deep vein thrombosis, pulmonary embolism, and ischemic stroke without residual deficit who presented with left calf pain after being bitten by her cat 3 weeks prior. A few days after the incident, she was seen by her primary care provider who prescribed a course of oral Trimethoprim/Sulfamethoxazole and Amoxicillin/Clavulanate. However, she began to have worsening swelling, erythema, and fevers, in which she decided to present to the emergency department.

On initial evaluation, the patient had noticeable erythema to the left calf with tenderness to palpation. Lab work showed a C-reactive protein of 12.4 mg/dl, erythrocyte sedimentation rate of 38 mm/h, lactic acid of 1.0 mmol/l, and white blood cell count of 15.1. Blood cultures were obtained and the patient was placed on intravenous (IV) vancomycin and cefepime by the primary team. Computed tomography imaging (CT) showed an irregularly shaped fluid collection at the junction of the calf musculature and upper aspect of the Achilles tendon measuring 10.8 cm × 2.1 cm × 2.0 cm (Figure 1).

Figure 1.

CT images showing left calf abscess.

Given this finding, the decision was made to perform an operative incision and debridement on the evening of her presentation. An incision was made at the medial border of the gastrocnemius muscle, and once the deep fascia was incised, frank purulence was encountered. The area was then copiously irrigated and thoroughly debrided, and closed loosely with drain placement. The drain remained until post operative day 2 in which there was no drainage. During this time, the operative cultures grew Pasturella multocida on postoperative day 2, and the patient’s blood cultures grew Coagulase negative staphylococcus in one out of two bottles, which was deemed a contaminant by the infectious disease team. The patient was discharged on postoperative day 4 with oral Amoxicillin/Clavulanate 875 mg twice daily for 4 weeks. On postoperative day 7, the patient’s operative cultures grew Baceroides pyogenes. She followed up with the infectious disease doctor who recommended she continue Amoxicillin/Clavulanate in for the full 4 weeks course based on updated sensitivities. At most recent follow-up, her pain was much improved and her left calf incision had healed. She has experienced no residual symptoms from her treatment.

Patient has given informed consent for this case report to be published.

Discussion

This unique case of an intramuscular abscess displays the importance of appropriate diagnostic studies based on suspected pathogenic organisms. Orthopedic literature has limited content regarding intramuscular abscess caused by B. pyogenes after a cat bite. Typically, P. multocida is considered the most common pathogen following a cat bite. A 1995 prospective study indicated that 75% of cat bites were infected with P multocida. ² However, most animal bites are polybacterial. ² P multocida is a facultative anaerobic, fermentative Gram-negative coccobacillus that is found in the oral cavities of many healthy animals. ⁴ Standard first-line treatment following a cat bite is Amoxicillin/Clavulanate. ⁴ The preferred culture medium to grow P multocida is 5% of sheep blood. The 5% sheep blood can be added to various agars such as tryptic soy yeast extract, dextrose starch, casein sucrose yeast, chocolate, Mueller–Hinton, or brain heart infusion agars^5,6 Ferreira et al. ⁵ achieved culture growth in 24 h by using tryptic soy yeast extract agar with 5% defibrinated sheep blood and incubated the specimen at 37°C. Blood agar provides a culture medium that can grow various organisms, so further serotyping or ribotyping is necessary for precise identification. Polymerase chain reaction (PCR) amplification of 16S ribosomal ribonucleic acid (rRNA) genes is the predominant and most reliable method to taxonomically identify P multocida isolates down to the genus and species. ⁶ Madsen et al. ⁷ had a similar patient scenario of a cat bite wound on the wrist that progressed after being treated with oral dicloxacillin. In this study, initial local wound cultures grew P multocida. The patient’s condition worsened after development of bacteremia at their second presentation to the emergency room and blood cultures were obtained grown under aerobic and anaerobic conditions. 16S rRNA gene sequencing was used to identify anaerobic culture growth and identified B. pyogenes. In this report, the wound swab that originally cultured P multocida was re-cultured under anaerobic conditions and grew an isolate identical to the B pyogenes found in the anaerobic blood culture. The patient required multiple debridement and vacuum-assisted wound healing. While inpatient, they received IV benzylpenicillin for 9 days. After discharge on day 9, the patient was prescribed oral phenoxymethylpenicillin for 11 days. At a follow-up appointment 3 weeks after discharge, the patient’s condition had resolved. ³ Although this patient did not present with abscess formation, it is an example of selecting appropriate culture techniques to identify specific pathogens.

Patients presenting with late complications of cat bites should cue clinicians to consider secondary pathogens. Repeat presentation warrants further consideration into appropriate diagnostic laboratory techniques. Informing the laboratory that B pyogenes is a possible suspect of infection can help direct time efficient and effective culturing. B pyogenes has excellent aerotolerance compared to most anaerobes and simple nutritional requirements. ⁷ Most Bacteroides species (spp.) have innate resistance to aminoglycosides and vancomycin, leading to the development that selective media should be used. ⁷ The Brain Heart Infusion Medium is a culture media used for fastidious organisms like Bacteroides, and with additional care to maximize anaerobic conditions, visible growth can occur within 36–48 h. ⁷ After growth is obtained, advanced testing can be used to specifically identify B pyogenes. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and 16S rRNA gene sequencing have previously been used for this species.^3,8,9 These ideal techniques may not have been considered in the case of our patient because B pyogenes has not been encountered in this form previously. The lack of specific culture techniques could explain why culture growth only appeared on day 7 as opposed to 36–48 h. As mentioned previously, Ferreira et al. ⁵ achieved culture growth of P multocida on a nonselective agar within 24 h. The shorter timeline for growth and the use of a nonselective agar provide more ease of identification compared to the selective agar that requires anaerobic care and a longer timeline for B pyogenes. The additional care required to achieve culture growth of B pyogenes may explain the delayed growth at 7 days compared to the growth of P multocida at 2 days. This delay emphasizes the importance of choosing appropriate culture techniques based on suspected pathogens. A wide range of culture techniques, including the use of aerobic versus anaerobic techniques, nonselective versus selective agars, and additional time for growth should be considered following animal bite wounds.

Antibiotic resistance in Bacteroides spp. is a growing concern, as mentioned in the study by Majewska et al. ⁸ Among anaerobes, Bacteroides spp. have had the highest growth of resistance over the past decade. Most Bacteroides spp. are known for innate resistance to aminoglycosides and vancomycin. ⁷ Fortunately, B pyogenes is known to have increased susceptibility to penicillins, attributed to its lack of beta-lactamase production. ⁸ Despite this feature, Majewska et al. ⁸ encountered multiple B pyogenes species that displayed resistance to penicillin, one species also displaying resistance to clindamycin. In this study, all of the species displayed sensitivity to Amoxicillin with Clavulanate. Future encounters of B pyogenes should be treated with caution based on the concern of further antibiotic resistance. Current literature supports that Amoxicillin with Clavulanate should be considered a first-line treatment for cat bites with its ability to cover both P multocida and B pyogenes.^4,8

Clinical message

An intramuscular abscess causes by B. pyogenes is a unique clinical presentation. It is important to maintain a broad differential for bacterial possibilities and to obtain appropriate culture techniques, as this guides proper patient treatment. A wide range of culture techniques should be considered following animal bite wound and the current literature supports that Amoxicillin with Clavulanate should be considered a first-line treatment for cat bites with its ability to cover both P multocida and B pyogenes. However, with the growing antibiotic resistance of Bacteroides species, it is imperative to use appropriate laboratory technique to isolate this species to treat properly according to antibiotic susceptibility.

Conclusion

In this case report, we have described an intramuscular abscess caused by B. pyogenes that, to our knowledge, has not been discussed in previous literature. We have emphasized the importance of maintaining a high clinical suspicion for less commonly isolated bacteria such as Bacteroides species in cat bite wounds with delayed presentation. This case report also emphasizes the importance of ordering both aerobic and anaerobic cultures and aiming antimicrobial therapy against both Pasteurella and anaerobic bacteria. Further studies can be performed to identify the most efficient ways to isolate and treat Bacteroides species related to animal bite wounds.