Capnocytophaga canimorsus Bacteremia Without Animal Bite: A Rare but Severe Infection to Consider in Older Adults

Introduction

Capnocytophaga canimorsus is a slow-growing, capnophilic, facultative anaerobic gram-negative bacillus that belongs to the normal oral microbiota of dogs and cats and can be transmitted to humans through bites, scratches, licking, or close contact with saliva [1–3]. Although uncommon, invasive infection can be severe and includes sepsis, septic shock, meningitis, and endocarditis [1,4]. Classic predisposing factors include asplenia, alcohol use disorder, and immunosuppression; however, 40% to 50% of cases occur in patients without obvious risk factors [1,3].

Diagnosis remains challenging because the organism is slow-growing and may require prolonged incubation of blood cultures; identification may rely on MALDI-TOF and, in culture-negative situations, broad-range or 16S rRNA PCR [2,5,6]. Here, we describe C. canimorsus bacteremia in an older woman without a documented bite or scratch, and we discuss the diagnostic and therapeutic challenges in the post-arthroplasty setting.

Case Report

A 75-year-old woman was admitted to the emergency department for fever and asthenia. Her medical history included hypertension, asthma, and breast cancer treated in 2000. Six weeks earlier, she had undergone right total hip arthroplasty for a femoral neck fracture, followed by inpatient rehabilitation with good functional recovery and no local symptoms.

Approximately 10 days after returning home, she developed progressive fatigue associated with urinary symptoms and right flank pain. On admission (Day 0), her temperature was 38.8°C without hemodynamic instability. Physical examination revealed right upper-quadrant abdominal tenderness, without erythema, swelling, pain on mobilization, or other signs of prosthetic joint inflammation. Laboratory tests showed an inflammatory syndrome. Empiric intravenous ceftriaxone was initiated for suspected acute pyelonephritis, and blood cultures were collected. Blood cultures were processed using an automated continuous monitoring system (BACT/ALERT®, BioMerieux, France). In our laboratory, the standard incubation period for aerobic bottles is 5 days. However, given the persistence of fever despite empirical antibiotic therapy and the absence of an identified source of infection, extended incubation was pursued in close collaboration with the microbiology laboratory.

Abdominopelvic computed tomography did not confirm a urinary tract infection but showed a nonspecific periprosthetic bursitis around the hip arthroplasty, without fluid collection. The patient was hospitalized for further investigations. Her fever persisted despite antibiotic therapy, and she developed an acute confusional episode; brain imaging ruled out acute stroke.

In the context of recent prosthetic surgery and persistent inflammatory syndrome, prosthetic joint infection became a major diagnostic concern. Whole-body 18F-FDG positron emission tomography/computed tomography (18F-FDG PET/CT) demonstrated hypermetabolism surrounding the prosthesis, which could not differentiate postoperative changes (eg, hematoma) from infection.

Following extended incubation, blood cultures became positive on Day 10 and grew a gram-negative bacillus identified as Capnocytophaga canimorsus by MALDI-TOF mass spectrometry. There was no history of animal bite or visible wound, although the patient reported close daily contact with dogs. Antibiotic susceptibility testing was inconclusive, likely due to the slow growth of the organism and prior antibiotic exposure. Transthoracic echocardiography showed no valvular vegetation or signs of infective endocarditis.

Given the absence of clinical evidence of septic arthritis or endocarditis, antimicrobial therapy was switched to high-dose amoxicillin for 14 days, with rapid clinical and biological improvement. C-reactive protein decreased to 8.0 mg/L at discharge.

The patient was discharged home after 3 weeks of hospitalization. As part of the post-hospitalization assessment, bone scintigraphy showed moderate uptake compatible with postoperative remodeling. A subsequent labeled leukocyte scan showed no significant leukocyte recruitment, arguing against active prosthetic infection.

Orthopedic follow-up confirmed a painless and functional prosthesis without radiologic abnormality and with normalization of inflammatory markers. No recurrence occurred during follow-up.

Discussion

This case highlights 3 clinically relevant points. Despite widespread exposure to dogs and cats, severe human infections due to Capnocytophaga species remain exceptionally rare, with a very low incidence reported in epidemiological studies [1,3,4].

Zoonotic Capnocytophaga species, including C. canimorsus, C. cynodegmi, C. canis, C. felis, and C. stomatis, are well-recognized commensals of the oral microbiota of dogs and cats [1,3]. Colonization occurs early in life, often within the first weeks, likely through maternal transmission. In parallel, humans harbor their own commensal Capnocytophaga species, such as C. ochracea, emphasizing the widespread presence of this genus in both animal and human microbiomes.

On the one hand, this case shows that invasive C. canimorsus infection can occur in older patients without additional risk factors and without documented bite or scratch. Recent epidemiologic and clinical series confirm that a dog or cat bite is absent in a substantial fraction of zoonotic Capnocytophaga infections [3], and exposures such as licking or close contact may be overlooked unless specifically investigated [1]. On the other hand, the diagnostic delay was driven by the organism’s fastidious growth, with blood cultures turning positive only after prolonged incubation. This underlines the need to alert microbiology laboratories when Capnocytophaga infection is suspected and to consider molecular diagnostics when cultures are negative after antibiotics, as emphasized by recent case reports [5,6]. Finally, the recent hip prosthesis raised the concern for hematogenous prosthetic joint infection (PJI), which has been previously described in the literature [7–9]. However, early postoperative nuclear imaging lacks specificity, and increased uptake can reflect postoperative inflammation rather than infection. In our patient, the absence of local signs, the favorable clinical course under antibiotics, and non-conclusive nuclear imaging argued against established PJI.

Primary prevention should emphasize thorough wound cleaning after animal bites or scratches and, in high-risk individuals, prompt medical assessment with consideration of antibiotic prophylaxis [1]. From a therapeutic perspective, beta-lactams (including penicillin/amoxicillin) are generally active against C. canimorsus and are recommended for targeted therapy in susceptible isolates [1]. Nonetheless, beta-lactamase-mediated resistance has been reported in animal-bite-associated Capnocytophaga species [10], supporting the use of third-generation cephalosporins or beta-lactam/beta-lactamase inhibitor combinations for empirical coverage in severe disease. In our case, ceftriaxone was chosen empirically based on the initial abdominal presentation; after identification, high-dose amoxicillin was selected because susceptibility testing was not interpretable and the patient was clinically improving.

Overall, this case highlights practical points for clinicians: (1) systematically inquire about dog/cat exposure in older patients with sepsis or persistent fever; (2) consider prolonged incubation and alternative identification methods (MALDI-TOF, 16S rRNA PCR) when standard cultures are negative or delayed; and (3) in patients with recent arthroplasty, evaluate for possible hematogenous seeding while interpreting early postoperative imaging cautiously.

Conclusions

This case emphasizes that Capnocytophaga canimorsus can cause bacteremia and sepsis in older, otherwise healthy individuals and may occur without obvious bite marks. Careful exposure history, prolonged blood culture incubation, and access to MALDI-TOF or molecular diagnostics are key to timely identification. Further prospective data are needed to better define optimal diagnostic strategies and treatment duration in immunocompetent older adults and in patients with orthopedic implants.