A Rare Case of Capnocytophaga canimorsus Meningitis in an Immunocompetent Patient: A Successful Outcome with a Short Course of Antibiotics

Capnocytophaga canimorsus is a long, non-spore-forming, slow-growing, Gram-negative, capnophilic, facultative anaerobic bacterium. It is part of the normal oral flora of dogs, and cats to a lesser extent. Humans can be infected by dog or cat bites, licks, or scratches. Cases of C. canimorsus infections have been reported worldwide [1‒3]. While C. canimorsus typically poses low risk to healthy individuals, it can cause severe, even life-threatening, illness in those with certain preexisting conditions. Infection can result in localized symptoms or spread throughout the body, especially in immunocompromised, asplenic, or alcoholic patients [2, 3]. Potential complications include bacteremia, sepsis, gangrene of the extremities, meningitis, and endocarditis [2, 4]. Diagnosis can be challenging due to the bacterium’s slow growth rate [5].

This case describes an immunocompetent patient with C. canimorsus meningitis. He recovered after only 1 week of antibiotic use.

A 46-year-old male patient presented at the emergency department with somnolence and an inability to respond coherently to questions or commands. The patient’s partner reported that he had been experiencing complaints of malaise, headache, nausea, vomiting, and diarrhea for the past 6 days while on a camping holiday with his partner and dog. On the most recent day, there was a notable change in his behavior; he appeared confused, and upon waking that morning, he was somnolent and was unresponsive to his partner. There was no notable medical history or current medication use. He was a smoker and consumed 1–2 L of beer daily.

At the time of presentation, his vital signs included a temperature of 39°C, a respiratory rate of 28/min, 95% oxygen saturation with 3 L of oxygen, a blood pressure of 120/60 mm Hg, and a pulse of 95/min. Laboratory results indicated signs of inflammation with leukocytosis and a CRP level of 124 mmol/L. COVID-19 test results came back negative, and imaging results of abdomen (echo and CT), head (CT), and lung (X-ray) showed no abnormalities.

On neurological examination, the patient showed signs of meningeal irritation, with fluctuating mental states alternating between moments of appropriate behavior and confusion. He was only able to perform one-step commands. He was oriented to person, place, and time with no aphasia or other focal neurological deficits. No papilledema was detected, and his extraocular movements were full and symmetric, with no evidence of diplopia or sixth cranial nerve palsy. Pupillary light reflexes were brisk and equal bilaterally, with no anisocoria. Deep tendon reflexes were normal with normal plantar flexor responses observed. Physical examination showed no petechia or infected wounds. A lumbar puncture was conducted, revealing cerebrospinal fluid (CSF) with a white blood cell count of 2,160 per cubic millimeter, glucose at 2.2 mmol/L, and protein at 2,062 mg/L (see Table 1). The Gram stain was negative. The patient was admitted to the neurology department and treated, in conformity with the Dutch guidelines, with a combination of amoxicillin 2 g every 4 h, ceftriaxone 2 g every 12 h, and dexamethasone 10 mg every 6 h for 4 days.

On the second day of admission, the patient already felt much better. The headache was minimal, with no neck pain present. He was alert and responsive, and neurological examination findings had normalized. On the fourth day of admission, the CSF culture results revealed an infection with C. canimorsus bacterium. The isolate was susceptible to penicillin, ceftriaxone, clindamycin, and ciprofloxacin. No beta-lactamase testing was performed. The blood cultures remained negative. Given the patient’s good clinical recovery and the fact Capnocytophaga species are generally susceptible to both agents, it was decided to continue with the current antibiotic regimen. On the sixth day of admission, ampicillin was discontinued. Ceftriaxone was continued for a total of 7 days, after which the patient was discharged home. The patient remained clinically symptom-free after discharge and at follow-up consultation at our outpatient clinic after 1 month. He reported to be content with the treatment he received.

C. canimorsus infections tend to occur more frequently in men than in women, with reported cases having an age range from 4 months to 77 years [1]. C. canimorsus infections are commonly caused by dog or cat bites (54% of cases), scratches (8.5% of cases), or close contact while having open wounds (27% of cases). In about 10% of cases, the source of the infection could not be determined. In people with normal immune function, these infections are usually asymptomatic due to the bacteria’s low virulence [3].

Our patient, despite not being bitten by a dog, as reported by most patients [5], had been licked by his dog several times in the week preceding the onset of symptoms. He is not known to be immunocompromised and rarely experiences infections, therefore screening for acquired immune deficiencies (e.g., HIV) was not deemed necessary. However, his alcohol use may have affected his immune system. Literature reviews note that alcoholism is present in 19–24% of cases with C. canimorsus meningitis [1, 5, 6].

The mortality rate for severe C. canimorsus infections is around 30%, whereas for C. canimorsus meningitis, it is significantly lower at 5%. After receiving treatment, the majority of patients recover without sequelae (75–81%), as was the case for our patient. Reported sequelae include hearing loss and cognitive impairment [5, 6]. The median time from exposure to onset of sepsis is 3 days, while for meningitis, it is 7 days [7]. Symptoms of C. canimorsus meningitis resemble those of bacterial meningitis, including headache and meningeal irritation. However, unlike conventional bacterial meningitis, fever is less frequently observed in cases of C. canimorsus meningitis [8]. The patient displayed many of the common symptoms, including headache, fever, nausea, neck stiffness, and altered mental status, usually seen in those with C. canimorsus meningitis [5]. Early identification can be difficult, due to the slow growth of this bacterium when using traditional culture techniques, potentially leading to diagnostic delays [5].

Capnocytophaga species (spp.) have growth requirements similar to those of Neisseria meningitides, so CSF cultures should be able to detect most infections caused by these organisms. However, growth is generally slower, and Capnocytophaga spp. can be missed with the standard 5-day incubation of CSF cultures. At least 1 week of incubation is advisable. In our case, the isolate showed growth after 4 days. The organism was identified as C. canimorsus through matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Although a PCR has been developed for C. canimorsus, it is not included in the currently available commercial Meningitis/Encephalitis Panels. Antimicrobial susceptibility testing was performed by an E-test. The resulting minimum inhibitory concentrations were interpreted according to the EUCAST guidance document on “When there are no breakpoints in breakpoint tables?” [9]. The susceptibly results are in line with the published literature on C. canimorsus.

The diagnosis of C. canimorsus can be challenging; however, the bacterium is typically susceptible to a wide range of antimicrobial agents. Our patient was empirically treated according to the Dutch guideline for meningitis (www.swab.nl) with amoxicillin, ceftriaxone, and dexamethasone. The isolate was susceptible to both antibiotics. There is no definitive evidence regarding the benefit or detriment of dexamethasone in treating C. canimorsus meningitis; however, a significant effect was found on reducing hearing loss and neurological sequelae in people of all ages with acute bacterial meningitis [10]. The duration of 1 week was chosen based on the good clinical response and the fact that there were no abnormalities on the CT scan as there are no specific guidelines for the treatment of C. canimorsus. The previous literature has reported a wide range of antibiotic therapy durations for C. canimorsus infections, spanning from 7 to 42 days, with a median treatment duration of 15 days [5]. Seven days is also the duration advised by the Dutch Working Party on Antibiotic Policy for the treatment of meningitis caused by N. meningitides and H. influenzae [11].

In conclusion, C. canimorsus infections, though rare, present diagnostic and therapeutic challenges due to their variable presentation and slow growth in culture. This case report underscores the importance of considering C. canimorsus in patients with relevant animal exposure, even in the absence of bites, and highlights the potential impact of alcohol use on the immune system. Early and accurate identification of C. canimorsus, using advanced techniques such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, is crucial for effective treatment. Empirical treatment with broad-spectrum antibiotics, guided by susceptibility testing, is recommended. The favorable outcome in this case, achieved with a 1-week course of antibiotics, supports the tailored approach to treatment duration based on clinical response, the outcomes of previously reported cases, and diagnostic findings.

The CARE Checklist has been completed by the authors for this case report, attached as online supplementary material (for all online suppl. material, see https://doi.org/10.1159/000541631).

Written informed consent was obtained from the patient for publication of the details of their medical case and any accompanying images. Ethical approval was not required for this study in accordance with local guidelines.

The authors have no conflicts of interest to declare.

This study was not supported by any sponsor or funder.

A.E.H. and T.J.L.: conceptualization and writing – original draft, review, and editing. J.J.K.: writing – original draft, review, and editing.

All data generated or analyzed during this study are included in this article and its online supplementary material. Further inquiries can be directed to the corresponding author.