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Capnocytophaga canimorsus meningitis diagnosed by 16S rRNA PCR
  1. T M J Beernink1,
  2. P C Wever2,
  3. M H A Hermans3,
  4. M G T Bartholomeus1
  1. 1Department of Neurology, Bernhoven Hospital, Uden, The Netherlands
  2. 2Department of Medical Microbiology and Infection Control, Jeroen Bosch Hospital, ‘s-Hertogenbosch, The Netherlands
  3. 3Laboratory for Molecular Diagnostics, Jeroen Bosch Hospital, ‘s-Hertogenbosch, The Netherlands
  1. Correspondence to T M J Beernink, Department of Intensive Care Medicine, Rijnstate Hospital, Wagnerlaan 55, Arnhem 6800 TA, The Netherlands; TBeernink{at}rijnstate.nl

Abstract

Capnocytophaga canimorsus is a common Gram-negative anaerobic bacterium from the oral flora of dogs, typically transmitted to humans by dog bites. We report a case of C. canimorsus meningitis where there was (on presentation) no apparent predisposing risk factor and in whom we used 16S rRNA PCR gene sequencing to identify the pathogen quickly and to switch to appropriate antibiotic therapy. Physicians should be aware of potential C. canimorsus meningitis if conventional cerebrospinal fluid bacterial culture is negative but Gram staining identifies bacteria, especially in patients with a recent dog bite or known immunodeficiency.

  • BACTERIOLOGY
  • CLINICAL NEUROLOGY
  • INFECTIOUS DISEASES
  • MICROBIOLOGY

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Introduction

Capnocytophaga canimorsus is a common anaerobic bacterium in the oral flora of dogs, typically transmitted to humans by dog bites.1 There have been only 30 reported cases of C. canimorsus meningitis in the English language literature but numerous cases of C. canimorsus sepsis, up to 30% of which were fatal.2 Routine bacterial cultures can miss C. canimorsus due to its need for a longer incubation time and specific agar requirements.2 Physicians should consider C. canimorsus infection if conventional cerebrospinal fluid (CSF) bacterial culture is negative but Gram staining identifies bacteria, especially in patients with a recent dog bite or known immunodeficiency. We report a case of meningitis caused by C. canimorsus in a man with no known risk factors at presentation, in whom 16S rRNA gene sequencing identified the pathogen despite no bacterial growth in a CSF culture.

Case report

A 52-year-old man presented to the emergency department with headache and dizziness; his wife reported a change in his behaviour. He had a history of chronic obstructive pulmonary disease. On examination, he was afebrile with no neck stiffness and no focal neurological deficits. Laboratory studies showed a white blood cell count of 12.0×109/L (4–11) with neutrophilia and a serum C reactive protein concentration of 334 mg/L (<10). Further laboratory findings were normal. Lumbar puncture revealed cloudy CSF, with a white cell count of 5210/0µL ≤5), a total protein concentration of 5.15 g/L (0.15–0.45) and glucose concentration of 0 mmol/L (3.3–4.4). No blood cultures were taken. We admitted him to the neurology ward and empirically treated him with intravenous ceftriaxone and dexamethasone.

CSF microscopy showed many leucocytes and a moderate number of Gram-negative bacilli (figure 1). With this result we added amoxicillin to the antibiotic regimen. Because there was no visible growth on his bacterial culture after 2 days, we performed 16S rRNA gene sequencing for bacterial identification directly on CSF. After proteinase K digestion, we isolated DNA from CSF, and then performed 16S rRNA PCR.3 Our comparison of the obtained sequence with sequences available in the NCBI database showed 100% homology to C. canimorsus Cc5.4 Thus, we established the diagnosis C. canimorsus meningitis and switched his antibiotic treatment to intravenous penicillin for a further 12 days. After 16 days we discharged him from the hospital without further neurological sequelae.

Figure 1

Gram-stained cerebrospinal fluid of a patient with Capnocytophaga canimorsus meningitis shows leucocytes and a moderate number of Gram-negative bacilli.

In hindsight, he recalled his dog licking scratches on his knee caused by a fall several days before admission; the scratches had healed at the time of admission.

Discussion

Bobo and Newton5 described the first case of C. canimorsus infection in 1976. C. canimorsus was first named Dysgonic fermenter 2, due to its slow growth (dysgonic) and its ability to ferment sugar (fermenter). In 1989, Brenner et al6 renamed the bacterium Capnocytophaga canimorsus: Capnocyto because of the high carbon dioxide concentrations needed during the incubation period and canimorsus because of its assumed pathway of infection—a dog bite.

C. canimorsus is common in the oral flora of dogs. According to Westwell et al,7 it is in the oral cavity of 24% of dogs and 17% of cats. Transmission usually occurs through bites (54%), scratches (9%) or close contact with dogs (27%). In a small number, the route of transmission is unknown. Infection is particularly virulent in patients with disorders in the mononuclear-phagocytic system and humoural immunity. Therefore, reported cases have typically been in patients with a splenectomy, functional asplenism, alcohol abuse, chemotherapy or use of immunosuppressive medication: our patient had none of these.8 The time from dog bite to hospitalisation is on average 7 days.9 Usually the bacteria do not cause a wound infection but do cause sepsis, endocarditis, meningitis or arthritis.8 Clinical symptoms vary from mild to severe, with vomiting, fever, diarrhoea, abdominal pain, dyspnoea and headache.8 In cases of sepsis, the mortality is as high as 30% compared with 5% in meningitis.10

C. canimorsus needs a longer incubation time and specific agar culture conditions before it will grow in routine CSF culturing.2 ,11 Even in these conditions, it can still take up to 7 days before cultures show signs of bacterial growth, making C. canimorsus infection difficult to diagnose.8 Furthermore, considering that it is often routine practice to discard negative culture media after 5 days, C. canimorsus infections may easily be missed within routine culturing. If there is a high suspicion of a C. canimorsus infection, one should inform the microbiology department so they can culture the sample in specific conditions.

A possible bite wound is the only feature in the history that helps to discriminate between a rare C. canimorsus meningitis and another more common cause of bacterial meningitis. Thus, it is particularly important to seek a history of a bite in patients with culture-negative meningitis.

Infected patients need rapid treatment with appropriate antibiotics and so it is important to diagnose C. canimorsus infection as quickly as possible. 16S rRNA identification is particularly useful in identifying slow-growing or fastidious bacteria, such as C. canimorsus, in otherwise sterile clinical materials (ie, blood, CSF, pleural fluid or synovial fluid).12 Conserved regions of the 16S rRNA gene allow the amplification and sequencing of variable 16S rRNA regions in between these conserved stretches that are species-specific. The vast majority of medical laboratories have standard PCR equipment that can isolate and amplify bacterial DNA from a clinical specimen. Many microbiological and pathological laboratories currently have a DNA sequencer for routine diagnostics. The costs of consumables of a 16S rRNA identification, including sequencing performed on an ABI 3100 instrument is around $65 (€60). The turnaround time is roughly 10 h, depending on the number of samples processed, and the time needed to compare the obtained sequence with sequences available in public databases.

C. canimorsus infection is an uncommon but possibly lethal condition especially in case of a septic patient. It has no pathognomonic symptoms and it is important to recognise patients with predisposing risk factors like a recent dog bite or immunodeficiency. Physicians should be aware of C. canimorsus meningitis as a possible cause for culture-negative meningitis with abnormal Gram-staining of CSF. If suspicion is high, a physician could consider requesting a 16S rRNA PCR on the CSF for diagnostic purposes. By creating more awareness about C. canimorsus meningitis and by early use of 16sRNA PCR, it is possible to decrease the time until diagnosis allowing a faster switch from empiric to directed antibiotic therapy.

Key points

  • Capnocytophaga canimorsus is a common Gram-negative bacillus in the oral flora of dogs.

  • Transmission to humans usually occurs through bites, scratches or close contact.

  • The slow-growing nature of the bacterium makes it a difficult to diagnose.

  • Consider requesting a 16S rRNA PCR on sterile material in case of a suspicion of slow-growing and fastidious bacteria such as C. canimorsus.

  • In cases where there is no bacterial growth in a conventional cerebrospinal fluid culture but the finding of organisms on Gram-staining, one should consider a C. canimorsus meningitis, especially in patients with a recent dog bite or immunodeficiency.

References

Footnotes

  • Contributors TMJB helped manage the case, drafted and revised the paper. PCW helped manage the case, was responsible for microbiological diagnostics, revised the paper and edited the image. MHAH was responsible for molecular diagnostics and revised the paper. MGTB helped manage the case and revised the paper. All authors were involved in final approval of the article.

  • Competing interests None declared.

  • Patient consent Obtained.

  • Provenance and peer review Not commissioned; externally peer reviewed. This paper was reviewed by Brendan Healy, Cardiff.

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