Answer: Capnocytophaga canimorsus. Asplenic patients are at risk for postsplenectomy sepsis with pathogens such as encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis), Babesia, and Plasmodium falciparum (1, 2). Capnocytophaga canimorsus is a zoonotic Gram-negative bacillus that is associated with fulminant infections in asplenic patients with histories of dog bites or scratches. Other risk factors include alcohol abuse and immunosuppression (3). Besides a thorough review of the patient's medical history, including animal exposure, Wright-Giemsa staining of a peripheral blood smear or a buffy coat provides a diagnostic clue of Capnocytophaga sepsis (4). The presence of Howell-Jolly bodies, which are the basophilic DNA remnants of red cells, nucleated red blood cells, and Pappenheimer bodies, all indicate inadequacy of splenic function for filtering these cells. Capnocytophaga is a slender, medium-to-long, Gram-negative bacilli with tapered ends, which is different from other common pathogens in the differential diagnoses of these patients.
Capnocytophaga is a fastidious bacterium often taking 48 to 72 h to reach 2 to 4 mm in diameter and usually requires incubation in 5 to 10% CO2. It can grow on blood and chocolate agars but not on MacConkey agar. Since it has no flagella, it is nonmotile on motility test medium. However, Capnocytophaga can move slowly with gliding motion, producing spreading edges and finger-like projections at the periphery of the colonies (5). Zoonotic strains like C. canimorsus and C. cynodegmi are distinguishable from human strains by positive catalase, oxidase, and arginine dihydrolase tests as well as lack of pigmentation (6). Phenotypic characterization, however, has limitations in identifying Capnocytophaga to the species level. 16S rRNA gene sequencing is an accurate identification method for a wide variety of bacterial species, including Capnocytophaga species (7). Our isolate exhibited 100% homology to C. canimorsus. Matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) is a recently developed diagnostic tool. Unfortunately, C. canimorsus is not included in the current database of the Vitek MS system (bioMérieux, Durham, NC). There is no standardized method of susceptibility testing, but Capnocytophaga is considered broadly susceptible to many antibiotics, including beta-lactam/beta-lactamase inhibitor and extended-spectrum cephalosporins (6). The patient was discharged after a 4-week course of ceftriaxone, but she still suffered from digital ischemia as sequelae of sepsis.
See page 1231 in this issue (https://doi.org/10.1128/JCM.01866-15 ) for photo quiz case presentation.
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