Case Presentation: A 75 year-old woman with a remote history of breast cancer and macrocytic anemia presented to the Emergency Department with chills, poor appetite and painful bilateral lower extremity rash for 3 days. Her vital signs were within normal limits and examination was most notable for non-blanching scattered purpuric macular lesions of her lower extremities. She was admitted to the hospital after being found to have platelets of 28,000 and serum creatinine of 3.4 mg/dL (both values were normal one week prior). Continued workup showed no evidence of hemolysis, peripheral smear showed only thrombocytopenia, and imaging revealed no abnormalities of her spleen or liver. Her urine studies were consistent with a prerenal AKI. Her creatinine normalized with intravenous fluids. However, her platelets decreased to 18,000, and she developed a new fever to 39° C along with headache, nausea, and encephalopathy. A chest X-ray, urinalysis and head CT where normal. Blood cultures were drawn, and, due to concern for meningitis, empiric antibiotic coverage with cefepime, vancomycin, ampicillin, and acyclovir was started after unsuccessful attempts at lumbar puncture.By hospital day 6, fever and encephalopathy resolved, and blood cultures from 72 hours prior became positive for anaerobic gram negative rods (GNRs). A detailed exposure history revealed that she had sustained several bites from a new kitten at home the week prior to admission. Over the ensuing days, her antibiotics were narrowed to ampicillin-sulbactam, and she remained afebrile with normalization of her labs. One week later, the identification of the anaerobic organism from her initial blood cultures performed via MALDI-TOF mass spectrometry assay returned as Capnocytophaga canimorsus.
Discussion: Capnocytophaga canimorsus is an anaerobic GNR that is a part of the normal oral flora of cats and dogs, and is usually transmitted by bites or scratches. This organism grows slowly in culture and has a requirement for an environment rich in iron and CO2. Because of its fastidiousness, the laboratory should be alerted if Capnocytophaga is suspected so that enriched agar can be used. In many cases, even with a culture environment tailored to growth of Capnocytophaga, culture data alone will be insufficient, and specialized molecular testing, such as 16s ribosomal sequencing or mass spectrometry, is necessary. This is consistent with the observation in our case wherein the culture data was only able to identify the presence of anaerobic GNRs, but the final results came from mass spectrometry testing.Infection with Capnocytophaga canimorsus can cause severe sepsis often accompanied by disseminated purpura, disseminated intravascular coagulation, encephalopathy and AKI, and is associated with a high mortality rate, with immunocompromised patients at greatest risk of infection. This patient was potentially immunocompromised from advanced age in addition to potential myelodysplastic syndrome from prior chemotherapy as the cause of her macrocytic anemia.
Conclusions: This case emphasizes the importance of obtaining an exposure history and continually reframing the differential diagnosis based on evolving clinical data. Having a high clinical suspicion for Capnocytophaga canimorsus is necessary to ultimately secure the diagnosis given the need for specialized lab testing. In this case, the positive zoonotic exposure history along with developing fever, purpuric rash, AKI, and thrombocytopenia ultimately led to a correct diagnosis.