Septic Arthritis due to Streptococcus dysgalactiae Subspecies equisimilis in a Healthy School Child
She was admitted to our hospital because of sustained pain and swelling of the left ankle 2 days before admission. The following laboratory examination results were obtained on admission: peripheral white blood cell count, 14,600/mm3; C-reactive protein, 4.07 mg/dL and polymorphonuclear leukocytes in peripheral blood smear, 65.3%. Synovial fluid obtained by arthrocentesis of the joint was grossly yellow and turbid with a cell count of 480/mm3. Gram stained smear revealed no identifiable bacteria, and the culture was negative for bacteria. Two days later, we performed an arthrotomy, and synovial fluid was reobtained. The synovial fluid obtained during the operation was still grossly yellow and turbid with a cell count of 10,060 /mm3. Gram stained smear revealed no identifiable bacteria. To identify the pathogen, we conducted conventional cultures including the use of enrichment medium and 16S ribosomal ribonucleic acid gene sequencing. Two days later, only the enrichment medium demonstrated bacterial growth that could be grown subsequently on sheep’s blood agar in 5% carbon dioxide. The isolates were identified as group G β-hemolytic streptococci. Analyses performed using the VITEK2 System (BioMérieux, Inc., Lyon, France) and a MALDI Biotyper (Bruker Daltonics GmbH, Bremen, Germany) identified the isolate as SDSE. In addition, it could be identified from the enrichment medium directly. 16S ribosomal ribonucleic acid gene sequencing results also supported our identification; Basic local alignment search tool was used to identify the isolate as SDSE. emm typing2 and multilocus sequence typing revealed the isolate to be stC36.0 and sequence type 8, respectively. This emm type has been identified in less than 3.0% of SDSE isolates globally.3 The isolate was susceptible to penicillin and all cephalosporins, including cefazolin, but resistant to erythromycin. After the operation, parenteral cefazolin was prescribed. The patient responded well to this management and improved symptomatically.
An increase in the white blood cell count in synovial fluid strongly suggests septic arthritis in a patient who presents with symptoms specific to septic arthritis. However, the inflammatory response within the synovial cavity may be milder in pediatric SDSE septic arthritis than in septic arthritis attributable to other common bacterial pathogens. A previous study reported that bone and joint infections caused by SDSE were associated with relatively poor clinical outcomes.4 Thus, septic arthritis should be considered in differential diagnoses to ensure that SDSE septic arthritis is identified for improved outcome.