[Ultra-late onset group B streptococcal disease--a report of two cases]

Group B Streptococcus (GBS) infection in infants aged over 90 days, known as ultra-late onset disease (ULOD), is extremely rare. We present 2 cases of ULOD and investigate etiology from both the host and microbiological aspects. Case 1, 99-day-old girl born in the late preterm, had a history of 6-hour fever. Bacterial meningitis was diagnosed and the patient was treated with intravenous ampicillin for 14 days. The isolate was serotype III GBS. Case 2, a 7-month-old girl with no medically significant history had an intermittent fever for 2 weeks before admission. Serotype Ia GBS was isolated from urine and blood, leading to a diagnosis of urosepsis. Intravenous cefotaxime was administered for 7 days. Both patients were discharged without any sequelae. We examined the host risk factors for ULOD development. (i) One subject had underlying preterm birth and the other had bilateral vesicoureteral reflux. (ii) Both had extremely low serum anti-serotype specific immunoglobulin levels, an important measure of protective immunity. The anti-type Ia antibody concentration was 0.24 microg/mL and the anti-type III IgG antibody concentration was 0.25 microg/mL. We employed multilocus sequence typing (MLST) to determine the genetic background of bacterial isolates. Sequence types (STs) of isolates were ST-335 (one allele variant of ST-19) and ST-23. ST-335 is an epidemic invasive GBS disease strain in Japan and is dominantly correlated with serotype III. ST-23 is highly associated with serotype Ia and is also a common invasive type in Europe, the United States and Japan. Our findings suggest that ULOD likely develops combined with underlying host disease, immunological factors and highly virulent strains. Continuous investigation of large numbers of cases is needed to better understand ULOD etiology.