Streptococcus agalactiae in adults at chiang mai university hospital: a retrospective study

Streptococcus agalactiae, a group B, β-hemolytic streptococcus, is a well-known cause of postpartum infection and neonatal sepsis[1, 2]. It colonizes in the gastrointestinal and urinary tract in healthy adults as well as the genital tract in healthy women[1, 2]. Recently, the number of cases of invasive infection caused by S. agalactiae in non-pregnant adults is increasing; [2‐10] the majority of patients had underlying medical conditions including diabetes, malignancy, genitourinary abnormalities, neurologic deficits, cirrhosis, renal dysfunction, steroid uses, and AIDS[2, 3, 6‐8, 11]. Clinical manifestations of invasive S. agalactiae infection vary widely depending on the sites of infection. Primary bacteremia without any obvious source is a common presentation in non-pregnant adults[2, 3, 6, 7, 10]. Endocarditis and meningitis have also been observed[3, 6‐9]. A myriad of virulence factors are crucial for its ability to cause invasive disease; these include but are not limited to the pore-forming toxins and the sialic acid-rich capsular polysaccharide[12, 13]. The mortality rate ranges from 3-47% and is highest in elderly patients with underlying medical conditions[2, 3, 5, 6, 8‐10]. This study aimed to describe the epidemiological, clinical, and microbiological data of invasive S. agalactiae infection in adult patients admitted to Chiang Mai University Hospital, a tertiary care center in Northern Thailand.

Streptococcus agalactiae is generally known to cause invasive infection in pregnant women and neonates since it commonly colonizes the vaginal and gastrointestinal tracts of healthy women[2]. However, infection in non-pregnant adults has been increasingly reported worldwide[5‐8, 10]. The incidence of invasive group B streptococcal diseases in the United States significantly increased from 3.4/100 000 population in 1999 to 5.0/100 000 population in 2005[10]. The same trend was observed in Asia[6, 8]. In our study, although the number of cases increased significantly from 2006 to 2007 (p = 0.017, data not shown), the difference was not observed from 2007 to 2009. However, we at least demonstrated that there were 5 times more cases within 4 years given the similar diagnostic method and data collection. This study also supports the evidence of increasing number of patients who were not pregnant. In addition, infections in non-pregnant adults occurred in similar proportion in male and female that corresponded to the previous studies [3, 6, 8, 9]

Other reports have not noted seasonal variation. However, our study revealed that the number was higher in September. We may need to further observe this phenomenon and explore the explanation for this finding.

Sixty percent of infected patients had underlying medical conditions including diabetes, chronic renal disease, and malignancy. Diabetes was the most common underlying medical disease in previous reports [2, 7, 10] which might be explained by the defect of bacterial engulfment by the neutrophils[19].

Isolation of S. agalactiae from urine were mostly from colonization; in our study, only 6.3% (5/79) of patients with positive urine culture were determined as true infection. This finding was similar to the report from the United States that specimens from urine and vaginal secretions were more likely to be colonization than infection[4].

The most common site of S. agalactiae infection was bloodstream infection which was also similar to other reports [2, 3, 6, 7, 10]. However, a study from Greece found that the most common site of infection was urinary tract[5]. In that study, the authors stated that there were two times more females than males, and it might be possible that faecal contamination during urine collection might have occurred[5]. Although echocardiogram was performed in all cases of bacteremia, no endocarditis was found in our study.

Clinical data comparing to other studies is shown in Table 5. Interestingly, in our study, meningitis was found in higher proportion than in other reports[3, 6‐8, 10, 11]. Seventy-five percent (6/8) of our patients with meningitis had concurrent bloodstream infection. The variation in infection sites among various reports might be attributed to the serotype of S. agalactiae. S. agalactiae is classified into serotype Ia/c, Ia/b, II, III, IV, V, VI, VII, and VIII[20]. Bolanos M, et al found that meningitis was correlated with infection with serotype Ia, whereas bloodstream infection, skin and soft tissue infections, urinary tract infection, and respiratory tract infections were correlated with infection with serotype III, V, III and V, and IV, respectively[21]. In addition, serotypes vary by geographic regions, e.g., serotype Ia, Ib, II, III, and V were accounted for 88% of adult cases in the United States and the most common serotype was serotype V[10]. In Japan, serotype Ia-V accounted for 69% of adult cases, and the most common serotype was serotype III[8]. This report also showed the virulence of serotype VI, VII, and VIII, which were not common in the United States[8, 10]. However, serotyping was not performed in our study.

The mortality rate in our study was 15% while those in other reports varied from 3 to over 30%[5, 7‐9, 11]. Other reports found that mortality was associated with bloodstream infection, hypotension, thrombocytopenia, other concurrent bacterial infections, infection with serotype Ia, and old age[2, 3, 7‐10]. Although we found that 83.3% (10/12) of death occurred in patients who had severe diseases, i.e., bloodstream infection and/or meningitis, we failed to demonstrate this association in the univariate analysis. In addition, we could not identify other factors associated with mortality, which is probably due to the small sample size.

All isolates in our study were susceptible to penicillin, ampicillin, and vancomycin, which was similar to other reports[5, 22, 23]. In general, group B streptococci remain uniformly susceptible to penicillins and cephalosporins in vitro, and penicillin G is still the drug of choice once the diagnosis is established[22]. However, ceftriaxone was frequently used in our study due to the fact that it could be administered once daily. Vancomycin, an alternative choice in patients who are allergic to β-lactams, was less frequently prescribed in our study.

S. agalactiae infection is a growing problem in non-pregnant patients, particularly in patients who had underlying medical conditions. Physicians should consider infection with S. agalactiae in the differential diagnosis of patients with meningitis or septicemia. It is noticeable from our study that no pregnant women with S. agalactiae chorioamnionitis had life-threatening conditions including bloodstream infection and meningitis, whereas non-pregnant patients had more serious infections; this might be explained by the fact that pregnant women are younger and have less comorbidities.