Group B streptococcal colonization in mothers and infants in western China: prevalences and risk factors

Streptococcus agalactiae, known as Group B streptococcus (GBS), is a common colonizing pathogen in pregnant women that can cause invasive infections during peripartum period [1]. The Vaginal and rectal tract are considered major reservoirs of GBS [2]. Mucosa surfaces such as oral, nasopharyngeal, vaginal and anal mucosa and skin of newborns at birth support the colonization of GBS [3]. It was reported that the risk of colonization of GBS was much higher in infants who were born to GBS positive mothers, and the likelihood was even higher if the mother was heavily colonized (growth of GBS with a density between 3⁺ to 4⁺) [4, 5]. GBS can easily be transmitted vertically from mothers to newborns in the peripartum period, especially during vaginal delivery [6]. GBS has been the principal cause of septicemia, meningitis and pneumonia for early and late onset diseases in newborns and was attributed to infant mortality in many parts of the world [4]. Little is known about the burden of perinatal GBS colonization and related infections in China. Specifically, the prevalence of maternal and infant GBS colonization and the risk factors for vertical transmission of GBS from mothers to infants have not been reported in western China. Since there is no screening for GBS or intrapartum administration of antibiotics (IAP) protocol was in place in China for prevention of GBS disease at the time of this study, the aims of this study were to determine the prevalence of and risk factors for maternal and infant GBS colonization at Liuzhou Maternity and Child Healthcare Hospital (LMCHH), one of the largest maternal and child health care hospitals in Guangxi in western China, in order to inform future prevention efforts and interventions.

Women and newborns were followed during the entire delivery hospitalization in LMCHH. The microbiological, clinical records and hospital databases were reviewed and recorded at the end follow-up for each pair of participants. Demographics, microbiological and clinical records from hospital databases were reviewed and extracted by study investigators in LMCHH. Data elements included in this study were: maternal age, previous obstetric history, gestational age, parity, mode of delivery, duration of membrane rupture (in hours), labor duration (in hours), antibiotic use of mother and newborn, newborn gender, birth weight, neonatal fever, newborn length of stay (LOS), occurrence of early onset diseases (EOD), and neonatal intensive care unit (NICU) admission.

Our study demonstrates that the GBS colonization rate in pregnant Chinese women in Liuzhou was 6.1%, which is similar to the reported rate of 7.1% in Beijing [9] and 8.2% in Dongguan [10]. Our rate is similar to those of other Asian countries including Philippines (7.5%) [11], Japan (8.2%) [12], Myanmar (7.1%) [11] and South Korea (8.0%) [13]. but lower than the global colonization rate of 17.9% reported by a recent meta-analysis study [14]. The GBS colonization rate in our study was also lower than the rate reported in Southeast Asia (11.1%) [14]. These results indicate that GBS colonization prevalence in pregnant women varies widely across the globe. Factors that may have contributed to this variation include the level of economic development, the availability of medical care and preventive services, varying clinical practice guidelines, as well as differing average child bearing age [15]. Our study found that ethnic minority pregnant women in Liuzhou were significantly more likely to be colonized with GBS when compared with Han and Zhuang ethnic groups. This is consistent with the previous study conducted in US which showed higher GBS colonization rates were more likely to have black and Hispanic maternal races than white race [16–19].

In the present study, we found that there was a significant association between maternal GBS colonization and newborn GBS colonization. Studies have found that the risk factors for infant GBS colonization include maternal rectovaginal colonization, preterm delivery, low birth weight, antepartum and intrapartum fever and prolonged membrane rupture [20–22]. Although we found no association between maternal GBS colonization and preterm birth, we did find the association between both membrane rupture (≥12 h) duration and labor duration (≥4 h) with infant GBS colonization risk. Our data was consistent with a previous study conducted in China which showed that prolonged rupture of membranes and increased duration of rupture of membranes were significant risk factors for neonatal GBS colonization [23].

Our data show that vertical transmission from mothers to their newborns was 7.6%. This is lower than what was reported in Europe and Asia, which have a transmission rate of 11.2% in Germany [24], 15.1% in Bangladesh [25] and 16.7% in Taiwan [26]. It is known that birth canal contact is a risk factor of GBS transmission to newborns [27]. The cesarean section rate at LMCHH was 54.1%, while the cesarean section rate was 42% in mainland China [28]. The high cesarean section rate may explain in part the low rate of vertical transmission in our study. It is also worth noting that despite the implementation of a universal protocol of GBS screening procedures in our institution, we still observed 7 infants with GBS colonization who were born to GBS-negative mothers. It was reported that false negative results are expected to occur based on several procedure of the implementation of screening test for GBS [16]. Improper collection methods, delay in processing, insufficient laboratory techniques, recent antibiotic use, or delay in GBS colonization may all have contributed to the negative GBS screening results in mothers who deliver GBS positive infants [18, 29, 30]. Several studies have examined the risk of GBS colonization in infants who were born to GBS negative mothers. The reported rates by these studies, ranged between 21.5 and 61.4%, [16, 31, 32] were consistent with our study observed rate of 30.4% (7/23). To improve the accuracy of screening by identifying the GBS status, a rapid polymerase chain reaction (PCR) test could be used at the time of labor to increase the sensitivity and specificity of the tests [33–35].

Our data demonstrated that GBS colonization remains an important cause of infant morbidity requiring NICU admission and longer LOS. Clinically, infants may be colonized with GBS through intra-amniotic infection by way of GBS ascending from the vagina or through birth canal contact during labor [27]. Exposure to GBS increased the risk of GBS-EOD, which often leads to NICU admission [26]. A previous study indicated that infants born to GBS positive mothers were at a higher risk of admission to the NICU [36], which is consistent with our study findings. Possible reasons for the increased morbidity of GBS positive infants include exposure to GBS in the perinatal period [27], hypervirulence of certain GBS strains [37], as well as the “maternal antibody” hypothesis [38]. The serotype III of GBS is considered hypervirulent which may cause a longer colonization duration throughout the pregnancy and is the most common serotype isolated in neonates less than 7 days of age [21]. In addition, maternal serum IgG antibodies against specific capsular polysaccharides of GBS has been shown to be protective against infection with virulence strain of GBS in their offspring [31]. Mothers with low titers of IgG antibodies against the GBS strains may be responsible for the hypervirulence of GBS [38], thus contributing to the GBS colonization and possible future infection in their newborns.

This study possessed two strengths. This is the first large-scale study conducted in western China with a cohort consisting of 3439 pairs of mothers and infants. Additionally, we collected surface culture on all newborns regardless of maternal GBS culture results.

There are some limitations to our study. First of all, the single-center design and the low prevalence of GBS positive infants may limit the generalizability of our study results. Secondly, the incidence of GBS-EOD may have been underestimated because we did not follow the infants who were discharged from hospital before 7 days. Lastly, due to budget constraints, the study utilized a culture-based testing procedure which is less expensive but not as sensitive as the PCR test.

Our study revealed that maternal age as well as ethnicity may play a role in maternal colonization of GBS. Maternal colonization with GBS, longer duration of membrane rupture and labor were significant risk factors for GBS colonization in Chinese newborns. Outcomes of infants colonized with GBS included fever, increased need for antibiotic, NICU admission as well as longer LOS. Future studies are needed to establish guidelines for GBS screening and Intrapartum Antibiotic Prophylaxis use in China. Prospective studies are also needed to evaluate the GBS disease burden in Chinese mothers and infants nationwide.