Prevalence of maternal group B streptococcus colonization and vertical transmission in low-risk women in a single institute

Abstract

Background: 

Intrapartum administration of antibiotics lowers the risk of neonatal group B streptococcus (GBS) infection based on recommended guidelines (a risk-based approach and a culture-based screening approach). However, many pregnant women do not undergo culture-based screening, nor treatment with prophylactic antibiotics after a GBS risk-based approach. Therefore, the value of GBS detection in asymptomatic low-risk pregnant women is controversial.

Methods: 

A cohort study of 354 asymptomatic pregnant women at more than 37 weeks’ gestation who were planning to undergo vaginal delivery, and 118 neonates (107 paired samples of both mother and newborn), was conducted to evaluate the GBS colonization rate of these pregnant women and the subsequent vertical transmission rate, using a culture method.

Results: 

The positive rate for GBS culture was 6.2% (22/354). Among 107 paired samples, six maternal samples and one neonatal sample were positive for GBS culture, with an estimated vertical transmission rate of 16.7% (1/6).

Conclusion: 

Although the positive rate of GBS culture was lower in asymptomatic low-risk pregnant women, the possibility of vertical transmission might be high. This finding is worthy of further investigation.

1. Introduction

Streptococcus agalactiae, known as group B streptococcus (GBS), is the leading cause of neonatal morbidity and mortality.¹ The gastrointestinal tract, especially the rectum, is considered the major reservoir for the colonized vagina.^2,3 Vertical transmission from the colonized mother to the newborn is the main route of infection, including ascending intra-uterine and intrapartum infection, especially during vaginal deliveries.⁴

Intrapartum administration of antibiotics has lowered the risk of neonatal GBS infection, especially early-onset diseases, and significantly continued the improvement of prenatal outcomes.^5–7 Therefore, the Centers for Disease Control and Prevention (CDC) issued recommendations for intrapartum prophylaxis to prevent perinatal GBS in 2010, as did the American College of Obstetricians and Gynecologists, the American Academy of Pediatrics, the American College of Nurse-Midwives, the American Academy of Family Physicians, and the American Society for Microbiology.⁸

The early guidelines recommended two prevention methods: a risk-based approach and a culture-based screening approach.⁹ The risk-based approach recommended that women who have obstetric risk factors, including delivery at <37 weeks’ gestation, duration of membrane rupture >18 hours, intrapartum temperature >100.4°F (>38.0°C), or intrapartum nucleic acid amplification tests positive for GBS, should receive screening and be administered intrapartum antibiotics based on the culture result.⁸ The culture-based screening approach recommends screening of all pregnant women for vaginal and rectal GBS colonization at between 35 and 37 weeks of gestation. Colonized women will be administered intrapartum antibiotics at the time of labor.

Although few reports have retrospectively revealed the incidence of neonatal GBS infection in Taiwan,^10–16 nearly all studies reported on the pediatric population. In addition, pregnant women with spontaneous vaginal delivery at term (>37 gestational weeks) within 18 hours after rupture of membrane (a low-risk population) might not fit the recommended guideline based either on risk or on culture. Actually, the intrapartum risk factors applied to circumstances in which screening results were not available at the time of labor and delivery. To clarify the GBS colonization and subsequent effect on newborns on these pregnant women in a low-risk population, the current study was conducted to estimate the GBS colonization rate of these women and calculate the subsequent effect on their newborns in a tertiary hospital in Taiwan.

2. Methods

2.1. Patients

Between September 2005 and November 2006, 472 samples, including 354 maternal and 109 neonatal samples (including two twins) were collected. Maternal samples were collected from the vagina before preparing for vaginal delivery. Vaginal delivery should occur in women with a “term pregnancy” (i.e., at more than 37 weeks of gestation). Neonatal samples were collected from the oral cavity immediately after vaginal birth. The study was designed with two groups; one was a screening-based method group (only maternal samples obtained) and the other was the vertical transmission screening group (both maternal and neonatal samples obtained).

In the vertical transmission screening group, only 107 pairs were analyzed, including 107 maternal and 109 neonatal samples, because some paired samples and some maternal samples without a neonatal sample were excluded. The reasons for exclusion were: (1) samples collected after rupture of the membrane; (2) mothers were treated with prophylactic antibiotics; or (3) cesarean section was performed for obstetric reasons. The hospital’s ethics committee approved the study, and informed consent was obtained from all patients.

2.2. GBS culture methods

A cotton swab was taken from the women’s upper vagina during the pelvic examination using a speculum, and a neonatal sample was taken from the secretion of the oral cavity. Then the cotton swabs were sealed and transported to a dedicated laboratory for GBS cultures. The swab for the GBS culture was inoculated onto trypticase soy agar containing 5% sheep blood and incubated with 5% CO₂ for 48 hours at 35°C.^17,18 Species identification of β-hemolytic colonies was performed by agglutination with a streptococcal grouping kit (Prolex Streptococcal Grouping Latex Kit; Pro-Lab Diagnostics, Richmond Hill, Ontario, Canada).¹⁸ The procedures were performed according to the manufacturer’s instructions. The final result of GBS infection was reported by the performing specialist.

3. Results

The mean age of the pregnant women was 28 ± 5 years, and the mean gestational age was 38 ± 1 weeks. Twenty-two of the 354 maternal vaginal samples were positive for GBS culture, with an estimated colonization rate of 6.2% (22/354). Among the vertical transmission screening group, only 107 paired samples (73.3%) were obtained, including 107 maternal samples and 109 neonatal samples (two twins). Among these paired samples, six maternal samples and one neonatal sample were positive for GBS culture, with an estimated vertical transmission rate of 16.7% (1/6) (Fig. 1). The neonate was found to have positive GBS culture, but was asymptomatic and was not treated with antibiotics. No newborns were diagnosed as having either early-onset (within 7 days) or late-onset GBS infection during the follow-up of 6 months.

Fig. 1: Flow chart showing assignment and results of the study subjects.

4. Discussion

Group B streptococcal infection is the main cause of neonatal morbidity and mortality. The mortality of reported neonatal cases reached nearly 50% in the 1990s.¹⁹ The prevalence of maternal colonization varied from 10% to 30%, depending on the regions of the world.⁸ Reports from Taiwan showed prevalence ranging from 4% to 21%, depending on the culture sites.^12,14–16 Although the formal announcement and routine clinical suggestion of the use of an intrapartum screening policy and intrapartum antibiotic prophylaxis are still under detabte,²⁰ competing strategies (risk-based vs. culture-based) for identifying chemoprophylaxis candidates have posed a challenge to implementation of antibiotic prophylaxis.²¹ In a risk-based strategy as an alternative to culture-based screening approaches, prematurity (<37 weeks of gestation), intrapartum fever [>100.4°F (>38.0°C)] and longer than 18 hours of ruptured membranes have been used as clinical indications for intrapartum antibiotic prophylaxis.²¹

In this study, the estimated prevalence of maternal colonization was only 6.21%, which was significantly lower than in other reports (ranging from 10–30%).^{8,12–16,22,23} The reasons are unclear, but contributory factors might include the specimen collection and culture medium. Some reports show different positive culture rates in the different culture media,^2,24,25 and in the different areas of specimen collection.^26–28

This study addressed the important issue of GBS colonization rates between mothers (6.21%) and their offspring (vertical transmission rate: 16.7%). The one infant with a positive GBS culture fortunately did not suffer from either early-onset or late-onset disease during the study period; however, the consequences of GBS infection in this newborn should not be overlooked, because the occurrence rate of an early onset of GBS disease in offspring in Taiwan ranged from 0.11 to 1.8 per 1000 live births, which contributed to 13–14% mortality rates.^10,15 Furthermore, up to three-quarters of pregnant women (72.7%) with positive GBS culture did not have a paired neonatal sample. That is to say, the data presented here evaluated only a relatively small study population. Finally, the majority of women enrolled into the current study had finished their delivery within 12 hours (data not shown) and the interval between rupture of the membrane and fetus delivery was less than 12 hours in the vertical transmission screening group.

Recent studies in the USA and Europe revealed that universal screening of pregnant women at 35–37 weeks’ gestation can reduce early-onset GBS disease in infants, but not late-onset disease.^29,30 In addition, even after screening, more than 60% of the term infants with GBS disease were born to women who had tested negative for GBS before delivery.¹ A recent review found that up to 6% of GBS carriers remained undetected in antenatal cultures.^2,31 Therefore, intrapartum antibiotic prophylaxis could be an alternative, although concerns about allergy or resistance to antibiotics for GBS were present.³² A recent study from France supported the current policy of GBS maternal prophylaxis, since it is not associated with an excessive risk of pathogen resistance.³³ Kaambwa et al tried to determine the cost-effectiveness of alternative screening and prevention strategies, including rapid intrapartum testing, for the prevention of early-onset neonatal GBS infection in the UK.³⁴ The authors found that the most cost-effective strategy was the provision of routine intrapartum antibiotic prophylaxis to all women without prior screening, but they also agreed that this was unlikely to be acceptable, given the broader concerns related to antibiotic use.³⁴ A 2009 Cochrane review critiqued the effectiveness of intrapartum antibiotic prophylaxis to reduce early-onset neonatal GBS infection.³⁵

Based on the potential risk of vertical transmission of GBS, routine intrapartum antibiotic prophylaxis might be more feasible. The discrepancy between the low prevalence of maternal GBS infection in low-risk asymptomatic women and the high vertical transmission rate is still unresolved. Large sample sizes and expanded eligibility criteria will be needed to clarify the limitations. Unfortunately, the opportunity to conduct such trials has likely been lost, as practice guidelines (albeit without good evidence) have been introduced in many jurisdictions, as mentioned by Ohlsson and Shah in 2009.³⁵

Acknowledgments

This work was supported in part by grants from Taipei Veterans General Hospital (V100E4 and V100C-054) and the National Science Council (NSC 99-2314-B-010 - 009 - MY3), Taiwan, R.O.C.