Currently, the only reliable way to prevent GBS-EOD is the administration of intrapartum
intravenous (IV) antibiotics before delivery. That is to say, intrapartum
antibiotic prophylaxis (IAP). IAP interrupts vertical transmission of GBS from the mother to the newborn and decreases the incidence of GBS-EOD. Administration of
intravenous (IV) antibiotics during labor. Intravenous
penicillin or
ampicillin given to GBS-colonized women at the onset of labor and then again every four hours until delivery have been proven to be very effective at preventing vertical transmission of GBS from mother to baby and GBS-EOD. Penicillin G, 5 million units IV initial dose, then 3 million units every 4 hours until delivery or ampicillin, 2 g IV initial dose, then 1 g IV every 4 hours until delivery. Penicillin-
allergic women without a history of
anaphylaxis (
angioedema,
respiratory distress, or
urticaria) following administration of a penicillin or a
cephalosporin (low risk of anaphylaxis) could receive
cefazolin (2 g IV initial dose, then 1 g IV every 8 hours until delivery) instead of penicillin or ampicillin. In women at high risk of anaphylaxis to penicillin, the use of
Erythromycin is not recommended today because the high proportion of GBS resistance to erythromycin (up to 44.8%). Penicillin administered to a woman with no history of
β-lactam allergy has a risk of anaphylaxis of 0.04 to 4 per 100,000. Maternal anaphylaxis associated with GBS IAP occurs, but any morbidity associated with anaphylaxis is offset greatly by reductions in the incidence of GBS-EOD. Neither oral nor intramuscular antibiotics are effective in reducing the risk GBS-EOD. Nevertheless, at present, there is no suitable approach for the prevention of late-onset GBS neonatal disease.
Identifying candidates to receive IAP Two ways are used to select female candidates to IAP: the culture-based screening approach and the risk-based approach. The culture-based screening approach identifies candidates using lower vaginal and rectal cultures obtained between 35 and 37 weeks of gestation (or 36–37), IAP is also recommended for women with intrapartum risk factors if their GBS carrier status is not known at the time of delivery, for women with GBS bacteriuria (in any colony count) during their pregnancy and for women who have had an infant with GBS-EOD previously. The risk-based approach is, in general, less effective than the culture-based approach, because in most cases, GBS-EOD develops among newborns who have been born to mothers without risk factors. IAP is not required for women undergoing planned cesarean section in the absence of labour and with intact membranes, irrespective of the known GBS carriage status. ACOG recommendations state, Routine screening of pregnant women is performed in most developed countries such as the United States, France, Spain, Belgium and Canada, and data have shown falling incidences of GBS-EOD following the introduction of screening-based measures to prevent GBS-EOD. The risk-based strategy is advocated, among other countries, in the United Kingdom and the Netherland. Other evaluations have also found the culture-based approach to be more cost-effective than the risk-based approach for the prevention of GBS-EOD. It has been reported that IAP does not prevent all cases of GBS-EOD; its efficacy is estimated at 80%. The risk-based prevention strategy does not prevent about 33% of cases with no risk factors. Up to 90% of cases of GBS-EOD would be preventable if IAP were offered to all GBS carriers identified by universal screening late in pregnancy, plus to the mothers in higher risk situations. Where insufficient intravenous antibiotics are given before delivery, the baby may be given antibiotics immediately after birth, although evidence is inconclusive as to whether this practice is effective or not.
Screening for GBS colonization in pregnant women Approximately 10–30% of women are colonized with GBS during pregnancy. Nevertheless, during pregnancy, colonization can be temporary, intermittent, or continual. In contrast, if the prenatal culture is carried out more than 5 weeks before delivery, it is unreliable for accurately predicting the GBS carrier status at delivery. Because of that, testing for GBS colonization in pregnant women is today recommended by the ACOG at 36–37 weeks of gestation. It is important to note that the
ACOG now recommends performing universal GBS screening between 36 and 37 weeks of gestation instead of at 35–37 as previously recommended by the CDC. This new recommendation provides a 5-week window for valid culture results that includes births that occur up to a gestational age of at least 41 weeks. These swabs should be placed into a non-nutritive transport medium.
Culture methods to detect GBS colonization in pregnant women Samples (vaginal, rectal, or vaginorectal swabs) should be inoculated into a selective enrichment broth, (
Todd Hewitt broth with selective antibiotics), enrichment culture. This involves growing the samples in a selective enriched broth medium to improve the viability of the GBS and simultaneously impairing the growth of other naturally occurring bacteria. Appropriate enrichment broths, commercially available, are Todd-Hewitt with gentamicin and nalidixic acid (Baker broth), or with colistin and nalidixic acid (Lim broth). After incubation the enrichment broth can also be subcultured to
granada medium agar where GBS grows as pinkish-red colonies, and further identification tests are not required. After incubation the enrichment broth can also be subcultured to chromogenic agars, where GBS grows as coloured colonies. This test is also available privately from around £35 per test for a home-testing pack, and it is offered by private clinics.
Point-of-care testing, POCT No current culture-based test is accurate enough and fast enough to be recommended for detecting GBS once labour starts. Plating of swab samples requires time for the bacteria to grow, meaning that this is unsuitable to be used as an intrapartum
point-of-care test (POCT or bedside testing). Testing women for GBS colonization using vaginal or rectal swabs and culturing them in an enriched media is not as rapid as a PCR test that would check whether the pregnant woman is carrying GBS at delivery. NAAT tests would allow starting IAP on admission to the labour ward in those women for whom it is not known if they are GBS carriers. POCT for detection of GBS carriers requires additionally that maternity units should provide 24/7 laboratory means required to perform rapid testing. However, point-of-care testing may be used for women who present in labor with an unknown GBS status and without risk factors for ascertaining the use of IAP. Therefore, treatment of these asymptomatic cases of bacteriuria with antibiotics at the time of diagnosis is just as important as treating symptomatic UTIs in pregnancy in order to reduce these risks. In addition to cases of GBS UTI and asymptomatic GBS bacteriuria with high CFU/mL, pregnant women with asymptomatic GBS bacteriuria, even with low CFU/mL counts at any time during the pregnancy, should receive IAP to protect the newborn; regardless of the results of the recto-vaginal screen later in pregnancy. This is because GBS bacteriuria, even asymptomatic, at any CFU/mL is an indication of heavy ano-genital colonization.
Missed opportunities for prevention of GBS neonatal infections The important factors for successful prevention of GBS-EOD using IAP and the universal screening approach are: • Proper sample collection • Using an appropriate procedure for detecting GBS • Administering a correct IAP to GBS carriers • Reach most pregnant women for antenatal screens Most cases of GBS-EOD occur in term infants born to mothers who screened negative for GBS coloniztion and in preterm infants born to mothers who were not screened, though some false-negative results observed in the GBS screening tests can be due to the test limitations, and to the acquisition of GBS between the time of screening and delivery. This shows that improvements in specimen collection and processing methods for detecting GBS are still necessary in some settings. False-negative screening test, along with failure to receive IAP in women delivering preterm with unknown GBS colonization status, and the administration of inappropriate IAP agents to penicillin-allergic women account for most missed opportunities for prevention of cases of GBS-EOD.
Home births and water birth Home births are becoming increasingly popular in the UK and elsewhere. Recommendations for preventing GBS infections in newborns are the same for home births as for hospital births. Around 25% of women having home births probably carry GBS in their vaginas at delivery without knowing, and it could be difficult to follow correctly the recommendations of IAP and to deal with the risk of a severe allergic reaction to the antibiotics outside of a hospital setting. The RCOG and the ACOG guidelines suggest that birth in a pool is not contraindicated for GBS carriers who have been offered the appropriate IAP if no other contraindications to water immersion are present ==Epidemiology of GBS neonatal infection==