What is Group B Streptococcus (GBS) prophylaxis?

What is GBS Prophylaxis?

GBS prophylaxis is the administration of intravenous antibiotics during labor to pregnant women who are colonized with Group B Streptococcus or have specific risk factors, aimed at preventing life-threatening early-onset neonatal infections that can cause meningitis, sepsis, and death. 1

Purpose and Rationale

Group B Streptococcus (GBS) emerged as the leading cause of serious bacterial infection in newborns in the United States since the 1970s, causing bacteremia, pneumonia, and meningitis with case-fatality rates of 5-20% in newborns. 1 Infants who survive may suffer permanent disabilities including hearing loss, visual impairment, or mental retardation. 1

The primary route of neonatal infection is vertical transmission from the mother's vagina during delivery, as approximately 25% of pregnant women are colonized with GBS in their genital or rectal areas. 1 Without intervention, about one-third of infants born to colonized mothers become colonized themselves, and approximately 3% of these colonized newborns develop serious early-onset infection within the first 7 days of life. 2

Who Receives GBS Prophylaxis

The CDC recommends intravenous antibiotics during labor for women with: 3

• Positive GBS screening culture at 35-37 weeks' gestation 1
• GBS bacteriuria at any concentration during the current pregnancy 3, 4
• Previous infant with invasive GBS disease 3
• Unknown GBS status at term with membrane rupture >18 hours 3
• Unknown GBS status at term with intrapartum fever ≥100.4°F (≥38.0°C) 1
• Preterm labor or rupture of membranes (<37 weeks) with unknown or positive GBS status 1, 3

Women do NOT need GBS prophylaxis if: 1

• Vaginal-rectal screen within 5 weeks was negative 1
• Planned cesarean delivery before labor onset with intact membranes, regardless of colonization status 3

Screening Protocol

GBS colonization status should be determined by collecting both vaginal and rectal specimens at 35-37 weeks' gestation, with a single combined vaginal-rectal specimen being acceptable. 1 Specimens must undergo 1-4 hour incubation at 35°-37°C in appropriate enrichment broth medium to enhance GBS recovery. 1 The screening culture remains valid for 5 weeks. 1, 5

Antibiotic Regimens

First-line therapy: 3

• Penicillin G: 5 million units IV initially, then 2.5 million units IV every 4 hours until delivery 1, 3, 4
• Penicillin G is preferred due to its narrow antimicrobial spectrum, making it less likely to select for antibiotic-resistant organisms 1

Acceptable alternative: 1, 4

• Ampicillin: 2 g IV initially, then 1 g IV every 4 hours until delivery 1, 4

For penicillin-allergic patients not at high risk for anaphylaxis: 4

• Cefazolin: 2 g IV initially, then 1 g IV every 8 hours 4

For penicillin-allergic patients at high risk for anaphylaxis: 4

• Clindamycin: 900 mg IV every 8 hours (if isolate is susceptible) 4
• Vancomycin: 1 g IV every 12 hours (if clindamycin resistance or susceptibility unknown) 4

High-risk allergy features include history of anaphylaxis, angioedema, urticaria, or asthma that would make anaphylaxis more dangerous. 4 Approximately 10% of persons with penicillin allergy also have immediate hypersensitivity reactions to cephalosporins. 4

Timing and Effectiveness

Antibiotics must be administered at least 4 hours before delivery to achieve optimal prevention of vertical GBS transmission and early-onset neonatal disease. 1, 3, 6 Shorter durations of ≥2 hours may confer some protection, but efficacy is reduced. 1

When administered appropriately, intrapartum antibiotic prophylaxis reduces early-onset GBS disease by 86-89%. 3, 6 The incidence of invasive early-onset GBS disease decreased by more than 80% from 1.8 cases per 1,000 live births in the early 1990s to 0.26 cases per 1,000 live births in 2010, with an estimated 70,000 cases prevented in the United States between 1994 and 2010. 6

Special Situations

Preterm labor: Women admitted with signs and symptoms of preterm labor with unknown GBS status or positive GBS screen within 5 weeks should receive GBS prophylaxis immediately at hospital admission. 1, 4 If true labor is not confirmed, GBS prophylaxis should be discontinued. 1

Preterm premature rupture of membranes (PPROM): Antibiotics given for latency that include ampicillin 2 g IV once, followed by 1 g IV every 6 hours for at least 48 hours are adequate for both latency prolongation and GBS prophylaxis. 1, 5 If other regimens are used, GBS prophylaxis should be initiated in addition. 1 For women with PPROM who are not in labor, GBS prophylaxis should be discontinued at 48 hours unless they enter labor. 1

GBS bacteriuria: Any concentration of GBS in urine during pregnancy requires immediate treatment of the UTI and mandates intrapartum antibiotic prophylaxis during labor, regardless of whether the UTI was treated earlier in pregnancy. 4 This is because GBS bacteriuria indicates heavy genital tract colonization and significantly increases the risk of early-onset neonatal disease. 4

Critical Pitfalls to Avoid

Oral antibiotics should never be used for GBS prophylaxis or to treat GBS colonization before labor. 1, 4 Such treatment is ineffective in eliminating carriage, does not prevent neonatal disease, and may cause adverse consequences including antibiotic resistance. 1, 4

The greatest risk of penicillin prophylaxis is anaphylactic reaction, estimated at approximately 5 cases per 10,000 treatments, which can have severe consequences for both mother and child. 1, 2 An additional 0.7-10% of women may experience less severe reactions. 1

Widespread antimicrobial use increases the risk for emergence of antimicrobial-resistant organisms, though GBS isolates have not developed clinically important resistance to penicillin. 1 The greater threat is development of antimicrobial resistance in other peripartum pathogens. 1

Limitations

IAP does not prevent late-onset GBS disease (occurring after 7 days of life), as these infections do not appear linked to intrapartum colonization. 6, 2 Additionally, disease incidence among preterm infants remains twice that of term infants despite prophylaxis. 6

Practical implementation challenges exist: in audit studies, adequate prophylaxis (≥4 hours before delivery) was achieved in only 32-42% of women with risk factors, primarily due to difficulty providing adequate prophylaxis to women with intrapartum maternal pyrexia due to time constraints. 7