Streptococcus Group B Infections 

Group B Streptococcus, also known as Streptococcus agalactiae, was once considered a pathogen of only domestic animals, causing mastitis in cows. S agalactiae is now best known as a cause of postpartum infection and as the most common cause of neonatal sepsis. More recently, numerous series have described S agalactiae as a cause of infection in nonpregnant adults, providing descriptions of the clinical spectrum of disease, including clinical features, risk factors, therapy, and outcome of group B streptococcal infection in nonpregnant adults.

Group B streptococci colonize the vaginal and gastrointestinal tracts in healthy women, with carriage rates ranging from 15%-45%. Neonates can acquire the organism vertically in utero or during delivery from the maternal genital tract. Although the transmission rate from mothers colonized with S agalactiae to neonates delivered vaginally is approximately 50%, only 1-2% of colonized neonates go on to develop invasive group B streptococcal disease.^[1]

Group B streptococcal neonatal sepsis is rare, but it is more common in the setting of prematurity and prolonged rupture of the membranes. Because of the ubiquity of S agalactiae colonization in women and the rarity of group B streptococcal neonatal sepsis, prevention of the disease is difficult. Many pregnant women require treatment to prevent a single neonatal infection. Immunoprophylaxis and chemoprophylaxis have both been studied as solutions to this problem.

Neonatal group B streptococcal disease is divided into early and late disease. Early group B streptococcal neonatal sepsis often presents within 24 hours of delivery but can become apparent up to 7 days postpartum. No specific clinical features differentiate early group B streptococcal disease from that caused by other pathogens. Pneumonia with bacteremia is common, while meningitis is less likely.

Late group B streptococcal neonatal sepsis is defined as infection that presents between one week postpartum and age 3 months. Late disease commonly involves group B Streptococcus serotype III, typically characterized by bacteremia and meningitis.

The absence of antibody to group B streptococci in infants is a risk factor for infection. Because antibodies to group B streptococci provide protection against disease in animal models, there is an ongoing interest in vaccination as an approach for reducing the incidence of group B streptococcal colonization in healthy women. Vaccine development was once promising, but shifting serotypes of group B streptococci responsible for clinical disease have limited this approach. Other factors that have made this approach less attractive include problems related to access to vaccination by women of childbearing age and the emotion and possible litigation associated with vaccination during pregnancy.

The current approach to the prevention of group B streptococcal infection in pregnancy approach requires intrapartum antimicrobial prophylaxis in term women with culture evidence of recent vaginal or rectal group B streptococcal infection. This has become a national standard owing to efforts by the Centers for Disease Control and Prevention (CDC) in 1995.^[2] Women without a known group B streptococci status delivering before 37 weeks' gestation with premature rupture of the membranes or intrapartum fever are also candidates for intrapartum antimicrobial prophylaxis. Penicillin or ampicillin is the initial approach. Clindamycin and erythromycin are standard in individuals with penicillin allergy, but group B streptococci are no longer always sensitive to these two drugs.

Only in the last 3 decades has the role of group B streptococci as a serious pathogen in the nonpregnant adult been well defined. Numerous studies have allowed description of the clinical spectrum of disease, including clinical features, risk factors, therapy, and outcomes.

S agalactiae infection is extremely rare in healthy individuals and is almost always associated with underlying abnormalities. Among published series, diabetes mellitus and malignancy are consistently the most common underlying diseases associated with infection.^[3] Other conditions associated with group B streptococcal infection in adults include cardiovascular and genitourinary abnormalities, neurologic deficits, cirrhosis, steroids, AIDS, renal dysfunction, and peripheral vascular disease. Relapse is not uncommon, with approximately 5% of nonpregnant adults eventually experiencing a second episode of group B streptococcal disease.^[4]

Group B streptococcal infection in elderly people (≥70 y) is strongly linked to congestive heart failure and being bedridden, with urinary tract infection, pneumonia, and soft-tissue infection as the most common manifestations of infection. Neurologic illness is associated with pneumonia in elderly people, possibly due to aspiration of group B streptococci from the upper respiratory tract. Nosocomial group B streptococcal infection was common in this group and is described in other series. The source of this infection is not always clear, but the genitourinary tract and skin are thought to be the sources of some nosocomial infections.

Group B streptococci are found commonly in the gastrointestinal tract and have been found to colonize the urethra in both men and women without causing infection. Group B streptococci can also colonize the upper respiratory tract. Colonization also is observed in wound and soft tissue cultures in the absence of obvious infection. Determining the acquisition and transmission of S agalactiae can be puzzling, as it is very invasive but produces little inflammation at the entry site.

Primary group B streptococcal bacteremia without an obvious source is a common presentation in adults. While one series suggests that group B streptococcal bacteremia is low-grade and easily controlled with little morbidity, other authors suggest that the clinical presentation may be that of classic sepsis with shock and may carry a high mortality. Sustained bacteremia may indicate endocarditis or an infected catheter. Group B streptococci can cause acute destructive endocarditis, which may require emergency valve replacement.

Urinary tract infections are a common manifestation of group B streptococcal disease and are observed in both pregnant and nonpregnant adults. Other presentations of group B streptococcal infection include pneumonia, skin and soft-tissue infections, septic arthritis, osteomyelitis, meningitis, peritonitis, and endo-ophthalmitis.

Group B Streptococcus remains sensitive to penicillin and ampicillin and was once also sensitive to cefazolin, erythromycin, and clindamycin. Although penicillin is the treatment of choice, it is unclear whether penicillin therapy provides a better outcome than other antibiotics.

S agalactiae is a gram-positive coccus that, when cultured on sheep blood agar, forms glistening gray-white colonies with a narrow zone of beta hemolysis. It is an invasive encapsulated organism capable of producing severe disease in immunocompromised hosts. Group B streptococcal infection in the absence of associated comorbid medical conditions is rare.

The virulence is of S agalactiae is related to the polysaccharide toxin it produces. Immunity is mediated by antibodies to the capsular polysaccharide and is serotype-specific. Several serotypes are known—Ia, Ib, Ic, II, III, IV, V, VI, VII, and VIII.

Group B streptococci colonize the vagina, gastrointestinal tract, and the upper respiratory tract of healthy humans. The portal of entry is not apparent, but possible areas include the skin, genital tract, urinary tract, and respiratory tract.

Frequency

United States

Group B streptococcal neonatal sepsis occurs in 1.8-3.2 per 1000 live births. In 2005, early group B streptococcal neonatal sepsis was observed in 0.35 per 1000 births, while late sepsis was observed in 0.33 per 1000 births.^[5] The incidence of early disease has decreased over the past decade, likely because of the CDC guidelines for the prevention of neonatal colonization with group B streptococci.

While the incidence of group B streptococcal disease in neonates appears to be decreasing, the rate in nonpregnant adults appears to be increasing, with an overall increase of 32% between 1999 and 2005.^[5] A recently published study of surveillance data from 10 states found that the incidence of group B streptococcal infection in persons aged 15-64 years increased from 3.4 per 100,000 population in 1999 to 5 per 100,000 in 2005. In adults aged 65 years and older, the incidence increased from 21.5 per 100,000 population in 1999 to 26 per 100,000 in 2005.^[5]

International

The role of group B streptococci in the developing world is not well defined. Carriage rates and serotypes in women in underdeveloped countries are similar to those observed in the industrial world. However, for unknown reasons, early group B streptococcal disease in infants is not documented in less-developed countries.

Mortality/Morbidity

Group B streptococcal disease results in significant mortality in both neonates and adults. While the mortality rate ranges from 9-47% in published reports, most studies find it to be approximately 20%.^[4] The mortality rate is highest in elderly patients with comorbid medical conditions, and the manifestations most likely to result in death include endocarditis, meningitis, and pneumonia. The high mortality rate in elderly people with group B streptococcal infection may not reflect the organism itself but the predisposing condition or conditions that put the individual at risk for group B streptococcal infection.

The mortality rate of neonatal group B streptococcal infection is much less than that in nonpregnant adults. An increasing awareness of group B streptococcal infection in infants has led to improved outcomes in recent years.

Postpartum group B streptococcal infection is associated with a low mortality rate because the group at risk is composed of healthy young or middle-aged women.

Race

Group B streptococcal infection is more common in African Americans than in whites and is much more common in older African Americans than in older whites. These differences are probably due to socioeconomic differences rather than race.

Sex

• Young and middle-aged women who undergo obstetrical and gynecological manipulation are at an increased risk of group B streptococcal infection.
• Among nonpregnant patients, group B streptococcal infection has no sexual predilection.

Age

• The mean age for group B streptococcal infection is 64 years.
• A bimodal distribution is well recognized. Young and middle-aged healthy women with group B streptococcal infection secondary to obstetrical or gynecological manipulation is one group, while the second group is elderly persons with group B streptococcal infection as a complication of preexisting illness.