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Original Studies

Maternal carriage of group B streptococci in developing countries

STOLL, BARBARA J. MD; SCHUCHAT, ANNE MD

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The Pediatric Infectious Disease Journal: June 1998 - Volume 17 - Issue 6 - p 499-503
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Abstract

INTRODUCTION

The global burden of neonatal disease caused by the group B streptococcus (GBS) is difficult to estimate because the importance of GBS varies around the world.1-4 Although GBS continues to be an important pathogen associated with neonatal sepsis in many developed countries,4-8 studies from developing countries suggest that GBS is a much less important cause of neonatal sepsis.1-4 In a review of 36 hospital-based studies of bacterial pathogens associated with neonatal sepsis in various geographic regions of the world, Stoll1 reported that the proportion of sepsis caused by GBS detected in hospitalized newborns ranged from 0 to 5% for India/Pakistan/Southeast Asia, 0 to 24% for North Africa/Middle East, 0 to 30% for Sub-Saharan Africa and 2 to 35% for the Americas/Caribbean. Gramnegative organisms were isolated more frequently than Gram-positive organisms in the majority of studies reviewed. Most studies made no distinction between early and late onset sepsis. In the United States important differences in colonization rates have been identified among women of different ethnic groups that appear to correlate with infection rates in newborns.5 In an attempt to understand the lower rates of invasive GBS disease among neonates in many developing countries, we reviewed 34 studies that evaluated GBS colonization rates in pregnant women.

METHODS

Studies evaluated for this paper were identified by computer searches of the scientific literature published between 1980 and 1996. Electronic databases used included Medline Express, 1980 to 1996 (National Library of Medicine, National Institutes of Health) and Abstracts on Tropical Agriculture and Rural Development in the Tropics, 1975 to 1995 (Royal Tropical Institute, Amsterdam, the Netherlands). Additional references were found by reviewing the references cited in all relevant articles. Studies included in the review were performed in developing countries but were published in English, Chinese or French (all abstracts in English).

Specimen collection and laboratory methods were noted for each study. Methods were classified as adequate if specimen collection sites included the vagina and if samples were inoculated into selective broth media. Proportions were compared using the chi square test.

RESULTS

We reviewed the data on female genital colonization with GBS in 34 reports (Table 1).9-43 These studies present data on 7730 women from 23 countries. The majority of women cultured were pregnant. Only 7 of the studies presented data on nonpregnant women.9,20,21,28,32,38,43 Studies varied in the detail with which culture methods were presented, particularly regarding the use of selective media and the number of sites cultured. Among all studies colonization rates ranged from 0 to 31%, with only 7 of the 34 studies reporting rates of 20% or greater (Table 1).

TABLE 1
TABLE 1

Analysis of data from all studies (regardless of methods) showed the regional carriage rates as follows: India/Pakistan, 9% (97 of 1114); Asia/Pacific, 8% (143 of 1867); Sub-Saharan Africa, 18% (249 of 1400); Middle East/North Africa, 17% (298 of 1787); Americas, 12% (196 of 1562). The overall carriage rate estimated from these studies was 12.7% (average of all data). However, analysis of data from only those studies that used selective broth media and cultured the vagina (among other sites) showed regional rates of: India/Pakistan, 12% (86 of 727); Asia/Pacific, 19% (32 of 168); Sub-Saharan Africa, 19% (219 of 1144); Middle East/North Africa, 22% (251 of 1144); Americas, 14% (87 of 618). The estimated overall carriage rate for these studies was 17.8%. A comparison of studies that did and did not use selective broth media revealed that the GBS carriage rate was significantly increased by the use of selective media (675 of 3801, 17.8% vs. 308 of 3929, 7.8%; relative risk, 2.3; 95% confidence interval, 2.0 to 2.6; P < 0.001; Fig. 1).

Fig. 1
Fig. 1

Thirteen studies (from nine countries) presented the serotype distribution of GBS isolates (Table 2). Although relatively few isolates were studied, serotype distribution appeared to vary among studies. In 7 of 13 studies serotype III was the most frequently identified serotype. In the recent Gambian26 and Peruvian studies,43 which used appropriate culture and serotyping methods, serotype V, a relatively newly recognized invasive serotype, was reported for 38% and 18% of isolates, respectively.

TABLE 2
TABLE 2

DISCUSSION

It is unclear why invasive neonatal GBS disease is reported less frequently from developing countries. The most important risk factor for invasive GBS disease in the neonate is exposure to the organism via the mother's genital tract. Other known risk factors include young maternal age, preterm birth, prolonged rupture of the membranes, maternal chorioamnionitis, exposure to a high inoculum of a virulent GBS strain and a low maternal serum concentration of antibody to the capsular polysaccharide of the colonizing GBS strain.44

In an attempt to understand the low rates of invasive GBS disease reported among neonates in many developing countries, we reviewed 34 studies that evaluated GBS colonization rates in women. Because of poor specimen collection and suboptimal laboratory techniques, some of these studies may have underestimated the true rate of maternal GBS colonization. It is well-known that use of selective media and culturing multiple sites (particularly culturing both the lower vagina and rectum) increase GBS isolation rates.45, 46 This review suggests that the rates of maternal GBS colonization may be underreported in many studies because of poor culture techniques and use of nonselective media. Nonetheless estimated developing country colonization rates (a composite of all country data reviewed) were similar to those reported for women in the United States and Europe.2, 3 The largest evaluation of GBS colonization among pregnant women in the United States, the Vaginal Infections and Prematurity Study,47 found GBS colonization rates of 21.2% for blacks, 20.9% for Hispanics and 13.7% for whites, values similar to those found in the studies reviewed in this article.

The distribution of GBS serotypes varied among studies. GBS serotype III, the most frequently identified invasive serotype in the West, was also identified in all studies reviewed and was the most frequently identified serotype in one-half of the studies. Serotype V, which has only been recognized as a cause of invasive disease in developed countries since the early 1990s,48 was identified in recent studies from Peru and the Gambia. Monitoring serotype distribution is important as candidate GBS vaccines are considered for areas with high rates of disease.

With estimated colonization rates among women in developing countries as high as 18%, one would expect higher rates of invasive neonatal disease than have been reported.1 Low rates of invasive GBS disease in some developing countries may be explained by less virulent strains, by genetic differences in susceptibility to disease, by as yet unidentified beneficial cultural practices or by high concentrations of transplacentally acquired protective antibody in serum (i.e. mother colonized but has protective concentrations of type-specific GBS antibody).

Hospital-based surveillance in developing countries may be insensitive for detecting sepsis in very young infants. In the United States ∼80% of infants with early onset GBS disease become ill within 24 h of birth.5 In developing countries, where most deliveries occur at home, infants with early onset sepsis often get sick and die at home or are taken to local health facilities where a diagnosis of possible sepsis may be missed (i.e. underrecognition) or where blood cultures cannot be performed. In this setting there may be underdiagnosis of early onset pathogens, including GBS. Hospital-based studies, which present data on organisms associated with neonatal sepsis, may be reporting results for late onset, as well as early onset cases.1

Increasing evidence suggests that heavy colonization with GBS increases the risk of delivering a preterm low birth weight infant.49 Population differences in the prevalence of heavy GBS colonization have been reported in the United States, where African-Americans have a significantly higher risk of heavy colonization.50 However, studies reviewed from developing countries did not report quantitative maternal genital colonization. If heavy colonization is more prevalent among women in developing countries and results in an increase in preterm low birth weight infants, GBS-related morbidity may appear as illness and death related to prematurity and neonatal GBS disease may be masked. By contrast heavy colonization could increase maternal type-specific GBS antibody concentrations, resulting in lower risk of neonatal disease. Further studies in developing countries are needed to explore these important issues.

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Keywords:

Group B Streptococcus; maternal colonization; neonatal sepsis

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