Streptococcus agalactiae in pregnant women in Zimbabwe: Epidemiology and serotype marker characteristics
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Background Streptococcus agalactiae (group B streptococcus, GBS) is one of the leading causes of serious bacterial infections such as septicaemia, pneumonia and meningitis in newborns, causing 0.5 to more than 2 cases per 1,000 live births. Children may be infected from their mothers during delivery from GBS colonizing the birth canal. GBS is also an important cause of premature delivery and may cause infections in parturient women. Significant differences in colonization rates have been shown among women of different ethnic groups or of the same ethnic group living in different parts of the same country. Knowledge obtained in Western countries related to maternal and neonatal colonization rates, incidence of disease and other GBS epidemiological data is based on detailed information that often includes serotyping and genotyping. Less data is available from African countries such as Zimbabwe. GBS colonization may have additional potential for adverse clinical outcome in sub-Saharan Africa since pregnant women in this region of the world are heavily burdened by co-infection with the human immunodeficiency virus. Aims The main aim of the study was to investigate the epidemiology of GBS colonization among pregnant women and neonates in Zimbabwe, and to characterize GBS isolates by molecular and immunological methods. In addition, the aim was to study co-infection between GBS, HIV-1 and Hepatitis B virus (HBV) in the study population. Materials and methods Pregnant women were recruited from three different communities in Zimbabwe, one urban, one rural-urban and one rural community, at 20 weeks gestation. Obstetric data were collected using questionnaires and hospital records. Rectal and vaginal swabs were collected for culture of GBS. Serum was collected for HIV-1 and HBV testing. The women were followed up at 26 weeks and at delivery with repeat collection of samples for GBS culture. Swabs for surface cultures were collected from the newborn babies. GBS surface polysaccharide and protein antigens were characterized by immunological techniques, and/or by PCR of encoding genes, in selected GBS isolates from the study. The value of GBS culture in early pregnancy for prediction of colonization status at delivery and in the newborn, as well as association with adverse perinatal factors, and coinfection with HIV-1 and HBV in pregnant women were analysed. Results Among 1037 women recruited at 20 weeks gestation and followed up at 26 weeks and at delivery, GBS culture results were obtained on one or more occasions for 780 women (75.2%). Altogether 470/780 (60.3%) women tested positive for GBS at least once; 47.0% at 20 weeks gestation, 24.2% at 26 weeks and 21.0% at the time of delivery. Colonization rates were significantly higher among women living in rural compared to urban communities (p<0.001). GBS colonization persistence, defined as positive GBS culture result at all three tests, was detected in 3.8% of the women. Positive GBS culture at 20 and 26 weeks gestation had low positive predictive values on colonization at delivery and in the newborn. Most socio-economic, demographic and obstetric factors analyzed were not significantly associated with GBS colonization. The GBS transmission rate from mother colonized by GBS at delivery to the baby was 61.7%. The HIV-1 and HBV prevalence rates among the pregnant women were 20.1% and 3.3%, respectively. Co-infection of GBS, HIV-1 and HBV was registered in 0.3% of the women. There was no association between GBS colonization and HIV-1 or HBV infection. The 121 GBS strains that were selected for further analysis were of capsular polysaccharide types (CPS) Ia, Ib, II, III, V, and two strains were nontypeable. The distribution of GBS CPS types was similar to that reported from Western countries. However, in comparison to other areas, the gene alp3 encoding the protein Alp3 occurred less frequently in Zimbabwean CPS type V isolates. Using polyclonal rabbit anti-R3 serum (R3 PAb) and monoclonal anti-R3 serum (R3 MAb) for analysis of the R3 protein, a putative novel strain-variable protein antigen was identified and was designated Z. Z was expressed by many CPS type V strains and was a high-molecular-mass antigen susceptible to degradation by pepsin and trypsin, but resistant to m-periodate oxidation. Conclusions The rate of GBS colonization was high among pregnant women and neonates in Zimbabwe. GBS colonization and persistence were significantly more common in women from rural than urban communities. GBS testing by culture in early pregnancy had a low predictive value for colonization at delivery and in the newborn. Most risk factors studied were not associated with GBS colonization. There was a very low frequency of co-infection with GBS, HIV-1 and HBV in pregnant Zimbabwean women. Zimbabwean CPS type V strains showed distinctive features with respect to the surface protein markers Alp3 and R3, and the surface protein antigens R3 and Z were found to be common serovariant markers in Zimbabwean GBS.