The magnitude and correlates of Parvovirus B19 infection among pregnant women attending antenatal clinics in Mwanza, Tanzania

Human Parvovirus B19 also known as B19 is a single stranded DNA virus commonly responsible for hydrops fetalis, intrauterine fetal death, aplastic crisis, spontaneous abortion, acute symmetric polyarthropathy and erythema infectiosum (5th disease) [1‐6]. A fetus is more susceptible to B19 infection during the first and second trimester of the pregnancy which coincides with the development of the erythroid precursors [7]. The decrease in the incidence of fetal loss in the third trimester is due to the fetal immune response to the virus [8]. The B19 is commonly transmitted through respiratory secretions, hand to mouth contact, blood transfusion and trans-placental transfer [9].

The magnitude of B19 have been studied in many developed countries [10, 11] whereby the prevalence of specific B19 antibodies among pregnant women has been found to range from 1 to 5% with transmission rate to the fetus of about 17–33% [12]. High transmission rates have been reported to occur during late spring to summer season [13], with other studies reporting the highest peaks during late winter to spring [14]. Previous studies suggest that 50–65% of the women develop natural immunity against B19 [15]. In developing countries; the epidemiology of B19 particularly in pregnant women is not well documented. Seroprevalence of IgM among pregnant women has been found to range between 3.3% in South Africa and 13.2% in Nigeria [16] while that of IgG was found to range from 24.9% in South Africa to 58.4% in Malawi [17].

Tanzania is among African countries where the magnitude of B19 is not known. No report on B19 infection is available in Tanzania therefore; this study for the first time in Tanzania provides the baseline information regarding the magnitude of this infection among pregnant women attending antenatal clinics.

The study was conducted in the city of Mwanza between December 2014 and June 2015. Sample size was calculated by Kish Leslie formula using a prevalence of 20% [18] at 95% confidence interval. The study included pregnant women at different gestation ages attending Makongoro (urban) and Karume (rural) antenatal clinics. The distance from urban to rural clinic is about 20 km. All pregnant women whose gestation ages were unknown were excluded from the study. A structured data collection tool was used to collect socio-demographic and obstetric characteristics. Gestational ages were extrapolated from the last normal menstrual period using pregnancy calculator.

A total of 258 pregnant women were enrolled into the study. The median age was 21 (IQR: 19–25) years. Out of 258 women 116(45.0%) were in the first trimester, 109(42.3%) in second trimester and 33(12.8%) in third trimester. A total of 168(66.4%) women represented the urban population. Only 253(98%) sera were available for specific parvovirus B19 IgM antibodies testing. The seroprevalence of IgM and IgG B19 antibodies were 83/253(32.8%) and 142/258(56.1%) respectively. Out 253 women, 79(30.6%) were negative for both IgG and IgM B19 antibodies while 50(19.4%) were positive for both IgG and IgM, giving the possibility that the true IgM positive was about 19.4%. Pregnant women in their first and second trimester had significantly higher IgG seropositivity rates than those in the third trimester (Fig. 1).

Out of 85 women from rural areas, 35(41.2%) were B19 IgM seropositive compared to 48(28.6%) of 168 women from urban areas (P = 0.045). On multivariate logistic regression analysis none of the factor was found to independently predict B19 IgM seropositivity (Table 1).

Of 234 pregnant women with no history of low birth weight baby; 121(51.7%) were IgG seropositive compared to 21(87.5%) of those with history of low birth weight baby (OR 6.5, 95% CI; 1.9–22.5), p = 0.003. Being at the third trimester (OR 0.38, 95% CI 0.16–0.92, p = 0.030) and a history of baby with low birth weight (OR 10, 95% CI 1.82–58.05, p = 0.010) were found to be associated with B19 IgG seropositivity on multivariate logistic regression analysis (Table 2).

To the best of our knowledge this is the first report on the magnitude of parvovirus B19 infection among pregnant women in Tanzania. The most important findings in this study was that more than half of pregnant women were B19 IgG sero-positive with more than a third of them being B19 IgM sero-positive indicating recent infections. The overall prevalence of both IgM and IgG antibodies which signifies true B19 recent infections was found to be slightly higher compared to many other studies [16‐20]. A slightly higher rate in this study could be due to variations in the geographical location, weather and season. The current study was conducted between December and June which is predominantly a rainy season associated with cold weather; all these factors have been found to influence transmission of B19 virus [13].

The high B19 IgM seropositivity found in this population could be due to false positive IgM results. In the current study, B19 IgM positive samples were not confirmed by IgM μ-capture ELISA or by specific B19 polymerase chain reaction (PCR) assay due to limited resources. Samples which were IgM positive and IgG negative, may suggest false-positive IgM reactivity [21]. Therefore, the true IgM positive results in this study could be 19.4% which is slightly higher compared to previous studies as detailed above. The other limitation was failure to collect additional information like history of fever and rashes which could have justified the high prevalence in case of possible outbreaks.

The B19 IgM sero-positivity observed in our study indicates the recent infections in this vulnerable population with the possibility of the adverse pregnancy outcome. This could further be explained by the fact that, in the current study women with history of a baby with low birth weight were more likely to be B19 IgG sero-positive. Further studies to investigate the role of maternal B19 infections in abortion, intra-uterine fetal death and other complications including premature labor and low birth weight are warranted in developing countries.

The majority of women in the present study were IgG sero-positive, our results are within 95% confidence interval of the magnitude of natural immunity reported in Libya, Malawi, Tunisia and Kuwait [16, 20, 22, 23]. However; the IgG sero-prevalence obtained in this study was higher than 24.9% which was observed in South Africa [19]. Our data suggest a relatively high transmission rate of B19 which could be explained by geographical variations. It should be noted that 30.6% of pregnant women in the current study were susceptible for acute parvovirus B19 infections hence at risk of the adverse pregnancy outcomes underscoring the routine screening of this virus during pregnancy.

In the present study; as gestational age increases the odds of being B19 IgG seropositive was found to decrease significantly. The decrease in B19 IgG seropositivity with gestation age observed in the current study might be due to pregnancy hemodilution as previously observed [24‐26]. The effect of hemodilution was further supported by the observation that IgG titres were found to significantly decrease as gestational increases. However; low sample in women in third trimester could contribute to the significant differences as selection bias. As in the previous study in Brazil [27], there were no association between B19 IgG seropositivity rates and rural or urban location.

A significant proportion of pregnant women in Mwanza city are B19 IgG seropositive. A history of baby with low birth weight was independent predictor of B19 IgG sero-positivity while being at the third trimester had a protective effect for being IgG seropositive. Further studies should be done to explore the impact of B19 infections in developing countries. This information may influence policy makers to consider the need for routinely B19 screening among pregnant women.