Open Access
Open Peer Review

This article has Open Peer Review reports available.

How does Open Peer Review work?

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

  • Mariam M. Mirambo1Email author,
  • Fatma Maliki1,
  • Mtebe Majigo2,
  • Martha F. Mushi1,
  • Nyambura Moremi1,
  • Jeremiah Seni1,
  • Dismas Matovelo3 and
  • Stephen E. Mshana1
BMC Pregnancy and ChildbirthBMC series – open, inclusive and trusted201717:176

https://doi.org/10.1186/s12884-017-1364-y

Received: 4 August 2015

Accepted: 31 May 2017

Published: 7 June 2017

Abstract

Background

Human parvovirus B19 (B19) infection has been associated with congenital infection which may result into a number of the adverse pregnancy outcomes. The epidemiology and the magnitude of B19 infections among pregnant women have been poorly studied in developing countries. This study was done to establish preliminary information about the magnitude of B19 among pregnant women attending antenatal clinics in the city of Mwanza, Tanzania.

Methods

A cross-sectional study was conducted between December 2014 and June 2015 among 258 pregnant women attending two antenatal clinics representing rural and urban areas in the city of Mwanza. Socio-demographic data were collected using structured data collection tool. Specific B19 IgM and IgG antibodies were determined using indirect enzyme linked immunosorbent assay kits (DRG Instruments GmbH, Germany). Data were analyzed using STATA version 11 software.

Results

The median age of study participants was 21 IQR (19–25) years. Of 253 pregnant women; 116(44.96%), 109(42.25%) and 33(12.79%) were in the first, second and third trimester respectively. The majority 168(66.4%) of women were from urban areas. Of 253 pregnant women, the overall prevalence of IgM was 83(32.8%) while that of IgG was 142(55.0%) among 258 women tested. A total of 50(19.4%) women were positive for both IgG and IgM indicating true IgM positive. History of baby with low birth weight (OR: 10, 95% CI: 1.82–58.05, P = 0.01) was independent predictor of B19 IgG seropositivity and being at the third trimester was protective (OR: 0.38, 95% CI: 0.16–0.92, P = 0.03). The IgG titers were found to decrease significantly as gestational age increases (Spearman’s rho = −0.2939, p = 0.0004)

Conclusion

More than a half of pregnant women in Mwanza city are B19 IgG sero-positive with about one third of these being B19 IgM seropositive. Further studies to determine the impact of B19 infections among pregnant women and their newborns are recommended in developing countries.

Keywords

Parvovirus B19 Pregnant women Tanzania

Background

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) [16]. 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.

Methods

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.

Specimen collection and laboratory procedures

About 4-5mls of blood sample were drawn from each participant and placed in a plain vacutainer tubes (BD, UK). Specimens were taken to the Catholic University and Health allied Sciences/Bugando Medical Centre (CUHAS/BMC) laboratory where sera were extracted and stored at −40 °C until further processing. Specific B19 IgG and IgM antibodies were detected by using commercial indirect enzyme linked immunosorbent assay (ELISA) kits (DRG Instruments GmbH, Germany). All procedures and interpretation were as per manufacturer’s instructions(http://www.drg-diagnostics.de/45-0-DRG+VirologieSerology+ELISAs.html?ItemPage=6).

Data analysis

The data were entered in the computer using excel software and analysed using STATA version 11(STATA Corp LP, USA). Categorical variables were summarized as proportions while continuous variables were summarized as median with interquartile ranges. Stepwise logistic regression model was done to determine predictors of B19 seropositivity among pregnant women whereby all factors with P value of less than 0.2 in univariate analysis were subjected were subjected into multivariate logistic regression analysis. In addition Kruskal-Wallis equality-of-populations rank test was done to compare the median IgG titres in different trimester followed by spearman correlation test to compare the correlation of the gestational age and IgG titres. Predictors with p-values of <0.05 at 95% confidence interval were considered statistically significant.

Results

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).
Fig. 1

Specific parvovirus B19 IgM and IgG seroprevalence in different trimesters

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).
Table 1

Univariate and multivariate logistic regression analysis of factors associated with IgM seropositivity

Characteristics

IgM seropositivity

(N, %)

Univariate

Multivariate

OR(95% CI)

P value

OR(95% CI)

P value

Age(years)a

21IQR (19–25)

0.97(0.92–1.02)

0.261

  

Gestation age

 First trimester (116)

41(35.3)

1

   

 Second trimester (109)

31(28.9)

0.74(0.42–1.31)

0.310

  

 Third trimester (33)

11(36.7)

1.05(0.45–2.44)

0.893

  

 Number in household

3IQR (2–5)

1.04(0.91–1.19)

0.528

  

Residence

 Urban (168)

48 (28.6)

1

   

 Rural (85)

35 (41.2)

1.75(1.01–3.02)

0.045

1.60(0.91–2.8)

0.098

Education

 High (50)

15 (30.0)

1

   

 Low (203)

68 (33.5)

1.175(0.60–2.30)

0.637

  

Occupation

 Employed (178)

59 (33.2)

1

   

 No employment (75)

24 (32.0)

0.949(0.53–1.68)

0.859

  

HIV status

 Negative(95)

31 (32.6)

1

   

 Positive (8)

4 (50.0)

2.81(0.54–14.66)

0.219

  

 Unknown (150)

48 (32.0)

1.20(0.72–2.00)

0.483

  

History of premature baby

 No (241)

75 (31.12)

1

   

 Yes (12)

8 (66.67)

4.42(1.29–15.15)

0.018

3.03(0.68–13.42)

0.129

History of miscarriage

 No (221)

71 (32.1)

1

   

 Yes (32)

12 (37.5)

1.057(0.50–2.22)

0.883

0.91(0.34–2.41)

0.847

History of baby with malformation

 No (247)

80 (32.4)

1

   

 Yes (6)

3 (50.0)

2.087(0.41–10.57)

0.374

  

History of a baby with low birth weight

 No (229)

71 (31.0)

1

   

 Yes (24)

12 (50.0)

2.22(0.95–5.14)

0.064

1.35(0.46–4.72)

0.098

High: College and above; Low: Primary education

aMedian

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).
Table 2

Univariate and multivariate regression analysis of factors associated with IgG seropositivity

Characteristics

IgG seropositivity (N, %)

Univariate

Multivariate

OR(95% CI)

P value

OR(95% CI)

P value

Age(years)a

21IQR(19–26)

1.000(0.956–1.046)

0.984

  

Gestation age

 First trimester (116)

66(56.9)

    

 Second trimester(109)

66(60.5)

1.1(0.68–1.97)

0.578

1.27(0.73–2.2)

0.391

 Third trimester(33)

10(30.3)

0.32(0.14–0.75)

0.009

0.38(0.16–0.92)

0.033

Number in household

4IQR(2–5)

1.12(0.98–1.281)

0.094

1.08(0.94–1.25)

0.130

Location

 Urban (168)

88 (52.38)

1

   

 Rural (90)

54 (60.0)

1.36(0.81–2.29)

0.242

  

Education

 High (50)

24 (48)

1

   

 Low (208)

118 (56.7)

1.42(0.76–2.64)

0.266

  

Occupation

 Employed (183)

102 (55.7)

1

   

 No employment (75)

40 (53.3)

0.91(0.53–1.56)

0.724

  

HIV status

 Negative (95)

49 (51.6)

1

   

 Positive (8)

6 (75.0)

2.82(0.54–14.67)

0.219

  

 Unknown (155)

87 (56.1)

1.20(0.72–2.00)

0.483

  

History of premature baby

 No (246)

133 (54.1)

1

   

 Yes (12)

9 (75.0)

2.54(0.67–9.64)

0.168

0.66(0.08–5.15)

0.697

History of miscarriage

 No (226)

124 (54.9)

1

   

 Yes (32)

18 (56.3)

1.06(0.51–2.22)

0.883

  

History of baby with malformation

 No (252)

138 (54.8)

1

   

 Yes (6)

4 (66.7)

1.65(0.30–9.18)

0.566

  

History of baby with low birth weight

 No (234)

121 (51.7)

1

   

 Yes (24)

21 (87.5)

6.54(1.90–22.51)

0.003

10(1.82–58.05)

0.010

High: College and above; Low: Primary education

aMedian

IgG titres by gestational age

The median IgG titres among IgG seropositive pregnant women were 27 IU/ml (IQR 17.8–42.2). The median IgG titres by trimesters were 32.7 IU/ml (IQR; 22.1–47.6) for the first trimester, 26 IU/ml (IQR; 17.3–35) for the second trimester and 16.7 IU/ml (IQR; 12.8.1–47.623.9) for the third trimester (Fig. 2). Significantly higher median titres were observed in the first trimester than in the second and third trimesters. Using Kruskal–Wallis equality of population rank test the differences observed were statistically significant (P = 0.001). In addition, it was further observed that as gestation age increases the titres were found to decrease significantly (Fig. 3; Spearman’s rho = −0.2939, p = 0.0004).
Fig. 2

Bar chart showing B19 IgG median titres by trimesters

Fig. 3

Scatter diagram showing the correlation of B19 IgG titres and gestational age

Discussion

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 [1620]. 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 [2426]. 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.

Conclusions

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.

Abbreviations

CI: 

Confidence interval

CUHAS/BMC: 

The Catholic University and Health allied Sciences/Bugando Medical Centre

ELISA: 

Enzyme linked Immunosorbent assays

IgG: 

Immunoglobulin G

IgM: 

Immunoglobulin M

IQR: 

Interquartile range

IU: 

International Unit

OR: 

Odd ratio

PCR: 

Polymerase chain reaction

Declarations

Acknowledgements

The authors would like to acknowledge the technical support provided by Mr. Seif Abdu, Mr. Vitus Silago, Ms. Maria Mwacha, Mrs. Damson Salema and Mrs. Esther Pastory. We thank all staff in Makongoro and Karume antenatal clinics for their technical support.

Funding

This study was supported by research grant from CUHAS to SEM. The funding body had no role in the study design, data collection, analysis, and interpretation of the data and in writing the manuscript.

Availability of data and materials

The datasets during and/or analyzed during the current study available from the corresponding author on reasonable request.

Authors’ contributions

MMM, MM and SEM participated in the design of the study. MMM, NM, MFM, DM and JS participated in the collection of specimens and clinical data. MMM, FM and SEM performed serological tests. MMM, FM and SEM analyzed and interpreted the data. MMM and SEM wrote the first draft of the manuscript. All authors read and approved the final version of the manuscript.

Competing interests

The authors declare that they have no competing interests.

Consent for publication

Not applicable.

Ethics approval and consent to participate

The study was approved by the joint CUHAS/BMC research ethics and review committee and permission to conduct study was obtained from Mwanza city administration and health facilities administrations. Written informed consent was sought from each participant prior enrolment.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Department of Microbiology and Immunology, Weill Bugando School of Medicine
(2)
Department of Microbiology and Immunology, Muhimbili University of Health and Allied Sciences
(3)
Department of Obstetrics and gynecology, Weill Bugando School of Medicine

References

  1. Rodis JF, Quinn DL, Gary GW, Anderson LJ, Rosengren S, Cartter ML, et al. Management and outcomes of pregnancies complicated by human B19 parvovirus infection: a prospective study. Am J Obstet Gynecol. 1990;163(4):1168–71.Google Scholar
  2. Gratacós E, Torres P-J, Vidal J, Antolín E, Costa J, de Anta MTJ, et al. The incidence of human parvovirus B19 infection during pregnancy and its impact on perinatal outcome. J Infect Dis. 1995;171(5):1360–3.Google Scholar
  3. Miller E, Fairley CK, Cohen BJ, Seng C. Immediate and long term outcome of human parvovirus B19 infection in pregnancy. BJOG. 1998;105(2):174–8.View ArticleGoogle Scholar
  4. Cioc AM, Sedmak DD, Nuovo GJ, Dawood MR, Smart G, Magro CM. Parvovirus B19 associated adult Henoch Schönlein purpura. J Cutan Pathol. 2002;29(10):602–7.View ArticlePubMedGoogle Scholar
  5. White FV, Jordan J, Dickman PS, Knisely A. Fetal parvovirus B19 infection and liver disease of antenatal onset in an infant with Ebstien's anomaly. Fetal Pediatr Pathol. 1995;15(1):121–9.Google Scholar
  6. Simchen MJ, Toi A, Bona M, Alkazaleh F, Ryan G, Chitayat D. Fetal hepatic calcifications: prenatal diagnosis and outcome. Am J Obstet Gynecol. 2002;187(6):1617–22.View ArticlePubMedGoogle Scholar
  7. Heegaard ED, Petersen BL, Heilmann CJ, Hornsleth A. Prevalence of parvovirus B19 and parvovirus V9 DNA and antibodies in paired bone marrow and serum samples from healthy individuals. J Clin Microbiol. 2002;40(3):933–6.View ArticlePubMedPubMed CentralGoogle Scholar
  8. Sukanya T, Pilaiwan K, Rattana K, Pakaphan K, Junya J, Thawlwong R. Hydrops fetalis caused by parvovirus B19 infection case report and literature. J Med Assoc Thail. 2006;89:1277–68.Google Scholar
  9. Anderson LJ. Role of parvovirus B19 in human disease. Pediatr Infect Dis J. 1987;6(8):711–8.View ArticlePubMedGoogle Scholar
  10. Mossong J, HENS N, Friederichs V, Davidkin I, Broman M, Litwinska B, et al. Parvovirus B19 infection in five European countries: seroepidemiology, force of infection and maternal risk of infection. Epidemiol Infect. 2008;136(08):1059–68.Google Scholar
  11. Röhrer C, Gärtner B, Sauerbrei A, Böhm S, Hottenträger B, Raab U, et al. Seroprevalence of parvovirus B19 in the German population. Epidemiol Infect. 2008;136(11):1564–75.Google Scholar
  12. Ergaz Z, Ornoy A. Parvovirus B19 in pregnancy. Reprod Toxicol. 2006;21(4):421–35.View ArticlePubMedGoogle Scholar
  13. Harger JH, Adler SP, Koch WC, Harger GF. Prospective evaluation of 618 pregnant women exposed to parvovirus B19: risks and symptoms. Obstet Gynecol. 1998;91(3):413–20.View ArticlePubMedGoogle Scholar
  14. Enders M, Weidner A, Enders G. Current epidemiological aspects of human parvovirus B19 infection during pregnancy and childhood in the western part of Germany. Epidemiol Infect. 2007;135(04):563–9.View ArticlePubMedGoogle Scholar
  15. Crane J. Parvovirus B19 infection in pregnancy. J Obstet Gynaecol Can. 2002;24(9):727–43. quiz 744–726View ArticlePubMedGoogle Scholar
  16. Emiasegen SE, Nimzing L, Adoga MP, Ohagenyi AY, Lekan R. Parvovirus B19 antibodies and correlates of infection in pregnant women attending an antenatal clinic in central Nigeria. Mem Inst Oswaldo Cruz. 2011;106(2):227–31.View ArticlePubMedGoogle Scholar
  17. Schwarz T, Gürtler L, Zoulek G, Deinhardt F, Roggendorf M. Seroprevalence of human parvovirus B19 infection in Sao Tome and Principe, Malawi and Mascarene Islands. Zentralblatt für Bakteriologie. 1989;271(2):231–6.View ArticlePubMedGoogle Scholar
  18. Abiodun I, Opaleye OO, Ojurongbe O, Fagbami AH. Seroprevalence of parvovirus B19 IgG and IgM antibodies among pregnant women in Oyo state, Nigeria. J Infect Dev Ctries. 2013;7(12):946–50.View ArticlePubMedGoogle Scholar
  19. Schoub B, Blackburn N, Johnson S, McAnerney J. Primary and secondary infection with human parvovirus B19 in pregnant women in South Africa. S Afr Med J. 1993;83(7):505–6.PubMedGoogle Scholar
  20. Elnifro E, Nisha A, Almabsoot M, Daeki A, Mujber N, Muscat J. Seroprevalence of parvovirus B19 among pregnant women in Tripoli, Libya. J Infect Dev Ctries. 2009;3(03):218–20.View ArticlePubMedGoogle Scholar
  21. Butchko AR, Jordan JA. Comparison of three commercially available serologic assays used to detect human parvovirus B19-specific immunoglobulin M (IgM) and IgG antibodies in sera of pregnant women. J Clin Microbiol. 2004;42(7):3191–5.View ArticlePubMedPubMed CentralGoogle Scholar
  22. Hannachi N, Marzouk M, Harrabi I, Ferjani A, Ksouri Z, Ghannem H, et al. Seroprevalence of rubella virus, varicella zoster virus, cytomegalovirus and parvovirus B19 among pregnant women in the Sousse region, Tunisia. Bull Soc Pathol Exot. 2011;104(1):62–7.Google Scholar
  23. Ma M, Pacsa A, Essa SS, Ahmed MA, Monem RA, Surkouh M. The prevalence of antibody to human parvovirus B19 in pregnant women in Kuwait. Acta Trop. 1999;73(3):225–9.View ArticleGoogle Scholar
  24. Eskild A, Jeansson S, Hagen J, Jenum P, Skrondal A. Herpes simplex virus type-2 antibodies in pregnant women: the impact of the stage of pregnancy. Epidemiol Infect. 2000;125(03):685–92.View ArticlePubMedPubMed CentralGoogle Scholar
  25. BABOONIAN C, GRIFFITHS P. Is pregnancy immunosuppressive? Humoral immunity against viruses. BJOG Int J Obstet Gynaecol. 1983;90(12):1168–75.View ArticleGoogle Scholar
  26. AMINO N, TANIZAWA O, MIYAI K, TANAKA F, HAYASHI C, KAWASHIMA M, et al. Changes of serum immunoglobulins IgG, IgA, IgM, and IgE during pregnancy. Obstet Gynecol. 1978;52(4):415–20.Google Scholar
  27. Barros DFR, Buarque DGS, Durigon EL, Linhares AC. Survey of parvovirus B19 infection in a cohort of pregnant women in Belem, Brazil. Braz J Infect Dis. 1999;3(1):6–14.Google Scholar

Copyright

© The Author(s). 2017