Elsevier

Acta Tropica

Volume 86, Issues 2–3, May 2003, Pages 125-139
Acta Tropica

Quantification of clinical morbidity associated with schistosome infection in sub-Saharan Africa

https://doi.org/10.1016/S0001-706X(03)00029-9Get rights and content

Abstract

Health policy making in developing countries requires estimates of the (global) burden of disease. At present, most of the available data on schistosomiasis is limited to numbers of individuals harbouring the infection. We explored the relationship between the presence of schistosome infection and clinical morbidity, in order to estimate numbers of individuals with disease-specific morbidity for Schistosoma haematobium and Schistosoma mansoni infection in sub-Saharan Africa. We searched the literature for cross-sectional data from field studies reporting both schistosome infection and morbidity. This was used to derive a functional relationship between morbidity and infection. After standardisation for diagnostic method, the number of individuals with specific types of clinical morbidity or pathology was predicted. As only aggregated prevalences of infection were available for countries or areas, we adjusted for heterogeneity in infection levels within communities in those countries. In total, 70 million individuals out of 682 million (2000 estimate) in sub-Saharan Africa were estimated to experience haematuria in the last 2 weeks associated with S. haematobium infection, and 32 million dysuria. Ultrasound detected serious consequences of S. haematobium, major bladder wall pathology and major hydronephrosis, were predicted at 18 and 10 million, respectively. Infection with S. mansoni was estimated to cause diarrhoea in 0.78 million individuals, blood in stool in 4.4 million and hepatomegaly in 8.5 million. As the associations between prevalence of S. mansoni infection and prevalence of diarrhoea and blood in stool were not very clear, the resulting estimates may be underestimations. Using the very limited data available, we estimated the mortality rates due to non-functioning kidney (from S. haematobium) and haematemesis (from S. mansoni) at 150 000 and 130 000 per year. Given the overall high number of cases with schistosomiasis-related disease and associated death, we conclude that schistosomiasis remains an important public health problem in sub-Saharan Africa.

Introduction

Health policy in developing countries is based on targeting diseases with high preventable burdens of disease. This requires estimates on the (global) burden of disease. Currently WHO estimates the burden of schistosomiasis on the basis of the number of individuals infected with Schistosoma haematobium, Schistosoma mansoni and Schistosoma japonicum (Murray and Lopez, 1996), with an associated low disability weight (0.005 compared with 0.172 for a malaria episode). However, not all infected individuals experience morbidity. Also, not all individuals with schistosomiasis-related disease are found positive at standard screening (De Vlas and Gryseels, 1992, Utzinger et al., 2001). Thus, estimates of the prevalence of morbidity are needed. Disability weights can then be assigned to symptoms to calculate the burden of disease, as is done for many other diseases in the burden of disease calculation (Murray and Lopez, 1997).

Often, epidemiological surveys only report the prevalence of schistosome infection (Brooker et al., 2000a, Chitsulo et al., 2000). Attempts to estimate the prevalence of specific symptoms from the prevalence of infection concluded that 20 million have serious clinical disease and 120 million are symptomatic, by using the well-known figure of 200 million infected individuals and assuming proportions of 10 and 60% for serious clinical disease and being symptomatic, respectively (WHO, 1993, Crompton, 1999, Savioli et al., 1997). Others assumed that intensity of infection is associated with morbidity and related the prevalence of morbidity in a population to intensity of infection (Chan et al., 1996, Gryseels and Polderman, 1991, Medley and Bundy, 1996). However, there is a dearth of data on intensity of infection, making it necessary to infer intensity from statistical associations between prevalence and intensity of infection (Guyatt and Bundy, 1991). Still others estimated morbidity due to helminth infection, by assuming a threshold value for the number of worms above which morbidity occurs (Chan et al., 1994, De Silva et al., 1997). These studies ignored that most symptoms and signs caused by helminth infection are non-specific (e.g. anaemia due to malaria infection and bloody diarrhoea due to amoebic dysentery).

This paper attempts to predict the number of individuals with morbidity associated with schistosome infection to serve as input for the Global Burden of schistosomiasis calculations. Since most accessible data are available for S. haematobium and S. mansoni infection and the majority of the individuals infected with schistosomes live in sub-Saharan Africa (165 vs. 28 million in the rest of the world, Chitsulo et al., 2000), we focussed on S. haematobium and S. mansoni infections in sub-Saharan Africa. We investigated the relationship between the presence of schistosome infection and clinical morbidity (or pathology) by using all published field studies. The parasitological data were standardised for differences in diagnostic sensitivity, we accounted for non-specific morbidity, and we adjusted our estimates for heterogeneity in infection.

Section snippets

Methods

In order to arrive at an estimate of the number of clinical cases from the prevalence of infection in a country, the following points were considered: (1) schistosomiasis causes many different signs and symptoms, out of which a selection had to be made; (2) morbidity is concentrated in individuals with current (or past) high intensity of infection; (3) most clinical morbidity associated with schistosomiasis is non specific; (4) prevalence of infection data were available on an aggregated level

Results

Table 1 summarises the types of morbidity and pathology for which data were available, the method of measurement used in the different studies and the number of articles concerned.

Table 2 shows the estimated number of individuals with signs and symptoms due to infection with S. haematobium in sub-Saharan Africa. Haematuria (in the last 2 weeks) is predicted to occur in 70 (51–87) million individuals. Haematuria is generally thought to result from bladder pathology, therefore, it is reassuring

Discussion

We attempted to estimate the numbers of individuals with morbidity or pathology due to schistosome infection by using all published data from field studies and taking into account ‘confounding’ factors. The quality of our estimates depends on the accuracy of the associations between prevalence of infection and morbidity, the quality of the available prevalence of infection data and the chosen degree of heterogeneity in prevalence of infection.

For some types of morbidity (major bladder wall

Acknowledgements

The authors acknowledge the co-operation of H. Feldmeier, C.F. Hatz, J.R. Lambertucci, R. Olds, J. Richter. We thank J. Utzinger for providing data for the heterogeneity analysis. This study was performed within the WOTRO/NWO funded multidisciplinary programme ‘Model-based decision support for schistosomiasis control in Ghana, Mali, Niger and Senegal’ and was also funded by the department of Communicable Diseases Control, Prevention and Eradication, World Health Organisation, Geneva,

References (65)

  • J.D. Kvalsvig

    The effects of parasitic infection on cognitive performance

    Parasitol. Today

    (1988)
  • C. Lengeler et al.

    Simple school questionnaires can map both Schistosoma mansoni and Schistosoma haematobium in the Democratic Republic of Congo

    Acta Trop.

    (2000)
  • C.J.L. Murray et al.

    Mortality by cause for eight regions of the world: Global Burden of Disease Study

    Lancet

    (1997)
  • C. Nokes et al.

    Does helminth infection affect mental processing and educational achievement

    Parasitol. Today

    (1994)
  • L. Savioli et al.

    Control of schistosomiasis—a global picture

    Parasitol. Today

    (1997)
  • J.E. Thomas et al.

    Relationship between bladder cancer incidence, Schistosoma haematobium infection, and geographical region in Zimbabwe

    Trans. R. Soc. Trop. Med. Hyg.

    (1990)
  • G.A. Traquinho et al.

    Schistosomiasis in Northern Mozambique

    Trans. R. Soc. Trop. Med. Hyg.

    (1998)
  • E. Abdel-Salam et al.

    Prevalence and morbidity of Schistosoma haematobium in Egyptian children

    Am. J. Trop. Med. Hyg.

    (1977)
  • M.F. Abdel-Wahab et al.

    The epidemiology of schistosomiasis in Egypt: Fayoum Governorate

    Am. J. Trop. Med. Hyg.

    (2000)
  • R. Barakat et al.

    Schistosoma mansoni in the Nile Delta, Egypt. A large scale epidemiological study in Kafr El Sheikh Governorate

    Trop. Geogr. Med.

    (1995)
  • R. Bedwani et al.

    Schistosomiasis and the risk of bladder cancer in Alexandria, Egypt

    Br. J. Cancer

    (1998)
  • M. Booth et al.

    Associations among multiple geohelminth species infections in schoolchildren from Pemba Island

    Parasitology

    (1998)
  • S. Brooker et al.

    Estimating the number of helminthic infections in the Republic of Cameroon from data on infection prevalence in schoolchildren

    Bull. WHO

    (2000)
  • M.G. Chen et al.

    Progress in assessment of morbidity due to Schistosoma mansoni: a review of recent literature

    Trop. Dis. Bull.

    (1988)
  • M.G. Chen et al.

    Progress in assessment of morbidity due to Schistosoma haematobium: a review of recent literature

    Trop. Dis. Bull.

    (1989)
  • M.S. Chan et al.

    The evaluation of potential global morbidity attributable to intestinal nematode infections

    Parasitology

    (1994)
  • M.S. Chan et al.

    Dynamic models of schistosomiasis morbidity

    Am. J. Trop. Med. Hyg.

    (1996)
  • D.W. Crompton

    How much human helminthiasis is there in the world

    J. Parasitol.

    (1999)
  • N.R. De Silva et al.

    Morbidity and mortality due to ascariasis: re-estimation and sensitivity analysis of global numbers at risk

    Trop. Med. Int. Health

    (1997)
  • S.J. De Vlas et al.

    A model for variations in single and repeated egg counts in Schistosoma mansoni infections

    Parasitology

    (1992)
  • Doumenge, J.P., Mott, K.E., Cheung, C., Villenave, D., Chapuis, O., Perrin, M.F., Reaud-Thomas, G., 1987. Atlas of the...
  • B. Efron et al.
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