Quantification of clinical morbidity associated with schistosome infection 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)
- et al.
Schistosoma haematobium infection in Egyptian schoolchildren: demonstration of both hepatic and urinary tract morbidity by ultrasonography
Trans. R. Soc. Trop. Med. Hyg.
(1992) - et al.
The use of morbidity questionnaires to identify communities with high prevalences of schistosome or geohelminth infections in Tanzania
Trans. R. Soc. Trop. Med. Hyg.
(1998) - et al.
Towards an atlas of human helminth infection in sub-Saharan Africa: the use of geographical information systems (GIS)
Parasitol. Today
(2000) - et al.
Tools from ecology: useful for evaluating infection risk models
Trends Parasitol.
(2002) - et al.
The global status of schistosomiasis and its control
Acta Trop.
(2000) - et al.
Underestimation of Schistosoma mansoni prevalences
Parasitol. Today
(1992) - et al.
Death attributed to kidney failure in communities with endemic urinary schistosomiasis
Lancet
(1970) - et al.
Morbidity, due to Schistosomiasis mansoni, and its control in Subsaharian Africa
Parasitol. Today
(1991) - et al.
Estimating prevalence of community morbidity due to intestinal helminths: prevalence of infection as indicator of the prevalence of disease
Trans. R. Soc. Trop. Med. Hyg.
(1991) - et al.
The incidence of squamous and transitional cell carcinomas of the urinary bladder in northern Tanzania in areas of high and low levels of endemic Schistosoma haematobium infection
Trans. R. Soc. Trop. Med. Hyg.
(1986)
The effects of parasitic infection on cognitive performance
Parasitol. Today
Simple school questionnaires can map both Schistosoma mansoni and Schistosoma haematobium in the Democratic Republic of Congo
Acta Trop.
Mortality by cause for eight regions of the world: Global Burden of Disease Study
Lancet
Does helminth infection affect mental processing and educational achievement
Parasitol. Today
Control of schistosomiasis—a global picture
Parasitol. Today
Relationship between bladder cancer incidence, Schistosoma haematobium infection, and geographical region in Zimbabwe
Trans. R. Soc. Trop. Med. Hyg.
Schistosomiasis in Northern Mozambique
Trans. R. Soc. Trop. Med. Hyg.
Prevalence and morbidity of Schistosoma haematobium in Egyptian children
Am. J. Trop. Med. Hyg.
The epidemiology of schistosomiasis in Egypt: Fayoum Governorate
Am. J. Trop. Med. Hyg.
Schistosoma mansoni in the Nile Delta, Egypt. A large scale epidemiological study in Kafr El Sheikh Governorate
Trop. Geogr. Med.
Schistosomiasis and the risk of bladder cancer in Alexandria, Egypt
Br. J. Cancer
Associations among multiple geohelminth species infections in schoolchildren from Pemba Island
Parasitology
Estimating the number of helminthic infections in the Republic of Cameroon from data on infection prevalence in schoolchildren
Bull. WHO
Progress in assessment of morbidity due to Schistosoma mansoni: a review of recent literature
Trop. Dis. Bull.
Progress in assessment of morbidity due to Schistosoma haematobium: a review of recent literature
Trop. Dis. Bull.
The evaluation of potential global morbidity attributable to intestinal nematode infections
Parasitology
Dynamic models of schistosomiasis morbidity
Am. J. Trop. Med. Hyg.
How much human helminthiasis is there in the world
J. Parasitol.
Morbidity and mortality due to ascariasis: re-estimation and sensitivity analysis of global numbers at risk
Trop. Med. Int. Health
A model for variations in single and repeated egg counts in Schistosoma mansoni infections
Parasitology
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