Assessment, mapping and prediction of the spatial distribution of parasitic infections in western Côte d'Ivoire and implications for integrated control
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AbstractAlmost half of the world’s population are at risk of malaria infection, and the disease kills more than one million people each year, mainly children under the age of five years living in sub-Saharan Africa. More than one-quarter of the world’s population are affected by schistosomiasis and soil-transmitted helminthiasis and approximately 300 million people suffer from associated morbidity. Recent estimates suggest that each year schistosomiasis alone causes more than 200,000 deaths due to kidney dysfunction and haematemesis in sub- Saharan Africa. Amoebiasis, a disease caused by the intestinal protozoan parasite Entamoeba histolytica, kills 40,000-100,000 people each year. An estimated 200 million people are currently infected with Giardia duodenalis, another intestinal protozoan infection, which can cause severe disease especially in children. The above-mentioned diseases are particularly prevalent in developing countries, especially in the poorest segments of rural communities. Underlying risk factors are lack of access to clean water and improved sanitation, inadequate hygiene behaviour, and lack of access to sound preventive measures and effective treatment. Because several of these parasitic infections are often prevalent in the same regions, it follows that people can harbour multiple parasite species infections concurrently. Individuals with multiple parasite infections are at an elevated risk of morbidity, hence the appraisal of the extent of this phenomenon and the underlying risk factors of single and multiple infections is important for the design and implementation of control strategies. The overarching goal of this thesis was to enhance our understanding of demographic, ecological, environmental and socio-economic factors that influence disease distribution in space in the region of Man, western Côte d’Ivoire. This knowledge base will facilitate the creation of risk maps and predictions of parasitic infections. For this purpose two different studies, namely (i) a community-based study in a single village with participants of all age groups and (ii) a regional school-based study with more than 4,000 schoolchildren, were carried out. The community-based survey was conducted in May-June 2002 in the village of Zouatta II. There, we first conducted a demographic survey among 561 individuals of 75 randomly selected households. Name, age and sex of household members were recorded, as well as the geographical coordinates of their houses. Then, we carried out a rigorous parasitological survey. From each study participant, stool samples were collected on three consecutive days. On the third day, finger prick blood samples were collected from each participant and thin and thick blood films were prepared. Stool and blood samples were analysed with standardized quality-controlled, methods for diagnosis of parasitic infections. The stool samples were processed with the Kato-Katz technique for the identification of Schistosoma mansoni and soil-transmitted helminth eggs (hookworm, Ascaris lumbricoides and Trichuris trichiura). The formalin concentration method was employed for the identification of intestinal protozoa cysts or trophozoites, including E. histolytica/E. dispar and G. duodenalis. Thin and thick blood films were stained with Giemsa and analysed by light microscopy for Plasmodium infections. In parallel, a questionnaire survey for the appraisal of perceived morbidity indicators was carried out among the same household members. In the regional school-based study, demographic data, i.e. age and sex of all schoolchildren attending grades 3-5, were obtained from official class lists in the school year of 2001/2002. First, a cross-sectional parasitological survey was carried out among more than 4,000 schoolchildren from 57 rural schools. From each child single stool and blood samples were collected and processed according to the same standardized, quality-controlled, methods mentioned above. Subsequently, a questionnaire survey was carried out for appraisal of selfreported morbidity indicators and schoolchildren’s socio-economic status. Questionnaires included 17 morbidity indicators and 12 household assets. Finally, a comprehensive geographical information system for the region of Man, including environmental data obtained from satellite imagery and digitised maps, was established. The results of the community-based parasitological survey confirmed that several parasitic diseases were common among rural dwellers of western Côte d’Ivoire, with all age groups concerned. The prevalences of P. falciparum, hookworm, E. histolytica/E. dispar and S. mansoni were 76.4%, 45.0%, 42.2% and 39.8%, respectively, and polyparasitism was very common. In fact, more than three-quarters of the population harboured three or more parasites concurrently. Several parasitic infections showed associations with age and sex. Furthermore, multivariate models revealed significant associations between several parasites and morbidity indicators. The administration of a single oral dose of praziquantel at 40 mg/kg against S. mansoni infections was efficacious, since 60.9% of the S. mansoni-positive study participants were cured. The egg reduction rate was 61.4%. Cure rates were strongly associated to the infection intensity pre-treatment, age of study participants, as well as the sampling effort. The school-based parasitological survey revealed that several parasites were common among schoolchildren. Laboratory examinations showed that the pathogens P. falciparum, S. mansoni, hookworm, G. duodenalis and E. histolytica/E. dispar were found in 64.0%, 38.7%, 30.5%, 17.4% and 11.0% of the schoolchildren, respectively. Strikingly, approximately 90% of the children were suffering from a polyparasitic infection and four out of five children harboured at least three parasite infections concurrently. Associations of different parasites with schoolchildren’s age and sex were found. The questionnaire survey revealed that in general, children did not perceive themselves as being healthy, as they responded to suffer, on average, from 5-6 different morbidity indicators concurrently. At present, only 10.4% of the schoolchildren reported to sleep under a bednet. While 22.4% of the least poor reported to have the opportunity to sleep under a bednet, none of the poorest schoolchildren gave a positive answer. Relationships to socio-economic status were further identified with parasitic infections and self-reported morbidity, as well as physical access to formal health care delivery services. In the case of S. mansoni infections, Bayesian geostatistical models revealed that age, sex, socio-economic status, rainfall and elevation were explaining part of the geographical distribution of this parasite in the region of Man. Boys, schoolchildren aged 11-16 years, and poorer children were more likely to be infected with S. mansoni than their respective counterparts. Further, schoolchildren living at elevations above 400 m were at an increased risk of having an S. mansoni infection. Interestingly, results showed that demographic factors and socio-economic status had stronger influence on the model fit than environmental factors. The results call for concerted efforts to reach the most disadvantaged segments of populations in this rural part of Côte d’Ivoire. This should include improved access to preventive and curative medicine, clean water and improved sanitation, coupled with sound hygiene behaviour education. The findings of the present investigations contribute to the planning of integrated control strategies of several human parasitoses and in particular to schistosomiasis, soil-transmitted helminthiasis and malaria control by providing risk maps that can guide decision makers in the region of Man, western Côte d’Ivoire.
Raso, Giovanna. Assessment, mapping and prediction of the spatial distribution of parasitic infections in western Côte d'Ivoire and implications for integrated control. 2004, PhD Thesis, University of Basel, Faculty of Science.