Assessment, mapping and prediction of the spatial distribution of parasitic infections in western Côte d'Ivoire and implications for integrated control

Almost 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.