Modeling and Validation of Environmental Suitability for Schistosomiasis Transmission Using Remote Sensing.

BACKGROUND: Schistosomiasis is the most widespread water-based disease in sub-Saharan Africa. Transmission is governed by the spatial distribution of specific freshwater snails that act as intermediate hosts and human water contact patterns. Remote sensing data have been utilized for spatially explicit risk profiling of schistosomiasis. We investigated the potential of remote sensing to characterize habitat conditions of parasite and intermediate host snails and discuss the relevance for public health. METHODOLOGY: We employed high-resolution remote sensing data, environmental field measurements, and ecological data to model environmental suitability for schistosomiasis-related parasite and snail species. The model was developed for Burkina Faso using a habitat suitability index (HSI). The plausibility of remote sensing habitat variables was validated using field measurements. The established model was transferred to different ecological settings in Côte d'Ivoire and validated against readily available survey data from school-aged children. PRINCIPAL FINDINGS: Environmental suitability for schistosomiasis transmission was spatially delineated and quantified by seven habitat variables derived from remote sensing data. The strengths and weaknesses highlighted by the plausibility analysis showed that temporal dynamic water and vegetation measures were particularly useful to model parasite and snail habitat suitability, whereas the measurement of water surface temperature and topographic variables did not perform appropriately. The transferability of the model showed significant relations between the HSI and infection prevalence in study sites of Côte d'Ivoire. CONCLUSIONS/SIGNIFICANCE: A predictive map of environmental suitability for schistosomiasis transmission can support measures to gain and sustain control. This is particularly relevant as emphasis is shifting from morbidity control to interrupting transmission. Further validation of our mechanistic model needs to be complemented by field data of parasite- and snail-related fitness. Our model provides a useful tool to monitor the development of new hotspots of potential schistosomiasis transmission based on regularly updated remote sensing data.

[Schistosomiasis caused by Schistosoma mansoni in the Kou valley: Characterization of the transmission system and socioeconomic impact]. / Endémie bilharzienne à Schistosoma mansoni à la vallée du Kou : caractérisation du système de transmission et impact socioéconomique.

Schistosomiasis is one of the waterborne diseases which benefit from environmental and behavioral changes induced by the mobilization of surface water resources in Sahelian countries, such as Burkina Faso. Studies have established the existence of human schistosomiasis in the Kou valley, one of the oldest hydro-agricultural zones in the country. However, the role of population behavior in the transmission pattern of this disease and its socioeconomic impact in this valley are poorly understood. It is in response to these questions that this study was undertaken. The objectives of this study were to identify activities that exposed most of the Valley's population to infection by schistosomiasis, and to contribute knowledge on the consequences of this disease. The study was conducted in the cold dry season at the Kou Valley, located in the South Sudanese area of Burkina Faso. It has adopted the strategy of direct observation to examine host-parasites interactions. The study of the socioeconomic consequences of the infection has been first to identify subjects that actually carry the parasite by screening the population by the Kato-Katz method. These were then subjected to a questionnaire. Data were analyzed using Epi Info 6.4. This work has revealed six activities at risk of infection for the residents of the Valley with an increased risk of factor for rice farming, household activities and swimming. In view of these activities, women and young people seem to be most vulnerable to infection. This disease causes significant economic losses as a function of socio-professional categories of infected persons.

[Parasitological profile of two forms of schistosomiasis (urinary and intestinal forms) at ten sites in Burkina Faso (Sub-Saharan Africa country)]. / Profil parasitologique de deux formes de schistosomiase (formes urinaire et intestinale) dans dix sites du Burkina Faso (pays d'Afrique sub-saharienne).

In spite of great progress in schistosomiasis control during the last decade in Burkina Faso, this disease remains a public health concern in the country. Indeed, our study consisted of the analysis of parasitological data related to Schistosoma haematobium and Schistosoma mansoni and in malacological investigations. The prevalence rate of Schistosoma haematobium varies from 3.3% to 50.4% and from 3.3% to 39.1% for Schistosoma mansoni, but only in the western part of Burkina Faso. Schoolboys are more infested than girls, but the phenomenon is reversed in adults. Biomphalaria pfeifferi, Bulinus truncatus, Bulinus senegalensis and Bulinus globosus were collected during this study. Thus, the behavioral factors as well as the dynamics and the distribution of the intermediate mollusks play a major role in the persistence of the disease.

Serodiagnosis of Schistosoma mansoni infections in an endemic area of Burkina Faso: performance of several immunological tests with different parasite antigens.

The performance of indirect haemagglutination assays (IHA), enzyme-linked immunosorbent assays (ELISA) and indirect immunofluorescent antibody tests (IFAT) were compared with 450 sera from a Schistosoma mansoni-endemic area in Burkina Faso. All participants in this survey provided at least one sample each of stool, urine and serum. From those with an egg-negative Kato-Katz thick smear, a second stool sample was examined. IHA was based on either extracts of adult S. mansoni worms (SmIHA) or S. japonicum egg antigen (SjIHA). For ELISA, three antigen preparations were used, namely: (i) soluble S. mansoni adult worm antigens (SWAP); (ii) soluble S. mansoni egg antigens (SEA); and (iii) a cationic exchange fraction of S. mansoni eggs (CEF6). IFAT was performed with S. mansoni male worm sections. Among the egg-excretors, the sensitivity of ELISA was high and egg antigens performed slightly better (SEA, 96%; CEF6, 97%) than worm antigen (94%). Sensitivity of IHA was satisfactory with homologous (Sm, >85%), but not heterologous (Sj, 56%) parasite antigen. In IFAT, the parenchyma-associated fluorescence showed high sensitivity (95%), but gut-associated fluorescence, which is known to be a sensitive diagnostic marker for schistosome-infected European travelers, was observed only in 76% of a sub-sample of 100 of the endemic sera. Among sera from egg-negative individuals, many gave positive reactions in several or all of the tests employed. These reactions (formally "false positive") are considered to represent true infections, since chemotherapy had not yet been delivered to this population. For the purpose of further surveys in Burkina Faso or other resource-poor settings, we suggest IHA as an accurate diagnostic test and propose to further improve its performance by including egg rather than worm antigens.

[Impact of water resource installations on the distribution of schistosomiasis and its intermediary hosts in Burkina Faso]. / Influences des hydro-aménagements sur la distribution des bilharzioses et de leur hôtes intermédiares au Burkina Faso.

Dams generally are a favourable biotope for the molluscs acting as intermediary hosts to schistosomiasis. The importance of the schistosomiasis endemic which follows depends on the interactions taking place between the parasites and their definitive (humans) and intermediary hosts. A preliminary sound knowledge of the prevailing epidemiological situations is therefore necessary to define an efficient programme to fight these infections. The extension of schistosomiasis following the installation of water resource facilities is significative of the part played by these hosts. In the hydroagricultural complex of Sourou, the prevalence of urinary schistosomiasis increased from 19% in 1954 to more than 70% in 1998-1999 in Guiédougou, the most ancient site. As to digestive schistosomiasis, almost unheard of until 1987, its prevalence ranged from 8% to 69% in 1998 in the villages located alongside the areas thus equipped. In the Kou Valley, the prevalence went up from 14% in 1957 to 80% in 1974 for urinary schistosomiasis and from 1.3% to 45% for intestinal schistosomiasis. The same tendencies are likely to appear in the hydraulic installations of Bagré, Ziga, and Kompienga. Dams thus constitute amplifying factors for the proliferation of species and for parasite-host interactions. All the actors (developers, populations and scientists) are faced with the challenge of finding a mean to control the development of schistosomiasis infections which are likely to seriously lessen the benefits expected from these hydraulic installations.