A Spatial-Temporal Model for Assessing the Effects of Intervillage Connectivity in Schistosomiasis Transmission

Abstract

Spatial interaction and connectivity are important factors in the spread of infectious diseases. We developed a spatial‐temporal model of schistosomiasis japonica transmission, a disease caused by parasites that are transported via surface water and that live in both snail and human hosts. The model employed a spatial interaction matrix based on neighborhood relationships and hydrologic connectivity to assess the effect of intervillage parasitic transport on disease transmission and control. Satellite remote‐sensing data served as input to the model for predicting snail density within each village, and for deriving a digital elevation model that was used to quantify hydrologic connectivity. Simulations of the model with varying levels of connectivity and in the presence and absence of chemotherapy control were run for 227 villages near Xichang City, in southwest Sichuan province, China. Increasing connectivity resulted in a geographic clustering of parasites within particular villages that produced higher levels of worm burden than in low and no connectivity simulations. Worm burden within a village could either increase or decrease with connectivity, depending on the degree to which parasites were imported and exported. Simulations of mass chemotherapy in select villages can result in a beneficial reduction in worm burden that extends to downstream neighbors. These findings suggest that better understanding of intervillage connectedness can be exploited in the design of cost‐effective control strategies.

Environmental effects on parasitic disease transmission exemplified by schistosomiasis in western China

“…Few villages in Sichuan are completely isolated, either hydrologically or socioeconomically, the latter, for example, exemplified by exchange of human and animal labor or the purchase of fertilizer from neighboring villages. In previous work, the potential impact of hydrological connectedness was explored by computer simulation, and the results support what practical experience and common sense suggest, that this form of connectedness can be quite important to the patterns of disease transmission and persistence (42). In this context, Clennon et al show how the transmission determinants at the village level are aggregated into a connected network and, by implication, how this combination of factors is likely to influence the scale of effective control programs.…”

“…The principal publications in the Western literature relate to epidemiological field methods and results (18,19,53), mathematical modeling and model calibration (34,42), snail distribution and population dynamics (56), and hydrological aspects of parasite transport (38,57,58). The new site-specific results reported here relate to the outcome of interventions in three villages in the same region that were the subject of earlier mathematical modeling studies.…”

Schistosomiasis as a disease and its prevalence in Sudan: An overview

“…The eggs of these parasites excreted with urine or faeces hatch when they come in contact with freshwater under favorable conditions of temperature (25-30 °C ), light, and osmotic pressure. Free swimming miracidia emerge from the egg shells and can remain infective for 8-12 h. The miracidium penetrates the specific cold-blooded intermediate host snails in the water, predominantly via the foot of the snail [37,41,42]. After penetration into the snail, the miracidium sheds its epithelium and begins development into a mother sporocyst, usually near its point of entrance, and within a period of 4 weeks the mother sporocysts give rise to daughter sporocysts which differentiate into cercariae by asexual multiplication.…”

Analysis of the Re-emergence and Occurrence of Cholera in Lagos State, Nigeria

“…Existing endemic infectious diseases are found to be transmitted in new areas; it is recognized that the transmission of many endemic diseases is being strengthened and accelerated due to the globalization of human activities and environmental change (Xu et al, 2006). According to WHO (2012), poor conditions of health and healthcare are among the factors responsible for the average life expectancy of 47 years in Nigeria.…”