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Table 2 Overview of parasite-, snail- and human-related factors that modify, retain or intensify the cycle of schistosomiasis transmission

From: Risk profiling of schistosomiasis using remote sensing: approaches, challenges and outlook

Parasite-related factors Effect on schistosomiasis transmission Reference(s)
Temperature Length of prepatent period; activity, survival and infection rate of free-living stages of the parasite [18,19]
Water flow velocity Passive transport of parasites in flowing water determines cercarial density [20]
Predators Fish and carnivorous invertebrates reduce parasite population as natural predators [20,21]
Sunlight Stimulation of cercarial shedding [19]
Pathogenicity Different strains of S. mansoni and S. haematobium result in geographical variations of disease severity [22]
Species Different efficiency in identifying and infecting snails [19]
Snail-related factors Effect on schistosomiasis transmission Reference(s)
Water temperature Fecundity, mortality and rate of reproduction [18,23-26]
Water flow velocity Flow velocity >0.3 m/s may dislodge and swep away snails [24,27,28]
Vegetation Food supply, surface to crawl and deposit egg masses; increase of dissolved oxygen [23]
Substratum Nature of substratum is related to snail abundance [23,24]
Water depth Snails generally found in shallow water near the margins of their habitats; below 1.5-2 m, snails have little importance for the transmission of schistosomiasis [29]
Fluctuations of water level Permanence of available habitats determines the distribution patterns of snails [23,24]
Rainfall Creation of temporary snail habitats; increase of water flow velocity; supports contamination of water and passively transports snails when rains are heavy [20,24,27]
Turbidity Turbidity can impact the reproduction cycle [24,30]
Water chemistry/quality Low pH, refuse from factories directly harm snails; high abundance where water is polluted with human excrements [23,24,27,31]
Sunlight Completely shaded pools provide unsuitable habitat and activity of snails is high in direct sunlight [23]
Predators/pathogens Natural predators, parasites and pathogens may limit the abundance of snails [23]
Species Variation of susceptibility to parasite and efficiency to produce cercariae [27,32]
Human-related factors Effect on schistosomiasis transmission Reference(s)
Water contact behaviour Exposure of the skin to parasite infested water is the prerequisite for human infection [33]
Hygiene Contamination of water due to excrements of infected humans in or aside water [20,34,35]
Gender Relationship between gender and risk of infection is culturally variable and a determinant of water contact activities [35-37]
Age Highest risk for children as consequence of degree of exposure and low level of immunity [33,35,38]
Immunity Resistance to reinfection can be developed by the human body as a consequence of previous infections [38]
Ethnic origin Variation in the susceptibility to infection [20]
Religion Religious rules related to water contact related to disease exposure [20,35]
Socioeconomic status Relation to hygiene, the availability of protected water supplies and ability to cope with the disease [39]
Migration Population movements can modify spatial patterns of disease transmission through both introduction of the parasite or the acquisition of infection [20,33,40]
Occupation Work related to water increases the exposure and risk of infection (fishermen, farmers, etc.) [34,35]
Location of house Location of house in relation to suitability of closest water source can influence infection status [35,41,42]
Prevention/control measures Spatial pattern of disease transmission can be highly modified by mass treatment campaigns and successful preventive measures [43-45]