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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]