Environmental Determinants
Snail ecology
Miricidia-Snail
interactions
Cercarial risk
Gating effects
The range of
topographic conditions in Sichuan Province, from flatlands along the
lake to steep and highly terraced landscapes at higher elevations,
provide an opportunity to identify environmental factors responsible for
differences in transmission intensity between villages.
A major focus of our research is assessing how variations in the
agricultural landscape, ecology, hydrology and microclimate in our study
villages mediate the high heterogeneity in environmental risk and
disease burden observed across the study region.
Snail Ecology
The geographic relationship
between snail intermediate hosts and humans plays an important role in
the epidemiology of schistosomiasis. In
Sichuan
Province,
the subspecies Oncomelania
hupensis robertsoni inhabits a complex network of small irrigation
channels in a mountainous agricultural region within an elevation range
of approximately 500 – 2000 m. Snails are often distributed along these
irrigation ditches, which supply water resources to rice fields and
terraces. Annual cross-sectional
snail densities have been traditionally collected in
China
as a part of larger surveys aimed at identifying high priority sites for
schistosomiasis disease surveillance based on their potential to support
snail populations. The
strong relationships between land-use, soil conditions, climate and
snail habitat suggest that environmental modification and changing
agricultural practices may be effective control strategies for
O. hupensis robertsoni.
Methods:
Snail sampling
Publications:
Snail Ecology
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Miricidia-snail interactions
The ecological interactions between miricidia and the
intermediate snail host are fundamental to understanding the link
between environmental contamination and subsequent environmental risk to
humans from water contact. We
assume that new snail infections by miricidia, and the subsequent
shedding of cercaria into surrounding waters, are proportional to the
product of the density of snails and the density of eggs along
irrigation ditches. While
it is feasible to measure snail density and approximate egg
contamination in the environment, quantifying the amount of eggs needed,
relative to snail population numbers, to produce and sustain infected
snail populations remains a challenge. The spatial coincidence of snails
and miracidia is extremely challenging to estimate, yet is a crucial
component of successful propagation of the disease cycle.
The interaction between parasite and
snail, as mediated by environmental factors, also operates on
evolutionary scales. Past studies reviewed
in
Davis
(1980, 1992) suggest a strong coevolutionary bond between the snail
hosts and Schistosoma japonicum.
It is hypothesized that over time, infected snails evolve defense
mechanisms to counter the parasite, and parasites then evolve a response
to evade the host’s defense, an escalating interaction often referred to
as The Red Queen Hypothesis. The effects of these interactions have been documented in the flood
affected areas of the
Yangtze River Valley
where migratory snails lacking resistance to new parasitic strains were
observed to be infected at higher numbers than were more stable snail
populations (Shi et al., 2002).
Methods:
Genetic analysis,
Landscape genetics
Publications:
Miricidia-Snail Interactions
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Cercarial Risk
Cercariae are the free-swimming form of the parasite
infective to mammals.
Location-specific variability in cercarial density has been shown to be
high in agricultural villages in the mountains of
Sichuan Province.
The high spatial and temporal
heterogeneity of cercaria density complicates the estimation of human
exposure and obscures the relationship between water contact and
infection intensity (Spear, Zhong, Mao, Hubbard, Birkner, Remais, Qiu,
2004). Cercarial risk in village
irrigation systems is associated with a number of village level
variables including snail density, village level human infection
intensity, and the application of manure-based fertilizers.
Based on our studies in Xichang
County we found that despite
the roughly 30-fold difference in the snail density between some
villages,
mean village snail density was not a significant independent predictor
of the cercarial risk as determined by mouse bioassay results. Other
findings suggest that once a threshold in snail density exists,
cercarial risk becomes highly variable.
Methods:
Molecular
detection / cercariometry
Publications:
Cercarial risk
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Gating
Effects
Gating effects are the time variable modifiers of transmission that
determine the endemic level of disease realized in an isolated village
in a semi-stable environment and in the absence of control activity. In
general terms these effects include the impact of environmental
variables like temperature and rainfall, but also the temporal patterns
of both uninfected snail population dynamics and human water contact
throughout the annual agricultural cycle. In the transmission model
these effects are realized as mathematical functions, for example, the
precipitation-and/or irrigation-dependent forces that modulate the
average daily production and transport of miricidia and cercariae.
Methods:
Transmission modeling
Publications:
Transmission modeling
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