Environmental Change
and Parasite Diffusion
Hydrological
Modeling
Intermediate
Host Snail Dynamics
Landscape Genetics
Sociometrics
Statistical and Mathematical Modeling
Schistosomiasis has long been associated with engineered environmental
changes, particularly dam construction. The construction of the Aswan
Dam in
Egypt,
the Tigay in
Ethiopia,
the Kossou and Taabo in
Cote d’Ivoire,
the Diama in
Senegal
and Manantali in
Mali
have all led to major outbreaks of schistosomiasis. While it is clear
that schistosomiasis transmission exhibits a strong response to
environmental change, the underlying mechanisms shaping this
relationship are unclear. We argue that transmission prediction, and
ultimately prevention, requires a more sophisticated understanding of
the mechanisms that alter transmission within a changing environment.
This project, supported by the
NSF Ecology of
Infectious Diseases program, aims
to answer two open questions about the epidemiology of human parasites:
how do parasites spread, and how does environmental change influence
parasite spread? In western
China,
little is known about the relative importance of the transport of snail
and S. japonicum larvae through waterways in relation to other
potential modes of schistosome spread, such as human and domestic animal
movement. With respect to the S. japonicum parasite, we term
these flows parasite diffusion, using the phrase to encompass all
diffusive pathways along which parasites are transported into new and
existing locales.
From our environmental change
research we hope to answer the following questions:
1)
What is the
relative importance of these diffusion pathways in spreading parasites
to new areas, or sustaining the parasite in endemic environments?
2)
How can
anthropogenic change modify diffusion parameters, and thereby influence
transmission?
This project brings together
leading experts in molecular ecology, schistosome genetics and
environmental and epidemiological modeling to comprehensively examine
the diffusion pathways that carry S. japonicum between and among
human populations in twenty-one villages in western
China.
Hydrological Modeling
Hydrological variability has been raised as a key factor in modifying
vector-borne disease transmission under climate change scenarios. In
Western China,
the water-based free-living stages and aquatic intermediate host of
S. japonicum have led us to
focus on hydrological corridors, including streams, rivers and
irrigation canals, and their impedances, such as weirs or dams, as key
pathways by which parasites can diffuse. Our aim is to integrate
dispersal characteristics of snails and parasites, habitat quality
estimators, and
GIS-based
hydrologic pathways into a least-cost model to estimate the most
probable dispersal paths between each pair of villages, and to quantify the degree to
which landscape features facilitate or impede snail migration and
parasite diffusion.
Examples of questions we hope to answer are:
1) Does the dominant direction of parasite diffusion follow a
hydrological gradient?
2) Is there a positive correlation between gradient in a watershed and
snail genetic heterogeneity?
3) Is contemporary dispersal among snail populations unidirectional
along the path of hydrological flow?
(Top)
| Methods:
GIS/RS,
Hydrology
Intermediate Host Snail Dynamics
Within
intermediate snail hosts, parasites are conveyed among and between
aquatic and riparian habitats. Much of our previous work has
focused on characterizing the response of snail population dynamics to
environmental factors (e.g. temperature, precipitation). We use
traditional (mark-recapture) and molecular (AFLP markers) methods to
estimate key population parameters and quantify dispersal among and
between sites. We recognize that traditional and molecular methods
measure somewhat different quantities and can disagree. Thus, we are
employing the simultaneous application of both direct and indirect
methods following the mass mark-recapture work of Remais et al. (2007)
and the AFLP markers developed by Wilke and Davis (2006).
Direct estimates of bidirectional dispersal will be used to validate the
indirect methods in a subset of study villages. Also to be collected in
the current study is the infection status, sex and age of each collected
snail.
With these data, we can potentially answer the following
questions:
1)
What characteristics (age, sex, infection status) are dominant among
migrant snails vs. non-migrants?
2)
What is the relationship between snail genetic
heterogeneity and susceptibility?
3) Between
susceptibility and isolation?
(Top)
| Methods:
Molecular Detection / Cercariometry
,
Genetic analysis
Landscape Genetics
Landscape
genetics is an emerging field integrating landscape ecology, population
genetics, and spatial statistics to better understand how physical,
biological, and chemical variation shape genetic diversity and
structure. We are currently applying landscape genetic tools to
model the influence of environmental variables such as riparian habitat
quality, topography, and hydrology on snail migration and parasite
diffusion, as measured using genetic assignment techniques. Correlations
between fine-scale genetic patterns and environmental variables are
being explored through the use of a
GIS-based
predictive hydrological platform, which allows us to model the dispersal
pathways of parasites and intermediate hosts, using the results of fully
Bayesian assignment tests, topographic barriers, hydrologic pathways,
and land use as model inputs.
(Top)
| Methods:
Genetic analysis,
hydrology,
GIS/RS
Sociometrics
The movements of human and animal hosts are being
reconstructed using a social network parameterized using sociometric
surveys of study participants and animal host tracking. We also
classify, through interviews and using extensive field surveys and
existing maps, the ease (or difficulty) of movement between villages by
systematically documenting the presence of roads, footpaths, public
transportation and the length (in time and distance) of common
between-village trips.
(Top)
| Methods:
GIS/RS
Statistical and
Mathematical Modeling
We use statistical
and mathematical models to achieve mechanistic insights into how
environmental change can modify parasite mobility, thereby altering
transmission. We regard the model as a platform for the synthesis of
general knowledge of the mechanisms of disease transmission,
quantitative estimates of biological parameter values, and the local
factors influencing transmission. To date, we have utilized a variant of
the Anderson-May model of schistosomiasis transmission, a compartmental
differential equation model modified to allow for multiple risk groups.
An extension of this form would be a stochastic compartmental model,
where the risk group structure would be maintained with each compartment
being comprised of a number of identical individuals. The static
representation of the aggregation of parasites in humans in this model
poses some limitations, and thus the predictive modeling carried out in
the present study will exploit the utility of stochastic models for
incorporating rare transmission events. Our general approach given any
model framework is to simulate transmission at the village scale, with
villages within and between watersheds exchanging parasites through the
major diffusion pathways. Our primary modeling outcomes are infection
intensity in humans and infected and uninfected snail densities.
To incorporate
spatial structure with respect to both the exposure process and snail
dynamics, we are currently exploring several model types, with our
primary interest in reaction-diffusion network models, which have a long
history of application to island models of metapopulation dynamics.
Theoretical explorations with a stochastic, individual-based model can
provide insight about the transmission relevance of the introduction of
an infected host, as well as answer questions related to parasite
population viability:
1) Can one
infected water-buffalo, alone (given their size and typically high worm
burden), initiate and sustain transmission in a new environment?
2) What is the impact on
transmission of rare diffusion events, such as long-distance snail
dispersal?
3) Given the
measured pathways, what influence does diffusion have in preventing
disease eradication? That is, how does diffusion influence parasite
population viability?
(Top) | Methods:
Disease
Modeling