Human Epidemiology
Water
Contact
Parasite Burden
Egg Excretion
While environmental factors may shape the suitability
of snail habitat and the resulting distribution of the intermediate
snail host, in the end, risk of transmission in humans depends on the
frequency and duration of contact with infective water and the level of
risk of infection upon contact. In the mountainous area of
Sichuan, exposure to infective water occurs
during farming, domestic, and recreational activities. The
resulting parasite burden falls unevenly on different demographic groups
both within and across endemic villages. Our research goals in
regard to human transmission are to understand what human activities
account for this heterogeneity in disease risk and burden and the degree
to which spatial and temporal patterns in environmental risk contribute
to human disease.
Water Contact
Data from individual level surveys in Xichang suggest that water contact
levels in a village vary based on demographic group.
A 25% random sample of the study population, stratified by
village and occupation, was selected to participate in an interview
aimed at assessing the nature, frequency, and location of their water
contact by month.
We used an index of water contact to quantify
the magnitude of water contact by risk-group based on duration and
frequency of different tasks weighted by the fraction of the body
surface estimated to be wetted.
We found clear differences in the magnitude and seasonal patterns of
water contact among children versus adults, which were robust across all
villages, particularly with children who had most of their contacts
during the summer months compared to farmers whose contacts were
coincident with major planting and harvesting times.
Water contact alone is a poor indicator
of infection risk.
Most
recently, we have found a better parameterization of individual-level
exposure based on a combination of the water contact and mouse bioassay
data, which has shown to be correlated with both infection status and
intensity of infection (Seto et al, 2007, Lee and Seto, in review).
This has allowed for a better understanding of within-village
risk.
Methods:
Field epidemiology
Publications:
Water contact
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Parasite Burden
We integrate a range of
epidemiological methods to determine baseline levels of disease and
measure spatial and temporal patterns of disease endemicity and
reemergence. Because human
parasite burden can accumulate over periods of years, we explored the
relationship of egg excretion in humans with various individual
predictor variables that might be expected to be relatively stable over
time. Such longitudinal studies
allow us to examine incidence and rates of re-infection.
In 2000 we surveyed twenty
villages in the
Anning
River
Valley
of southwestern
Sichuan,
PRC
for Schistosoma japonicum infections in humans and domestic
animals. We also surveyed human water contact patterns, snail
populations, cercarial risk in irrigation systems, and agricultural land
use. Except for occupation and education, we found that individual
characteristics were not strong determinants of infection intensity
within a village. We also found that differences in human infection
intensity between these villages was strongly associated with crop type,
with low-intensity villages principally growing rice, in contrast to
villages devoting more land to vegetables and tobacco.
Methods:Transmission modeling
Publications:
Parasite Burden
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Egg excretion
The
egg excretion parameter in our model is central to understanding the
relationship between village level worm burden and the resulting village
level infection intensity. Through
extensive field work we have tracked mean worm burden among various
subsets of different sample populations to better understand the
relationship between worm burden and the resulting variability in egg
excretion. Most
of the observed variability in egg excretion could be attributed to
differences in worm burden across the different risk groups. A large
fraction of egg excretion within similar risk groups on the other hand,
could be attributed to within-person variability, as was observed in the
high variability in eggs in stool.
It
is also becoming increasingly clear that the intensity of transmission
is closely connected to the types of crops grown and associated
fertilizer use demands.
Night soil, a mixture of animal and human waste, is a common fertilizer
spread over highly valued agricultural land.
The use of night soil is a major pathway through which
S. japonicum eggs spread to
local waterways and snail habitat. We use village administration records
and local interviews with village leaders and farmers to assess night
soil use. We are also
researching the potential for anaerobic biogas digesters, which we have
shown to inactivate S. japonicum
eggs in human and animal waste, to reduce environmental contamination of
village waterways. See
interventions for more detailed
information on this topic.
Methods: Field
epidemiology
Publications:
Field epidemiology,
Parasite burden,
Transmission modeling,
Interventions
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