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Memórias do Instituto Oswaldo Cruz
Fundação Oswaldo Cruz, Fiocruz
ISSN: 1678-8060 EISSN: 1678-8060
Vol. 90, Num. 2, 1995, pp. 205-209
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Memorias Instituto Oswaldo Cruz, Vol. 90(2):205-209
mar./apr. 1995
The Geographic Understanding of Snail Borne Disease in Endemic
Areas Using Satellite Surveillance
John B Malone
Department of Veterinary Microbiology and Parasitology,
Louisiana State University, Baton Rouge LA 70803, USA
Code Number:OC95041
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The current status of research on use of earth observing
satellite sensors and geographic information systems for
control program management of schistosomiasis and fascioliasis
is reviewed.
Key words: Schistosoma - Fasciola - epidemiology -
geographic information systems - satellites - Biomphalaria -
Bulinus - Lymnaea
Geographic information systems (GIS) technology provides a
powerful new tool for epidemiologic studies on vector-borne
diseases with strong environmental determinants. By use of
statistical and image analysis methods GIS allows computer
based analysis of multiple layers of mapped data in digital
form, including earth observation satellite data, agroclimatic
databases, and maps of host populations, vector distribution
and disease prevalence. GIS data layers are registered to the
identical scale and geographic projection of a reference base
map. This allows analysis of all information by location,
including descriptive data sets that are 'attached' to
specific locations or areas (Burrough 1986).
Once created, GIS provides a dynamic, easily updated mapping
system that can be used by health management officers to plan
and monitor control programs. By virtue of its potential to
'match' the relative suitability of various environments to
the life cycle and transmission dynamics of host-parasite
systems, GIS provides a new way to address classic concepts of
'landscape epidemiology' and the essential nidality of disease
(Pavlovskii 1945). Recent applications include
schistosomiasis (Cross & Bailey 1984, Malone et al. 1994),
fascioliasis (Zukowski et al. 1992, 1993, Malone et al. 1992),
rift valley fever (Linthicum et al. 1987), African
trypanosomiasis (Rogers & Randolph 1993) and east coast fever
(Lessard et al. 1990).
Digital data from several polar orbiting and geostationary
(fixed view) earth observation satellites systems are
available for purchase by investigators for development of GIS
models of disease. Three of the most commonly used are listed
in Table, with earth surface resolution for each pixel
(picture element) and the swath width of imagery that is
available. This review focuses on the potential for
application of satellite sensor and GIS technology in control
programs for two snail borne diseases, schistosomiasis and
fascioliasis.
TABLE
Data collection interval, resolution of each pixel
(picture element) and swath width ibr data from
polar-orbiting satellites commonly used for remote
sensing and geographic information systems
Interval Resolution Swath
----------------------------------------------
AVHRR 12 hours 1.1 km 2800 km
Landsat MSS 18 days 79 m 185 km
TM 16 days 30 m 185 km
SPOT 26 days 20 m (infrared) 60 km
10 m (visible) 60 km
SCHISTOSOMIASIS
Dramatic recent success in schistosomiasis control has been
realized in Egypt by a campaign of mass chemotherapy
supplemented by molluscicide use and a unique public education
program (El Khoby 1994). Schistosomiasis typically has a
patchy distribution in endemic areas. This has been
attributed to environmental effects, local snail-parasite
genetic compatibility or to sociologic factors, especially
agricultural practices or the proximity of a particular
community to strong transmission foci. Identification of
environmental indicators of high infection risk may facilitate
provision of more frequent chemotherapy and focal molluscicide
for areas of high prevalence that may serve as reservoirs for
well controlled areas. Future success in the consolidation and
maintenence phases will depend on broader intervention efforts
to target areas where high-intensity Schistosoma
mansoni infections occur and on development of cost-
effective strategies for eliminating residual foci of
Schistosoma haematobium.
Polar orbiting environmental satellites operated by the U.S.
National Atmospheric and Oceanographic Administration (NOAA)
acquire daytime and nighttime thermal infrared measurements of
the earth s surface around the world, at 1.1 km spatial
resolution. This sensor has an across track swath 2800 km
wide beneath the satellite, measuring daytime reflected solar
radiation in the visible and near mid-infrared spectral bands
and radiation both day (Tmax) and night (Tmin) in the mid-
infrared and thermal infrared portions of the spectrum. A
vegetation index can be calculated from the visible and near
infrared sensors to identify vegetation and estimate the
'greenness' or health of foliage. Global AVHRR imagery records
are available at least four times per day. AVHRR data are
thus useful for studies that require repetitive, regional
scale analysis of climate, vegetation and environmental change
(Huh 1991). By use of day-night surface temperature
difference (dT) images, it is possible to define moisture
domains in agricultural areas because water buffers the
amplitude of the diurnal fluctuation of surface temperatures
(Shih & Chen 1987). The frequent clear skies and dry
atmospheric conditions of Egypt favor radiometric surface
temperature measurements by remote sensing.
Patterns in the Nile delta were identified in a classification
of Tmax, Tmin and dT images of 16 Aug 90 that reflected the
classic S. mansoni prevalence maps described by Scott
in 1935 (Malone et al. 1992); a transition zone of high dT
values was seen in the southwestern delta that generally
conformed to the outline of Scott's low S. mansoni
prevalence region (Scott 1937). The broad thermal domains
seen in the 16 Aug 90 image were seasonally stable in 18 Oct
90 and 14 Feb 91 dT images, suggesting that permanent
features, not vegetation or climate, were responsible for
patterns seen.
Detailed analysis was done on dT AVHRR images of 16 Aug 90 and
14 Feb 91 to further study the relationship of regional
thermal domains to historical S. mansoni and S.
haematobium distribution in the Nile delta (Malone et al.
1994). In both dT images, a series of transect profiles
revealed a decrease in dT values of approximately 2 C at
points approximating Scott's transition from low to high
prevalence of S. mansoni.
Median dT values were then calculated for a 5 X 5 pixel area
(28 km^2) centered on the latitude and longitude of 41 rural
survey sites named in 1935, 1983 and 1990 surveys (Scott 1937,
Cline et al. 1989, Michelson et al. 1993). For both 16 Aug 90
and 14 Feb 91, there was a significant inverse association
between median dT values and S. mansoni prevalence in
1935 and 1983, suggesting that lower dT values reflect wetter
hydrologic regimes that are more suitable for S.
mansoni. A consistent trend was observed between dT and
prevalence in 1990, but these values were not significantly
correlated. Similar analysis for S. haematobium
revealed a positive relationship of 16 Aug 90 and 14 Feb 91 dT
and prevalence in 1935.
To substantiate regional environmental influences on current
infection risk, S. mansoni prevalence data from the
Nile delta surveys of 1983 and 1990 were compared with the
rank order of Scott s 1935 data for 41 sites. Spearman rank
correlation coefficients revealed a significant relationship
between 1935 and 1983 data and between 1990 and 1935 data. By
1983, extension of S. mansoni into formerly low
prevalence zones of the southwestern delta was observed as
well as a dramatic decrease in the prevalence of S.
haematobium; this trend was maintained at the time of the
1990 survey, when a diminished prevalence of both S.
mansoni and S. haematobium was observed. S.
haematobium prevalence in 1935 diminished as S.
mansoni prevalence increased; no correlation was found
between the ranked 1935 prevalence data and the greatly
decreased S. haematobium prevalence rates of 1983 and
1990. Results indicate that AVHRR thermal-moisture domains
represent stable environmental features in the Nile delta that
reflect historical risk of S. mansoni on a regional
scale and suggest that these features continue to influence
moisture regime and S. mansoni risk 55 years after
Scott's 1935 survey in spite of delta structural changes and
hydrologic consequences of the Aswan High Dam. In Florida
studies on citrus freeze damage, stable terrestrial thermal
patterns observed by GOES climate satellites were found to be
generally governed by broad soil types, soil depths, soil
drainage classes, surface vegetation and land use (Shih & Chen
1987). Moisture domains seen in the 16 Aug 90 and 14 Feb 91
AVHRR images may reflect water retention characteristics
related to underlying geologic formations, the texture and
depth of the agricultural soil layer or elevation above sea
level.
Bulinus truncatus, the S. haematobium
intermediate host, is able to tolerate several months of
drought and high temperatures in its biotope. Biomphalaria
alexandrina, the snail host of S. mansoni, is more
sensitive to extreme temperature variations and does not
survive seasonal drought well; it tends to be found in slowly
moving waters of shallow drains in irrigation networks where
it may be more sensitive to drought periods, annual irrigation
system closure and dryness between irrigation cycles (Abdel-
Wahab 1982).
Additional work is needed to elucidate factors underlying
regional thermal domains and the possibility that similar
thermal domain associations can be found at local community
scales in the Nile delta. This can be addressed in future
studies by developing geographic information system (GIS)
models that include higher resolution Landsat TM imagery,
detailed agricultural and climatic databases, snail population
distribution maps and accurate data on infection prevalence.
A successful environment-based GIS for schistosomiasis in
Egypt can provide a vehicle for later incorporation of
community based sociologic and prevalence data and maps of
water contact, water supply and sewage disposal as they affect
control management at the local level.
FASCIOLIASIS
The unique biology and life cycle strategy of Fasciola
hepatica make it amenable to effective use of GIS control
models in several respects:
Climate Sensitivity - The high environmental
sensitivity and focal nature of F. hepatica
transmission typically results in wide variation in infection
prevalence in animals in fluke enzootic regions. Explosive
outbreaks of fatal disease due to fascioliasis were documented
in sheep in Europe as early as the 18th century. In cattle, a
100-fold difference in parasite burdens can occur between
years owing to the effect of climate variation alone on snail
host populations, intramolluscan asexual multiplication,
survival of fluke eggs and persistence of metacercariae on
pastures. Climate forecast models for fascioliasis,
originally developed for use in Europe, have been adapted for
use the United States, Australia and elsewhere to advise
stockmen on the relative need for flukicide treatment each
year. In the southern United States it has also been
possible, using a forecast model and 30-year average climate
records, to develop a profile on the seasonal pattern of
transmission, relative severity and range of fascioliasis in
divergent climate zones of that region (Malone & Zukowski
1992).
Climate risk data can be included in a GIS as separate layers
on long-term climate patterns (eg. 30-year-average data), maps
of annual values or even as surrogates of climate. In unique
work in Africa, the distribution of TseTse vectors of
trypanosomiasis and tick vectors of East Coast Fever
(Theileria) were successfully characterized by
supplementing long-term climate average records with monthly
vegetation index values derived from NOAA environmental
satellite imagery. Vegetation index was strongly correlated
with long term average rainfall values and saturation deficit
(humidity) and with important biological variables of vector
populations, such as population density, mortality rate and
size (Rogers & Randolph 1993).
Vector Habitat Nidality - Climate forecasts provide a
comparison of annual variation in Fasciola transmission
on a regional scale, but no provision is made for
potential infection pressure related to the amount of snail
host habitat present on specific premises grazed. The
differential suitability of pastures for lymnaeid snail
habitat can result, like climate variation, in a 100-fold
variation in infection prevalence between individual cattle
operations in a given agroclimatic zone (Malone et al. 1992).
As compared to more ephemeral vectors such as mosquitoes,
snails tend to be present year after year in the same habitats
and population generation times are relatively long. Stocking
rate and the proportion of a farm occupied by habitat have
been considered to be the two most important factors
influencing cost benefit of control of fascioliasis in sheep
in Australia (Meek & Morris 1981). A similar relationship has
been described between snail habitat extent and prevalence of
human schistosomiasis in Iran (Rosenfield 1987).
Soil-hydrology GIS models for estimating farm-specific risk of
fascioliasis in cattle have been developed for use in the
Chenier Plain (Zukowski et al. 1992) and Red River basin
(Malone et al. 1992) ecologic zones of Louisiana. Image
overlays of soil type maps, hydrologic features shown in 7.5'
United States Geologic Survey quadrangle maps and farm
boundaries were compared to snail habitat maps or herd egg
shedding prevalence. Farm boundaries were derived from aerial
photographs or pasture vegetation seen in a post-harvest
Landsat MSS satellite infrared image. Fluke egg shedding
indices (mean number of eggs per 2 g of feces in 12-15 random
samples per herd X prevalence %) were placed in rank order and
iteratively fit to soil-hydrology parameters by regression
analysis. The egg shedding index is an expression of egg
abundance.
In the Red River basin, soil types ranged from sandy loams to
hydric clays. The rank of herd egg shedding indices regressed
significantly against a snail habitat risk factor derived from
the proportion of soils present, slope and the length of
interfaces of pastures with water bodies and other major
hydrologic features. In the chenier plain region, the ranked
egg shedding indices correlated with the area of Hackberry-
Mermentau soils on cheniers (relict beaches), associated
Mermentau soils and the length of chenier-marsh interfaces. A
general association of Fasciola and certain soils has
also been reported in Wales (Wright & Swire 1984). Japanese
workers associated soil type with prevalence of S.
japonicum (Nihei et al. 1981). Such wide variation in
site-specific snail habitat risk must be considered, with
climate data, in GIS models aimed at making treatment and
control decisions for fascioliasis. GIS models based on
environmental features can be expected to be extrapolated with
validity only within the same ecologic zone; different factors
or different weighting of data may be required to adapt risk
indices to other scenarios, such as the altiplano of Bolivia
or the southern Caspian region.
Control of Livestock Populations - Livestock are
ordinarily confined in herds or flocks of known number in
specific grazing areas. Even in unfenced areas, producers can
often identify areas used by animals under their control (Sol-
lod & Stem 1991). This facilitates effective treatment
programs and makes it possible to estimate relative risk of
fascioliasis in a given grazing area by infection prevalence
in resident animal populations. For zoonotic fascioliasis,
pasture and range boundaries could be defined in a GIS, with
livestock prevalence data, and used: (1) to plan and monitor
control based on reducing fecal egg contamination of snail
habitats by animal populations and (2) to identify high risk
areas for watercress and other vegetation used for human
consumption.
Potential Integration of Geographic, Mathematical and Cost-
benefit Analysis Models - Adoption of a geographic
approach by GIS can provide the environmental context for
fascioliasis and a systematic way to evaluate variation in
parasite distribution on both a broad scale and a local scale.
GIS results can then be used with mathematical models that
suggest and compare control strategies on given premises based
on intrinsic life cycle reproduction and mortality rates of
F. hepatica. Mathematical models are relevant, like
the parasite population described, in the context of a given
environment.
Preventive treatment of livestock populations is realistic
only if treatment is of recognized cost-benefit to producers.
An appropriate approach in cost-benefit analysis is to
identify a target economic threshold of infection below which
morbidity or production losses do not occur and then monitor
success based on that criterion. This is currently a weak
link for practical control models: (1) there is limited data
on economic thresholds for fascioliasis in cattle; (2)
monitoring infection levels in control programs is dependent
primarily on unprecise fecal sedimentation egg count data and
(3) the same level of parasitism can have dramatically
different effects on animal production depending on
nutritional state and animal husbandry practices.
Computer expert systems can store and process large quantities
of complex data that enhance the ability of the mind to make
sound judgements and they allow transfer of evaluation
criteria to the novice that were gained by years of field
experience. Available information suggests that geographic
models can be developed, using GIS, that can be integrated
with site-specific mathematical models and cost-benefit
analysis components to construct comprehensive decision-making
systems. Computer systems needed, including complete image
analysis and GIS capabilities, are now available for use at
the microcomputer level at reasonable costs of less than
$25,000.
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Copyright 1995 Fundacao Oswaldo Cruz (Fiocruz)
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