VOLUME 16 NUMBER
6 November/December 2003
Worms and satellites: [PDF]
Brooker S, Beasley M, Ndinaromtan M, Madjiouroum
EM, Baboguel M, Djenguinabe E, Hay SI, Bundy DAP. (Department
of Infectious Diseases Epidemiology, Imperial College School
of Medicine, London, UK; Hôpital Général
de Référence, N’Djamena, Chad; Ministry of
Health and Ministry of Education, N’Djamena, Chad; Department
of Zoology, University of Oxford, Oxford, UK; International School
Health Initiative, Human Development Division, The World Bank,
Washington DC, USA.) Use of remote sensing and a geographical
information system in a national helminth control programme in
Chad. Bull World Health Organ 2002;80:783–9.
This article highlights the utility of the geographical information
system (GIS) and remote sensing method to determine the target
population for helminthic infection. The study was carried out
in Chad which has a diverse climatic and population distribution.
The objective of the study was to provide rapid feedback to healthcare
providers, so that a school-based antihelminthic programme could
be developed at the national level. For this purpose, meteorological
information on the characteristics of soil, land surface temperature,
rainfall, etc. was necessary to predict the distribution of helminths.
However, the meteorological infrastructure in Chad was inadequate
to provide the relevant spatial data on climatic variation for
the whole country. This led to the utilization of remote sensing
data for this purpose. Data for 18 years (1982–2000) were
collected from the National Oceanic Atmospheric Administration’s
(NOAA) American satellite. The annual mean, minimum and maximum
values of land surface temperature, rainfall and vegetation index
were calculated. On the basis of the satellite findings, the
entire country was mapped into 7 ecological zones with the help
of GIS (computer-based technology for spatial mapping) and a
prediction was made of the target population exposed to helminthic
infection. This proxy prediction of the distribution of helminths
was followed by a field survey to test the validity of the former
For the purpose of the field survey, 20 schools were selected
from the country on the basis of the proportion of population
in each ecological zone. From each school, 25 boys and 25 girls
of class III who were 9–10 years of age were randomly selected.
In several schools as it was difficult to get the desired number
of subjects (particularly girls); students from other classes
were also recruited to obtain an adequate sample size. The local
school committee and parents approved the study and the students
participated voluntarily. Stool and urine samples were collected
from each child for the laboratory investigation of helminths.
A total of 1023 students were examined of whom 626 were boys
and 397 were girls; 1017 provided urine samples and 22.5% were
found to have Schistosoma haematobium infection. Out of the 1000
students who gave stool samples, 32.7% were found to be infected
by hookworms. Both S. haematobium and hookworm were present in
7.1% of children. The distribution of S. haematobium and hookworm
showed a significant relationship with the environmental variables.
Among all ecological zones the highest prevalence of S. haematobium
was found in the area where the mean land surface temperature
was maximum and the total rainfall was minimum. The highest prevalence
of hookworm infection was associated with low mean land surface
temperature and high rainfall. The prevalence of hookworm was
lowest or nil with ³47 oC mean land surface temperature.
There was a significant positive association of probability of
infection by S. haematobium in relation to male sex (OR 2.4,
p<0.001), mean land surface temperature (OR 0.66, p<0.001)
and rainfall (OR 1.01, p<0.001). In contrast, there was an
inverse relationship between the probability of getting infected
by hookworm and the mean land surface temperature (OR 0.07, p<0.01).
The GIS is a rapidly emerging computer-based technology that
incorporates graphical features with tabular data to assess the
world’s problems. Its application in solving public health
problems has been encouraged globally, particularly in socioeconomically
poor countries with diverse and complex health problems, rapidly
growing populations and severe resource constraints.1,2 Its role
has already been studied in the surveillance and control of vector-borne
diseases such as malaria and dengue in many countries including
India.3–10 Several other communicable and non-communicable
diseases are being studied.11–15
As the sample units (schools) in this study were derived on the
basis of proportion of population of each ecological zone and
the schools were selected randomly from each zone, a selection
bias in the study sample may have been taken care of. The statistical
tests for significance were appropriately applied and the presentation
of the data was simple and easily comprehensible. But it is not
clear why a sample of 25 boys and 25 girls was selected from
The present study is particularly important because helminthic
infection in children is a public health problem in many developing
countries including India. The prediction of GIS showed a variable
accuracy for different types of helminths. It accurately predicted
that Chad would not have A. lumbricoides and T. trichiura infection
as these two helminths are unlikely to survive at high temperatures
(>37 oC), which was supported by similar findings from the
field survey. However, the prediction for S. haematobium was
less reliable, as the GIS predicted that 9 schools would have
an infection rate exceeding 50%, but the field survey detected
only one such school. However, no prediction model for hookworm
could be developed from this study as there were dissimilarities
between the field survey results and previous experimental studies
on the development of eggs of this particular species of helminth.
This encourages more research on the epidemiology of hookworm
in relation to ecological and climatic differentials.
Though the study provides variable reliability on the prediction
of helminthic distribution, GIS and remote sensing may have a
role in carrying out rapid, valid epidemiological surveys and
control of diseases. It is particularly important in remote,
inaccessible areas of developing countries where the meteorological
and health sector infrastructure is inadequate. It may be helpful
to rapidly implement interventions, bypassing time-consuming
and costly field surveys.17
In India, there is no health programme at the national level
for the control of helminths though it is a serious health problem.
India has a strong satellite network which is being successfully
used in many fields.16,17 However, it has not been used for public
health problems, except to some extent in controlling malaria.7,8 Hence, this tool could be used more often to conduct studies
which could help in planning the control of various endemic diseases.
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Radhakrishana K, Jayaraman
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T. K. Ray
Department of Community Medicine
Maulana Azad Medical College