Planning Disease Control in Cameroon, West Africa
Simon Brooker, Simon I. Hay, Louis-Albert Tchuem Tchuente
1995a). Praziquantel is used to treat the schistosomes (Schisto-
and S. mansoni
), and intestinal nema-
Human helminth infections (intestinal nematode infections
todes (Ascaris lumbricoides
, Trichuris trichiura
Trichuris trichiura, and hook-
are treated by the benzimidazole drugs, albendazole and meb-
worm, and schistosome infections such as
endazole (WHO, 1995b). Studies have shown that these treat-
S. mansoni) affect more than a quarter of
ments can be safely and effectively combined (Savioli et al.
the world’s population, with potential consequences for the
1997; Olds et al.
, 1999), and the WHO recommends joint deliv-
health and nutritional and educational development of infected
ery in areas where both groups of parasites occur (WHO,
individuals. The advent of broad-spectrum anthelminthic
1995a). It has been suggested that the overlap in the geographi-
drugs that are cheap, safe, and simple to deliver has meant
cal distribution of each group of species is sufficiently large to
that control has become a viable option for many countries.
justify combined treatment (Bundy et al.
, 1991), but more de-
Because helminth infections patterns are highly heterogeneous,
tailed analysis suggests uneven and often non-overlapping dis-
methods to identify priority areas for intervention against
tributions within countries (Brooker et al.
, 1999). This indi-
intestinal nematode and schistosome will enhance the efficacy
cates a more refined approach to combined control is required,
of control. This paper describes the use of NOAA-AVHRR data
whereby target communities are identified separately for inter-
to develop logistic regression models that predict the prob-
vention against schistosomes and intestinal nematodes, and
ability of infection prevalence greater than 50 percent, and
drugs are distributed according to local needs, thus reducing
thus warrant mass treatment for intestinal nematodes and
delivery costs and the prospect of drug resistance.
schistosomes, according to WHO’s criteria. Moreover, by
In an effort to better understand the distribution of species,
overlaying the resulting risk maps on population surfaces, it
geographic information systems (GIS) are increasingly being
is possible to estimate the school-aged population size
used to collate and map available helminth survey data avail-
requiring mass treatment and also provide an estimate of
able from the formal and “grey” literature (Brooker et al.
2000a). Such information on the distribution of infection willbe central to successfully addressing the key operational ques-
tions of reliably estimating the target population numbers at
More than fifty years ago, a seminal paper entitled “This
risk (Brooker et al.
, 2000b), stratifying areas by prevalence to
Wormy World” (Stoll, 1947) showed that helminth infections,
prioritize areas for control, and estimating overall drug needs
including schistosome and intestinal nematode species, were
and costs. Although there is comprehensive information on
among the most common of human infections. Today, these
helminth distributions in some African countries, empirical
infections still affect more than a quarter of the world’s popula-
survey data are available for only a third of administrative dis-
tion (Chan et al.
, 1994; Bundy, 1997), with potential conse-
tricts across the continent (Brooker et al.
quences for children’s physical and intellectual development
To help fill the gap in empirical data, remotely sensed (RS)
(Stephenson, 1987; Watkins and Pollitt, 1997).
satellite sensor data and interpolated meteorological surfaces
The advent of broad-spectrum anthelminthic drugs that
are being used to predict the distributions of a variety of infec-
are cheap, safe, and simple to deliver has meant helminth con-
tious diseases (Malone et al.
, 1997; Hay et al.
, 2000; Rogers,
trol has become a viable option for many countries. The World
2000; Lindsay and Thomas, 2000; Malone et al.
, 2001). For hel-
Health Organization (WHO) presently recommends mass an-
minth species, years of field studies have documented the
thelminthic treatment in areas where infection prevalence (pro-
influence of climate and environmental variables on the distri-
portion of community infected) is 50 percent or greater (WHO,
bution of helminth infections (Appleton, 1978; Brown, 1994;Crompton, 1994), and RS-derived environmental variables andmeteorological variables are of potential use in predicting the
S. Brooker is with the Department of Infectious Disease Epide-
occurrence of significant transmission (Brooker and Michael,
miology, Imperial College School of Medicine, Norfolk Place,
2000). The present study uses environmental data derived
London WG 1PG, United Kingdom (email@example.com).
from meteorological satellite sensors and interpolated meteoro-
S. Hay is with the Trypanosomiasis and Land Use in Africa(TALA) Research Group, Department of Zoology, Universityof Oxford, South Parks Road, Oxford OX1 3PS, United Kingdom.
Photogrammetric Engineering & Remote Sensing
L.-A. Tchuem Tchuente´ is with the Centre for Schistosomiasis &
Vol. 68, No. 2, February 2002, pp. 175–179.
Parasitology, P.O. Box 7244, Yaounde´, Cameroon.
R. Ratard is with the Louisiana Department of Health and Hos-pitals, Office of Public Health, 1201 Capitol Access Road, P.O.
᭧ 2002 American Society for Photogrammetry
Box 629, Baton Rouge, LA 70821-0629.
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
logical data to model the distribution of schistosome and intes-
Logistic regression models were developed to identify signifi-
tinal nematode infections in Cameroon. These distributions are
cant environmental variables affecting the transmission of
then used to quantify the population requiring treatment and
infection. A potential problem in developing regression mod-
to estimate the financial costs of school-based programs using
els using environmental vari-ables is that many are highly inter-
single species or combined control options.
correlated so that it is difficult to separate the effects of theindependent variables statistically (Morgenstern, 1998). Toreduce the dimensionality of these collinear variables, we first
selected those variables likely to have greater biological signifi-
cance on infection transmission (Brooker and Michael, 2000).
Prevalence data were collected during a nation-wide survey of
Second, the remaining variables were added to the models in a
helminth infection in 1985–1987 originally aggregated at the
stepwise fashion, and the statistical fits of alternative models
district level (Ratard et al.
, 1990; Ratard et al.
, 1991). These data
were compared using the residual deviance of models includ-
are stratified at the school level here because schoolchildren
ing and excluding correlated variables using a 2 distribution
are the primary targets for treatment and the educational infra-
(Venables and Ripley, 1999). Analysis was done using S-Plus
structure is usually used to deliver treatment. The original
4.5 Professional Release 2 (Math Soft, Seattle, Washington).
study was designed to provide data on the nationwide distribu-
The best-fit logistic regression models were then used to
tion of helminth infections using a stratified, random-cluster
map the probability of infection prevalence being greater than
sampling procedure with the primary school as the basic sam-
50 percent using Idrisi Version 2 (The Idrisi Project, Worcester,
pling unit. In the north of the country where school enrollment
Massachusetts). To define whether a district would be a prior-
was low, all children from the appropriate age group living in
ity area for control, we have used an arbitrary criteria based on
the community were invited to participate (Ratard et al.
whether the average logistic regression probability is greater
Urine and stool samples were examined by sedimentation and
than 0.5 within a district. On this basis, the number of school-
the Kato-Katz thick smear technique, respectively. The loca-
aged children who would receive mass treatment for intestinal
tion of schools was obtained by transcribing coordinates from
nematodes was estimated. The population size for praziquantel
1:25,000-scale maps used in the original survey. Data on intes-
and albendazole treatment was calculated by overlaying the
tinal nematode species (Ascaris lumbricoides
predictive maps of infection prevalence on a population map.
) for 18,260 school children aged 10 to 19 years in 402
For albendazole we used a combined estimate of infection pre-
schools and data on schistosome species (Schistosoma haema-
diction for either A. lumbricoides
or T. trichiura
) for 19,524 children in 303 schools were collected.
Detailed prospective cost analyses have been conducted for
Land Surface Temperature (LST) and the n
school-based anthelmintic programs in Ghana and Tanzania
ndex (NDVI) information were derived from the
(Partnership for Child Development, 1999a). The cost of mass
Advanced Very High Resolution Radiometer (AVHRR) on board
distribution of a single dose of albendazole for intestinal nema-
the National Oceanic and Atmospheric Administration’s
todes to schoolchildren by their teachers was US$0.03 in both
(NOAA) polar-orbiting meteorological satellites (Cracknell,
countries. The cost of delivering praziquantel for schisto-
1997) using Price (1984) and Tucker (1979) procedures, respec-
somes—which required targeting schools by a questionnaire,
tively. Daily data at 8- by 8-km spatial resolution were first pro-
and required a calculation to determine the dose based on the
cessed for the period 1985 through 1998 to exclude unreliable
height of the child—was US$0.67 in Ghana and US$0.21 in
pixels due to extreme sun and sensor viewing angles and cloud
Tanzania. The figures for Tanzania and Ghana were used to
contamination (see Hay and Lennon, 1999). Single monthly
estimate the lower and upper costs of implementing a school-
images were then maximum-value composited (Holben, 1986).
based helminth control program in Cameroon.
Minimum, mean, and maximum values of these data wereextracted for each pixel that corresponded to the location of the
parasitological surveys. Image processing was performed
using the Earth Resources Data Analysis System (ERDAS) Imag-
A number of different logistic regression models were fit to the
ine 8.4 (ERDAS, Inc., Atlanta, Georgia).
data, and residual deviances were compared to identify thebest-fit models. The variables available to the regression analy-
sis were mean, minimum, and maximum LST and NDVI; total
Interpolated rainfall surfaces were taken from the Spatial Char-
annual rainfall; and altitude. Studies show that maximum tem-
acterization Tool (Corbett and O’Brien, 1997), and an interpo-
perature is an important variable in determining helminth dis-
lated digital elevation model (DEM) of Africa was obtained from
tribution because of the effect of heat and low humidity on the
the Global Land Information System (GLIS) of the United States
embryonation, development, and survival of free-living infec-
Geological Survey (EROS Data Center, 1996).
tive stages and intermediate hosts (Brooker and Michael, 2000).
Consequently, this variable was entered into the regression
model first; next, minimum and mean LST were included and
District population data were derived from a 1990 national
the additional model improvement was assessed. Added next
population forecast based on the 1987 national census (Deich-
to the model analysis was NDVI (minimum, maximum, and
mann, 1996), and were projected to 2001 using annual specific
mean), rainfall, and altitude. The derived species-specific
growth rates obtained from the United States Census Bureau
models (Table 1) indicate (1) the importance of maximum LST,
(2) the influence of rainfall, and (3) the influence of NDVI.
The results for A. lumbricoides
and T. trichiura
negative effect of maximum LST. For helminth species, temper-
To examine the relationship between environmental variables
ature is a density-independent factor effecting parasite trans-
and the need for mass treatment, schools were classified as
mission, as measured by the basic reproductive number (R0)
having prevalence above or below 50 percent, WHO’s treatment
(Anderson and May, 1991; Brooker and Michael, 2000), and
threshold (WHO recommends that mass treatment is warranted
thus observed patterns of infection prevalence. As temperature
if the prevalence in a school exceeds 50 percent infection).
increases, transmission and infection prevalence decrease.
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
REGRESSION COEFFICIENTS DESCRIBING THE LOGISTIC REGRESSION
no districts would warrant mass treatment at the 50 percent
MODELS. LST ϭ LAND SURFACE TEMPERATURE; NDVI ϭ NORMALIZED
threshold. However, because there will be heterogeneity of
prevalence within districts and there should be flexibility inthe treatment thresholds to suit local needs, the analysis was re-
run using a 20 percent prevalence threshold. On this basis, we
estimate that 1.8 million school-aged children in nine districts
would receive mass treatment with praziquantel. For S.
, an effective approach to help locate high-risk
communities/schools requiring mass treatment within at-risk
areas has been the use of blood in urine questionnaires (Red
Urine Group, 1995; Partnership for Child Development,
1999b). To identify priority areas for questionnaire surveys, the
present broad-scale ecological predictions can provide rele-
Estimates of treatment costs were developed based on the
predicted target population size. Using the Tanzania cost data
as a lower estimate and the Ghana cost data as an upper esti-mate, we suggest that the cost of control for A. lumbricoides
and T. trichiura
(using albendazole) would be US$ 0.18 millionand for S. haematobium
(using praziquantel) would be
In contrast to the models for A. lumbricoides
and T. trichi-
, maximum LST has a positive effect on S. haematobium
. InCameroon, the predominant snail species are Bulinus senega-
in the north of the country and Bu. globosus
These results indicate that RS and meteorological data offer the
in the south (Wright, 1959; Greer et al.
, 1990). The
opportunity to investigate the distribution of intestinal nema-
former is found principally in semi-permanent water bodies
tode and schistosome infection and some of the ecological fac-
and can survive the dry season by aestivation. By contrast, the
tors that limit transmission, for purposes of predicting infec-
two southern species tend to occur in more permanent water
tion distributions. This will prove valuable for health planners
bodies. These features of snail distributions suggest that the
in the majority of low-income countries where there is a lack of
model for S. haematobium
is in fact predicting the distribution
detailed empirical survey data. Equally important for national
of Bu. Senegalensis
. Specifically, the model is predicting habi-
health planning, the results will help provide estimates of con-
tats suitable for this species, i.e., areas with semi-permanent
trol program costs of delivering anthelmintics through the
water bodies which arose from periodic drying out due to high
school system. This has important implications for the effi-
temperatures. Moreover, the rarity of water points in the north
cient allocation of scarce health resources.
leads to a concentration of human water contacts with fewer
Previously, the sensitivity of helminth transmission to cli-
water points available, thus increasing the risk of transmis-
mate variation, and the use of models using NOAA-AVHRR data
sion. In the south, by contrast, human water contacts are more
to predict infection patterns, have been described for S. man-
dispersed among numerous water bodies, decreasing the risk
(Malone et al.
, 1994; Malone et al.
, 2001). In Egypt,
Malone et al.
(1994) used 1-km resolution data to derive mapsof diurnal temperature differences (dT), which indicate sur-
Estimates of Treatment Population Size and Program Costs
face and sub-surface moisture contained in soil and plant can-
The best-fit regression models were then used to generate prob-
opy and hence may act as a surrogate for the abundance of the
ability maps of infection prevalence greater than 50 percent for
snail vector, Biomphalaria alexandrina
, whereby wetter and
, A. lumbricoides
, and T. trichiura
more suitable habitats for Bi. alexandrina
These maps indicate that different areas would warrant mass
lower dT values. They found that low values of dT are associ-
treatment with albendazole than those requiring mass treat-
ated with increased snail abundance in wet areas with a slow
ment with praziquantel—combined control would not be jus-
current flow, and is closely mirrored in the patterns of S. man-
prevalence. Malone et al.
(2001) also used 1-km AVHRR
We estimate that 5.8 million school-aged children in 33 of
data to produce maps of LST and NDVI to study the distribution
49 districts in Cameroon would warrant mass treatment with
of S. mansoni
in Ethiopia. They found that annual composite
albendazole. Using the model for S. haematobium
, we estimate
maximum LST values of 20 to 33ЊC and wet season values of 18to 29ЊC defined the distribution of S. mansoni
prevalencegreater than 5 percent in Ethiopia, and used these limits to pre-dict infection risk within the country. In an analysis of surveydata from Tanzania (Brooker et al.
, 2001), we have used 8-kmAVHRR to develop predictive models of S. haematobium
. Wefound that the model allows reasonable discrimination be-tween high- and low-prevalence schools, at least within thosegeographical areas in which they were originally developed,and performs reasonably well in other coastal areas, but per-forms poorly in comparison in the Great Lakes area of Tanzania.
Despite these applications for S. mansoni
and S. haematob-ium
, we believe that there are no published studies using satel-lite sensor data to predict distributions of intestinal nematodesin Africa.
Figure 1. Predicted probability of having infection preva-
Although the present analysis uses the example of Camer-
lence greater than 50 percent. (a) S. haematobium
. (b) A.
oon because of the geographically detailed data available for
. (c) T. trichiura
the country, the approach can be extended to other countries inAfrica. The potential of such an approach will, however, re-
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
main undefined until further studies are undertaken which
Appleton, C.C., and E. Gouws, 1996. The distribution of common
intestinal nematodes along an altitudinal transect in Kwa-Zulu
consider several issues. The first issue relates to the problem of
Natal, South Africa, Annals of Tropical Medicine and Parasitol-
spatial scale (Wiens, 1989; Levin, 1992; Walsh et al.
, 1999). The
problem is that many biological responses are scale-dependent(Wiens, 1989), and observed associations between disease
Brown, D.S., 1994. Freshwater Snails of Africa and Their Importance
Taylor and Francis, London, England, 608 p.
transmission and environmental variables vary as the scalechanges. In the context of helminth control, the rationale for
Brooker, S., M. Booth, and H. Guyatt, 1999. Comparisons of schisto-
prediction is to provide information on the spatial patterns of
some and geohelminth infection prevalence in school-aged chil-
infection and disease at the administrative level at which con-
dren from selected areas of Africa: Implications for rapid assess-ment and combined control, Transactions of the Royal Society of
trol resources are likely to be mobilized, usually the district
Tropical Medicine and Hygiene
level. Furthermore, although RS data are available at fine spatialscales, the satellite systems most widely used in the RS com-
Brooker, S., and E. Michael, 2000. The potential of geographical infor-
mation systems and remote sensing in the epidemiology and con-
munity are those with a broad spatial scale of 1 to 8 km. We used
trol of human helminth infections, Advances in Parasitology
AVHRR 8-km satellite sensor data to model the probability of
areas having infection of 50 percent or greater and warrantingmass treatment with anthelmintics. Other studies conducted
Brooker, S., M. Rowlands, L. Haller, L. Savioli, and D.A.P. Bundy,
2000a. Towards an atlas of human helminth infection in sub-
at different scales may reach different conclusions (Walsh et
Saharan Africa: The use of geographical information systems (GIS),
A further issue is that different environmental variables
may impact upon helminth transmission in different areas
Brooker, S., C.A. Donnnelly, and H. Guyatt, 2000b. Estimating the
number of helminthic infections in the Republic of Cameroon
(Appleton and Gouws, 1996; Brooker and Michael, 2000) and,
from data on infection prevalence in schoolchildren, Bulletin of
in the case of schistosomiasis, different snail species may be
the World Health Organization
differently affected by environmental variability responsiblefor disease transmission (Malone et al.
, 2001; Brooker et al.
Brooker, S., S.I. Hay, W. Issae, A. Hall, C.M. Kihamia, N.J.S. Lwambo,
W. Wint, D.J. Rogers, and D.A.P. Bundy, 2001. Predicting the
2002). The development of separate or modified models of spa-
distribution of urinary schistosomiasis in Tanzania using satellite
tial distribution of infection will provide the basis for a wider
sensor data, Tropical Medicine and International Health
and more detailed analysis of the population size at risk of
infection and allow for the more targeted and rational imple-
Brooker, S., S.I. Hay, and D.A.P. Bundy, 2002. Tools from ecology to
mentation of control programs in Africa. The challenge lies
evaluate infection risk models, Trends in Parasitology
, in press.
however in defining the spatial envelope in which developedmodels can be applied and where different models are re-
Bundy, D.A.P., 1997. This wormy world—then and now, Parasitology
quired. This is an area of ongoing research.
In summary, the models developed here provide health
Bundy, D.A.P., S.K. Chandiwana, M.M.A. Homeida, S. Yoon, and K.E.
planners with a means of predicting the geographical distribu-
Mott, 1991. The epidemiological implications of a multiple-infec-
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With the development of further models, the approach couldprovide, for the whole of Africa, reasonable predictions of the
Chan, M.-S., G.F. Medley, D. Jamison, and D.A.P. Bundy, 1994. The
need to mass treat with albendazole and praziquantel, and
evaluation of potential global morbidity attributable to intestinalnematode infections, Parasitology
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Corbett, J.D., and R.F. O’Brien, 1997. The Spatial Characterization
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, Texas Agricultural Experiment Station, Texas
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Crompton, D.W.T., 1994. Ascaris lumbricoides, Parasitic and Infectious
providing statistical and GIS advice. We are also grateful to Don
Diseases: Epidemiology and Ecology
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work in Cameroon was funded by USAID. S. Brooker and S.I.
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Hay are supported by a Wellcome Trust Prize Fellowship
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(#062692) and Wellcome Trust Advanced Training Fellowship
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(#056642), respectively. Data used in this study include data
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Blackburn with Darwen Borough Council Initial Screening When should I do an Equality Impact Assessment (EIA)? An EIA should be done when revising, introducing or measuring a new activity . What is an activity? ‘ activity ’ throughout the document EIA Flow Chart Service details / overview (Complete sections 1 – 2) Initial Screening (Complete se