Exploring 30 years of malaria case data in KwaZulu-Natal, South Africa: part I. The impact of climatic factors.

Large parts of Africa are prone to malaria epidemics. Advance epidemic warning would give health services an opportunity to prepare. Because malaria transmission is largely limited by climate, climate-based epidemic warning systems are a real possibility. To develop and test such a system, good long-term malaria and climate data are needed. In KwaZulu-Natal (KZN), South Africa, 30 years of confirmed malaria case data provide a unique opportunity to examine short- and long-term trends. We analysed seasonal case totals and seasonal changes in cases (both log-transformed) against a range of climatic indicators obtained from three weather stations in the highest malaria incidence districts, using linear regression analysis. Seasonal changes in case numbers (delta log cases, dlc) were significantly associated with several climate variables. The two most significant ones were mean maximum daily temperatures from January to October of the preceding season (n=30, r2=0.364, P=0.0004) and total rainfall during the current summer months of November-March (n=30, r2=0.282, P=0.003). These two variables, when entered into the same regression model, together explained 49.7% of the total variation in dlc. We found no evidence of association between case totals and climate. In KZN, where malaria control operations are intense, climate appears to drive the interannual variation of malaria incidence, but not its overall level. The accompanying paper provides evidence that overall levels are associated with non-climatic factors such as drug resistance and possibly HIV prevalence.

Exploring 30 years of malaria case data in KwaZulu-Natal, South Africa: part II. The impact of non-climatic factors.

Malaria transmission is a multifactorial phenomenon. Climate is a major limiting factor in the spatial and temporal distribution of malaria, but many non-climatic factors may alter or override the effect of climate. Thirty years of monthly malaria incidence data from KwaZulu-Natal province, South Africa, reveal strong medium and long-term trends, which were not present in the climate data. This paper explores various non-climatic factors that may have contributed towards the observed trends. The development of antimalarial drug resistance, available information on human immunodeficiency virus (HIV) prevalence, cross-border people movements, agricultural activities, emergence of insecticide resistance and the case reporting system are reviewed and their potential effect on malaria transmission examined. Single-variable linear regression analysis showed significant association between seasonal case totals (log-transformed) and the measured level of drug resistance (log-transformed) (r2=0.558, n=10, P=0.013) as well as relative measures of HIV infection since 1990 (r2=0.846, n=11, P=0.001). The other factors appear to have affected the level of malaria transmission at certain periods and to some degree. The importance of surveillance and inclusion of non-climatic variables in analysis of malaria data is demonstrated.

Malaria control--two years' use of insecticide-treated bednets compared with insecticide house spraying in KwaZulu-Natal.

OBJECTIVES: The objective of this study was to produce data indicating whether insecticide-treated bednets should replace insecticide house spraying as a malaria control method in South Africa. We report 2 years of preliminary data on malaria incidence comparing areas receiving insecticide-treated bednets and those subjected to house spraying in northern KwaZulu-Natal. DESIGN, SETTING AND SUBJECTS: In order to measure significant reductions in malaria incidence between the two interventions, a geographical information system (GIS) was used to identify and create seven pairs of geographical blocks (areas) in the malaria high-risk areas of Ndumu and Makanis in Ingwavuma magisterial district, KwaZulu-Natal. Individual blocks were then randomly allocated to either insecticide-treated bednets or house spraying with deltamethrin. Malaria cases were either routinely recorded by surveillance agents at home or were reported to the nearest health facility. RESULTS AND CONCLUSIONS: The results show that 2 years' use of insecticide-treated bednets by communities in Ndumu and Makanis, KwaZulu-Natal, significantly reduced the malaria incidence both in 1997 (rate ratio (RR) = 0.879, 95% confidence interval (CI) 0.80-0.95, P = 0.04) and in 1998 (RR = 0.667, CI 0.61-0.72, P = 0.0001). Using a t-test, these significant reductions were further confirmed by an assessment of the rate of change between 1996 and 1998, showing a 16% reduction in malaria incidence in blocks using treated bednets and an increase of 45% in sprayed areas (t = 2.534, P = 0.026 (12 df)). In order to decide whether bednets should replace house spraying in South Africa, we need more data on the efficacy of treated bednets, their long-term acceptability and the cost of the two interventions.

A spatial statistical approach to malaria mapping.

BACKGROUND: Good maps of malaria risk have long been recognized as an important tool for malaria control. The production of such maps relies on modelling to predict the risk for most of the map, with actual observations of malaria prevalence usually only known at a limited number of specific locations. Estimation is complicated by the fact that there is often local variation of risk that cannot be accounted for by the known covariates and because data points of measured malaria prevalence are not evenly or randomly spread across the area to be mapped. METHOD: We describe, by way of an example, a simple two-stage procedure for producing maps of predicted risk: we use logistic regression modelling to determine approximate risk on a larger scale and we employ geo-statistical ('kriging') approaches to improve prediction at a local level. Malaria prevalence in children under 10 was modelled using climatic, population and topographic variables as potential predictors. After the regression analysis, spatial dependence of the model residuals was investigated. Kriging on the residuals was used to model local variation in malaria risk over and above that which is predicted by the regression model. RESULTS: The method is illustrated by a map showing the improvement of risk prediction brought about by the second stage. The advantages and shortcomings of this approach are discussed in the context of the need for further development of methodology and software.

Distribution of African malaria mosquitoes belonging to the Anopheles gambiae complex.

The distribution of malaria vector mosquitoes, especially those belonging to species complexes that contain non-vector species, is important for strategic planning of malaria control programmes. Geographical information systems have allowed researchers to visualize distribution data on maps together with environmental parameters, such as rainfall and temperature. Here, Maureen Coetzee, Marlies Craig and David le Sueur review our current knowledge on the distribution of the members of the Anopheles gambiae complex.

A preliminary continental risk map for malaria mortality among African children.

Approaches to global public health are increasingly driven by an understanding of regional patterns of disease-specific mortality and disability. Current estimates of disease risks associated with Plasmodium falciparum in sub-Saharan Africa remain poorly defined. Through the integration of high-resolution population and climate probability models of P. falciparum transmission, geographical information systems have been used to define the spatial limits of populations exposed to the risk of infection in Africa. These estimates were combined with a range of annual malaria-specific mortality rates, derived from a variety of epidemiological approaches, among children aged 0-4 years. The best estimates of malaria-attributable mortality using this approach ranged between 0. 43 million and 0.68 million deaths per annum among an exposed population of approximately 66 million children in 1990. Despite the limitations of modelled transmission and population distributions, these empirical approaches to probabilities of infection risk and epidemiological data on mortality provide a novel approach to present and projected burdens of malaria mortality, as discussed here by Bob Snow, Marlies Craig, Uwe Deichmann and Dave le Sueur.

A climate-based distribution model of malaria transmission in sub-Saharan Africa.

Malaria remains the single largest threat to child survival in sub-Saharan Africa and warrants long-term investment for control. Previous malaria distribution maps have been vague and arbitrary. Marlies Craig, Bob Snow and David le Sueur here describe a simple numerical approach to defining distribution of malaria transmission, based upon biological constraints of climate on parasite and vector development. The model compared well with contemporary field data and historical 'expert opinion' maps, excepting small-scale ecological anomalies. The model provides a numerical basis for further refinement and prediction of the impact of climate change on transmission. Together with population, morbidity and mortality data, the model provides a fundamental tool for strategic control of malaria.

Mapping malaria transmission intensity using geographical information systems (GIS): an example from Kenya.

That there are so few examples of the use of epidemiological maps in malaria control may be explained by the lack of suitable, spatially defined data and of an understanding of how epidemiological variables relate to disease outcome. However, recent evidence suggests that the clinical outcomes of infection are determined by the intensity of parasite exposure, and developments in geographical information systems (GIS) provide new ways to represent epidemiological data spatially. In the present study, parasitological data from 682 cross-sectional surveys conducted in Kenya were abstracted and spatially defined. Risks of infection with Plasmodium falciparum among Kenyan children, estimated from combinations of parasitological, geographical, demographic and climatic data in a GIS platform, appear to be low for 2.9 million, stable but low for another 1.3 million, moderate for 3.0 million and high for 0.8 million. (Estimates were not available for 1.4 million children.) Whilst the parasitological data were obtained from a variety of sources across different age-groups and times, these markers of endemicity remained relatively stable within the broad definitions of high, moderate and low transmission intensity. Models relating ecological and climatic features to malaria intensity and improvements in our understanding of the relationships between parasite exposure and disease outcome will hopefully provide a more rational basis for malaria control in the near future.

Models to predict the intensity of Plasmodium falciparum transmission: applications to the burden of disease in Kenya.

There is an increasing need to provide spatial distribution maps of the clinical burden of Plasmodium falciparum malaria in Africa. Recent evidence suggests that risk groups and the clinical spectrum of severe malaria are related to the intensity of P. falciparum transmission. Climate operates to affect the vectorial capacity of P. falciparum transmission and this is particularly important in the Horn of Africa and parts of East Africa. We have used a fuzzy logic climate suitability model to define areas of Kenya unsuitable for stable transmission. Kenya's unstable transmission areas can be divided into areas where transmission potential is limited by low rainfall or low temperature and, combined, encompass over 8 million people. Among areas of stable transmission we have used empirical data on P. falciparum infection rates among 124 childhood populations in Kenya to develop a climate-based statistical model of transmission intensity. This model correctly identified 75% (95% confidence interval CI 70-85) of 3 endemicity classes (low, < 20%; high, > or = 70%; and intermediate parasite prevalences). The model was applied to meteorological and remote sensed data using a geographical information system to provide estimates of endemicity for all of the 1080 populated fourth level administrative regions in Kenya. National census data for 1989 on the childhood populations within each administrative region were projected to provide 1997 estimates. Endemicity-specific estimates of morbidity and mortality were derived from published and unpublished sources and applied to their corresponding exposed-to-risk childhood populations. This combined transmission, population and disease-risk model suggested that every day in Kenya approximately 72 and 400 children below the age of 5 years either die or develop clinical malaria warranting in-patient care, respectively. Despite several limitations, such an approach goes beyond 'best guesses' to provide informed estimates of the geographical burden of malaria and its fatal consequences in Kenya.

An atlas of malaria in Africa.

PIP: The Mapping Malaria Risk in Africa (MARA) project will use computerized geographic information systems (GIS) to create an atlas of malaria risk in Africa. This is the first time GIS will be used to predict such risk and the first attempt to map malaria risk on a continental scale. Many heterogenous data sets relevant to the transmission dynamics of the disease will be combined, including the manipulation of the climatic factors which affect vector distribution and malaria transmission into a new index of malaria risk, and its validation against actual data. Specifically, the MARA project will collate measures of malaria risk, mainly the parasite ratio and annual incidence, to obtain an index for Africa and to create a continental, spatial, and temporal database. Five planned regional centers will work with a coordinating center in Durban, South Africa. The effort recently received its first funding. Canada's International Development Research Center, the World Health Organization/TDR bednet task force, the Wellcome Trust, and the South African Medical Research Council are supporting the effort.^ieng

Schistosoma mansoni in migrants entering South Africa from Moçambique--a threat to public health in north-eastern KwaZulu-Natal?

OBJECTIVES: To identify possible public health consequences of Schistosoma mansoni infections in migrants entering north-eastern KwaZulu-Natal and Mpumalanga and Northern Province from southern Moçambique. DESIGN: (i) Intestinal parasite surveys, one sample per person, within a 4-month period; (ii) temperature recordings and snail collections in an irrigation system. SETTING: North-eastern KwaZulu-Natal and Kruger National Park, Mpumalanga and Northern Province. PARTICIPANTS: Ninety-seven migrants (1-68 years) from north-eastern KwaZulu-Natal and 47 from the Kruger Park (1 - 70 years). MAIN OUTCOME MEASURES: The relatively high S. mansoni prevalence may cause problems on the rice paddy scheme in north-eastern KwaZulu-Natal. Ranges and prevalence rates of other parasites were recorded. RESULTS: S. mansoni prevalence in Moçambicans entering north-eastern KwaZulu-Natal was 14.4%, seven times more than in local residents. S. mansoni is not endemic to this region because most bodies of water become too warm for either the parasite or its snail host, Biomphalaria pfeifferi, to develop. Recordings in paddies suggest, however, that the rice modifies the temperature pattern as it grows and provides a suitable habitat for transmission. Migrants entering the Transvaal lowveld where S. mansoni is endemic may become more severely infected. Thirteen other parasite species were recorded from migrants entering KwaZulu-Natal and 8 from those entering Mpumalanga and Northern Province. CONCLUSIONS: The relatively high S. mansoni prevalence among migrants entering north-eastern KwaZulu-Natal may have public health implications as it could encourage transmission in a non-endemic area. The range of parasites carried by migrants entering KwaZulu-Natal, Mpumalanga and Northern Province was similar to that harboured by local people but prevalence rates were generally lower.

[Malaria in South Africa: past, present and perspectives]. / Le paludisme en Afrique du Sud: passé, présent et perspectives.

This paper outlines a brief historical perspective on malaria which is considered essential to an understanding of the gains made in the control of the disease, followed by an emphasis on the fact that control is a dynamic process requiring research back-up, private and public sector and national and international collaboration. Malaria control is based on scientific principles and ongoing success requires continual research input, government commitment to control of the disease and appropriately skilled and trained personnel. This overview cannot do justice to malaria control and research in South Africa in its entirety, but looks at some of the major factors facing malaria control that have motivated the Medical Research Council's research initiative, which includes vector and parasite research, the use of geographical information systems and the epidemiology of the disease, with a view to sustaining control in the future.

Assessment of the residual efficacy of lambda-cyhalothrin. 2. A comparison with DDT for the intradomiciliary control of Anopheles arabiensis in South Africa.

There are several factors that support the need to assess the efficacy of potential alternative insecticides to DDT for malaria vector control. The objectives of this study were to evaluate the persistence and efficacy against Anopheles arabiensis of lambda-cyhalothrin used as an intradomiciliary insecticide in daub huts and to compare its efficacy in this regard to DDT. Exit trap catches showed the population of An. arabiensis was high during the months of January to March, with a peak in February. During all months, the number caught leaving lambda-cyhalothrin-sprayed huts was markedly less than the number from both control and DDT-sprayed huts. The percentage survival of bloodfed mosquitoes ranged from a low of 55% caught leaving the lambda-cyhalothrin-sprayed huts, to 82% of those caught leaving DDT-sprayed huts. The percentage of bloodfed mosquitoes caught leaving huts was high (> 60%). The survival of unfed mosquitoes was low, even from the control huts (43%).

Assessment of the residual efficacy of lambda-cyhalothrin. 1. A laboratory study using Anopheles arabiensis and Cimex lectularius (Hemiptera: Cimicidae) on treated daub wall substrates from Natal, South Africa.

Laboratory assessment of the residual efficacy of lambda-cyhalothrin (Icon) 10% AI against Anopheles arabiensis and Cimex lectularius was carried out. The insecticide was applied to daub substrates, simulating the wall surface of houses from 3 areas within the endemic malaria area of Natal, South Africa. Variability in residual efficacy was found between different areas and appeared to correlate to organic content of the substrate. Residual efficacy against An. arabiensis ranged from as little as 2 wk in some areas to in excess of 14 wk in others. Residual efficacy against C. lectularius was 4 wk for all 3 areas, but was 10 wk in only 2 areas.

Temperature-dependent variation in Anopheles merus larval head capsule width and adult wing length: implications for anopheline taxonomy.

Seasonal variations in mosquito larval head capsule width and adult female wing length were investigated in a field population of Anopheles merus Dönitz at Nceswana Lake, Ophansi, within the endemic malaria area of Natal, South Africa. An inverse relationship was detected between each of these morphological characters and seasonal fluctuations in air/water temperatures. Mean head capsule width in all larval instars decreased by 4.8-7.9% in summer, while mean wing length decreased by 19.6%. These changes are discussed in relation to the annual range in mean air temperature in southern Africa and the distribution of An.merus. Implications for the use of such morphological characteristics in existing taxonomic keys are discussed.