Spatiotemporal mapping of malaria prevalence in Madagascar using routine surveillance and health survey data

Abstract

Malaria transmission in Madagascar is highly heterogeneous, exhibiting spatial, seasonal and long-term trends. Previous efforts to map malaria risk in Madagascar used prevalence data from Malaria Indicator Surveys. These cross-sectional surveys, conducted during the high transmission season most recently in 2013 and 2016, provide nationally representative prevalence data but cover relatively short time frames. Conversely, monthly case data are collected at health facilities but suffer from biases, including incomplete reporting and low rates of treatment seeking. We combined survey and case data to make monthly maps of prevalence between 2013 and 2016. Health facility catchment populations were estimated to produce incidence rates from the case data. Smoothed incidence surfaces, environmental and socioeconomic covariates, and survey data informed a Bayesian prevalence model, in which a flexible incidence-to-prevalence relationship was learned. Modelled spatial trends were consistent over time, with highest prevalence in the coastal regions and low prevalence in the highlands and desert south. Prevalence was lowest in 2014 and peaked in 2015 and seasonality was widely observed, including in some lower transmission regions. These trends highlight the utility of monthly prevalence estimates over the four year period. By combining survey and case data using this two-step modelling approach, we were able to take advantage of the relative strengths of each metric while accounting for potential bias in the case data. Similar modelling approaches combining large datasets of different malaria metrics may be applicable across sub-Saharan Africa.

Fine-scale variation in malaria prevalence across ecological regions in Madagascar: a cross-sectional study

“…This differs from the pattern typically seen in endemic settings in sub-Saharan Africa where infections are generally most common in younger children and decrease quickly after age 5 [ 34 ]. Also of note is that previous analyses (e.g., [ 4 , 35 ]) of prevalence in Madagascar have focused on the 0–5 year age group and as a result may underestimate population-wide prevalence. The age and sex distribution of infections in the central plateau (CP) region communities sampled could not be determined due to the few RDT positive individuals ( n = 6) observed.…”

Genetic diversity of Plasmodium falciparum populations in three malaria transmission settings in Madagascar

“…One of the major challenges facing policy makers is the tremendous variability of malaria transmission across Madagascar, depending on regional variations in rainfall, temperature and elevation. The country is typically divided into five epidemiological strata: (i) the Equatorial stratum on the east coast, where malaria transmission is highest and perennial; (ii) the Tropical stratum on the west coast, with seasonal transmission spanning around 6 months (October–April); (iii) the Sub-desert stratum in the south, characterized by a dry and hot climate prone to episodic outbreaks; (iv) the Highland; and, (v) Fringes stratum in the centre of the country, where malaria transmission rates are low and unstable between January and April [ 3 , 4 ].…”

“…No specific strategies for malaria control tailored to the epidemiological context have been designed and implemented locally or regionally. Furthermore, the impact of strategies has been assessed only on the basis of the estimated number of malaria cases recorded by hospital and health centre staff and community workers [ 4 ].…”

Genetic Diversity of Plasmodium Falciparum Populations in three Malaria Transmission Settings in Madagascar

“…One of the major challenges to the policy makers is that malaria transmission greatly varies across Madagascar, depending on regional variations in rainfall, temperature and altitude. The country is typically divided into ve epidemiological facies, including (i) the equatorial facies on the east coast, where malaria transmission is the highest and perennial, (ii) the tropical facies on the west coast, with a seasonal transmission spanning around six months (October-April), (iii) the sub-desert facies in the south characterized by a dry and hot climate prone to episodic outbreaks and (iv) the highland and the fringes facies in the center of the country, where malaria transmission is low and unstable between January and April [3,4].…”

“…No speci c and tailored strategies for malaria control are designed and implemented at regional or local levels according to the epidemiological context. Furthermore, the impact of the strategies is assessed only on estimates of the malaria cases recorded by hospital and health center staff and malaria community workers [4].…”