Spatial prediction of Plasmodium falciparum prevalence in Somalia

Abstract

Background: Maps of malaria distribution are vital for optimal allocation of resources for anti-malarial activities. There is a lack of reliable contemporary malaria maps in endemic countries in sub-Saharan Africa. This problem is particularly acute in low malaria transmission countries such as those located in the horn of Africa.

Methods: Data from a national malaria cluster sample survey in 2005 and routine cluster surveys in 2007 were assembled for Somalia. Rapid diagnostic tests were used to examine the presence of Plasmodium falciparum parasites in finger-prick blood samples obtained from individuals across all age-groups. Bayesian geostatistical models, with environmental and survey covariates, were used to predict continuous maps of malaria prevalence across Somalia and to define the uncertainty associated with the predictions.

Results: For analyses the country was divided into north and south. In the north, the month of survey, distance to water, precipitation and temperature had no significant association with P. falciparum prevalence when spatial correlation was taken into account. In contrast, all the covariates, except distance to water, were significantly associated with parasite prevalence in the south. The inclusion of covariates improved model fit for the south but not for the north. Model precision was highest in the south. The majority of the country had a predicted prevalence of < 5%; areas with > or = 5% prevalence were predominantly in the south.

Conclusion: The maps showed that malaria transmission in Somalia varied from hypo- to meso-endemic. However, even after including the selected covariates in the model, there still remained a considerable amount of unexplained spatial variation in parasite prevalence, indicating effects of other factors not captured in the study. Nonetheless the maps presented here provide the best contemporary information on malaria prevalence in Somalia.

Figure 1

Map of Somalia showing the self-declared states of Puntland, Somaliland and South Central; their capital cities; and the two main rivers of Juba and Shabelle.

Figure 2

Map and variograms of north and south of Somalia showing the distribution PR data locations (the X-axis shows distance as degrees latitude and longitude). The data were distributed as model (n = 113 north; n = 336 south) and outlier (n = 2 north; n = 1 south) locations. Variograms are of the data after outliers were removed.

Figure 3

Maps of north and south of Somalia showing: a) posterior median PfPR; b) endemicity classes based on the posterior probability of PfPR class membership PfPR; c) probability that a prediction location is correctly classified to an endemicity class: <5%; 5–39%; and ≥40% PfPR. Probabilities < 0.33 are considered no better than chance class assignment. In Somalia probabilities were all ≥ 0.45.