Factors associated with high heterogeneity of malaria at fine spatial scale in the Western Kenyan highlands.

Amrish Y Baidjoe,Teun Bousema,Chris Drakeley,Victor Osoti,Jennifer Stevenson,William Stone,Euniah Makori,Wycliffe Odongo,Shehu Shagari,Pauline China,Philip Knight,Simon Kariuki,Gillian Stresman,Chrispin Owaga,Jonathan Cox

doi:10.1186/s12936-016-1362-y

Abstract

BackgroundThe East African highlands are fringe regions between stable and unstable malaria transmission. What factors contribute to the heterogeneity of malaria exposure on different spatial scales within larger foci has not been extensively studied. In a comprehensive, community-based cross-sectional survey an attempt was made to identify factors that drive the macro- and micro epidemiology of malaria in a fringe region using parasitological and serological outcomes.MethodsA large cross-sectional survey including 17,503 individuals was conducted across all age groups in a 100 km2 area in the Western Kenyan highlands of Rachuonyo South district. Households were geo-located and prevalence of malaria parasites and malaria-specific antibodies were determined by PCR and ELISA. Household and individual risk-factors were recorded. Geographical characteristics of the study area were digitally derived using high-resolution satellite images.ResultsMalaria antibody prevalence strongly related to altitude (1350–1600 m, p < 0.001). A strong negative association with increasing altitude and PCR parasite prevalence was found. Parasite carriage was detected at all altitudes and in all age groups; 93.2 % (2481/2663) of malaria infections were apparently asymptomatic. Malaria parasite prevalence was associated with age, bed net use, house construction features, altitude and topographical wetness index. Antibody prevalence was associated with all these factors and distance to the nearest water body.ConclusionAltitude was a major driver of malaria transmission in this study area, even across narrow altitude bands. The large proportion of asymptomatic parasite carriers at all altitudes and the age-dependent acquisition of malaria antibodies indicate stable malaria transmission; the strong correlation between current parasite carriage and serological markers of malaria exposure indicate temporal stability of spatially heterogeneous transmission.

Highlights

The East African highlands are fringe regions between stable and unstable malaria transmission
These studies were based on passively detected malaria cases [24, 25] and active surveillance in children [26, 27]; a more comprehensive, community-based, assessment of parasitological and serological outcomes in all age groups is needed to establish the macro- and micro-epidemiology of P. falciparum and to identify factors associated with exposure and infection so that more targeted and specific interventions can be locally deployed
Altitude and malaria risk The proportions of individuals with fever, clinical malaria, parasites detected by PCR, malaria-specific antibody responses (Fig. 2) and anaemia (Hb

Summary

Introduction

The East African highlands are fringe regions between stable and unstable malaria transmission. It appears that this picture has been changing [23] with studies describing instances of relatively stable malaria transmission in the Kenyan highlands [24, 25], characterized by age-dependent acquisition of anti-malarial immunity [24, 26] and a substantial reservoir of asymptomatic malaria infections [27] These studies were based on passively detected malaria cases [24, 25] and active surveillance in children [26, 27]; a more comprehensive, community-based, assessment of parasitological and serological outcomes in all age groups is needed to establish the macro- and micro-epidemiology of P. falciparum and to identify factors associated with exposure and infection so that more targeted and specific interventions can be locally deployed

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Results

Discussion

Conclusion