Abstract
BackgroundThe relationship between entomological measures of malaria transmission intensity and mortality remains uncertain. This is partly because transmission is heterogeneous even within small geographical areas. Studying this relationship requires high resolution, spatially structured, longitudinal entomological data. Geostatistical models that have been used to analyse the spatio-temporal heterogeneity have not considered the uncertainty in both sporozoite rate (SR) and mosquito density data. This study analysed data from Kassena-Nankana districts in northern Ghana to obtain small area estimates of malaria transmission rates allowing for this uncertainty.MethodsIndependent Bayesian geostatistical models for sporozoite rate and mosquito density were fitted to produce explicit entomological inoculation rate (EIR) estimates for small areas and short time periods, controlling for environmental factors.ResultsMosquitoes were trapped from 2,803 unique locations for three years using mainly CDC light traps. Anopheles gambiae constituted 52%, the rest were Anopheles funestus. Mean biting rates for An. funestus and An. gambiae were 32 and 33 respectively. Most bites occurred in September, the wettest month. The sporozoite rates were higher in the dry periods of the last two years compared with the wet period. The annual EIR varied from 1,132 to 157 infective bites. Monthly EIR varied between zero and 388 infective bites. Spatial correlation for SR was lower than that of mosquito densities.ConclusionThis study confirms the presence of spatio-temporal heterogeneity in malaria transmission within a small geographical area. Spatial variance was stronger than temporal especially in the SR. The estimated EIR will be used in mortality analysis for the area.
Highlights
The relationship between entomological measures of malaria transmission intensity and mortality remains uncertain
Malaria in Ghana is transmitted by two main vectors: Anopheles gambiae and Anopheles funestus, whose peak activities occur at the end of the wet season
To clarify the relationship between malaria transmission and mortality, the Malaria Transmission Intensity and Mortality Burden Across Africa (MTIMBA) project was established in 10 INDEPTH network sites between 2001 and 2004 [21,22]
Summary
Description of density data The mean biting rates per person and night for An. funestus were 34 in the first year, 32 in the second and 19 in the third. The fraction of infected An. gambiae mosquitoes was higher in dry season than wet for the second (2.0%) and third (1.5%) year. For An. funestus, distance to water bodies, NDVI, season, day temperature and second year of data collection were related to density. For An. gambiae, distance to water bodies, NDVI, day temperature and night temperature were associated with mosquito density. Model-based results: sporozoite rate data Lag analysis shows that An. funestus SR was related to total rainfall of the survey month, average NDVI, average night temperature for the two months preceding the survey, and average day temperature of current and previous month. It is evident from the maps that areas close to water bodies experienced high EIR
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