Age, spatial and temporal variations in hospital admissions with malaria in Kilifi County, Kenya: A 25 year longitudinal observational study

Background Encouraging progress has been seen with reductions in Plasmodium falciparum malaria transmission in some parts of Africa. Reduced transmission might lead to increasing susceptibility to malaria among older children, which has implications for ongoing control strategies. Methods and findings We conducted a longitudinal observational study of children admitted to Kilifi County Hospital in Kenya and linked to data on residence and Insecticide Treated Net (ITN) ownership. This included data from 69,104 children admitted to Kilifi County Hospital aged from 3 months to 13 years between 1st January 1990 and 31st December 2014. The variation in malaria slide positivity among admissions was examined in logistic regression models using the predictors; location of residence, calendar time, child’s age, ITN use and Enhanced Vegetation Index (a proxy for soil moisture). The proportion of malaria slide positive admissions declined from 0.56 with 95% confidence interval (95%CI) 0.54 to 0.58 in 1998 to 0.07 95%CI 0.06 to 0.08 in 2009, but then increased again through to 0.24 95%CI 0.22 to 0.25 in 2014. Older children accounted for most of the increase after 2009 (0.035 95%CI (0.030 to 0.040) among young children compared to 0.22 95%CI 0.21 to 0.23 in older children). There was a non-linear relationship between malaria risk and prevalence of ITN use within a 2km radius of an admitted child’s residence such that the predicted malaria positive fraction varied from ~0.4 to <0.1 as the prevalence of ITN use varied from 20% to 80%. In this observational analysis we were unable to determine the cause of the decline in malaria between 1998 and 2009, which pre-dated the dramatic scale-up in ITN distribution and use. Conclusion Following a period of reduced transmission a cohort of older children emerged who have increased susceptibility to malaria. Further reductions in malaria transmission are needed to mitigate against the increasing burden among older children and universal ITN coverage is a promising strategy to achieve this.