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Observational Study
. 2016 Jun 28;13(6):e1002047.
doi: 10.1371/journal.pmed.1002047. eCollection 2016 Jun.

Age, Spatial, and Temporal Variations in Hospital Admissions with Malaria in Kilifi County, Kenya: A 25-Year Longitudinal Observational Study

Polycarp Mogeni  1 , Thomas N Williams  1   2 , Gregory Fegan  1   3 , Christopher Nyundo  1 , Evasius Bauni  1 , Kennedy Mwai  1 , Irene Omedo  1 , Patricia Njuguna  1 , Charles R Newton  1   3 , Faith Osier  1 , James A Berkley  1   3 , Laura L Hammitt  1   4 , Brett Lowe  1 , Gabriel Mwambingu  1 , Ken Awuondo  1 , Neema Mturi  1 , Norbert Peshu  1 , Robert W Snow  3   5 , Abdisalan Noor  3   5 , Kevin Marsh  1   3 , Philip Bejon  1   3
Affiliations
Observational Study

Age, Spatial, and Temporal Variations in Hospital Admissions with Malaria in Kilifi County, Kenya: A 25-Year Longitudinal Observational Study

Polycarp Mogeni et al. PLoS Med. .

Abstract

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 due to lower acquired immunity, and this 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 it to data on residence and insecticide-treated net (ITN) use. This included data from 69,104 children aged from 3 mo to 13 y admitted to Kilifi County Hospital between 1 January 1990 and 31 December 2014. The variation in malaria slide positivity among admissions was examined in logistic regression models using the following predictors: location of the residence, calendar time, the child's age, ITN use, and the enhanced vegetation index (a proxy for soil moisture). The proportion of malaria slide-positive admissions declined from 0.56 (95% confidence interval [CI] 0.54-0.58) in 1998 to 0.07 (95% CI 0.06-0.08) in 2009 but then increased again through to 0.24 (95% CI 0.22-0.25) in 2014. Older children accounted for most of the increase after 2009 (0.035 [95% CI 0.030-0.040] among young children compared to 0.22 [95% CI 0.21-0.23] in older children). There was a nonlinear relationship between malaria risk and prevalence of ITN use within a 2 km 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 the increasing burden among older children, and universal ITN coverage is a promising strategy to achieve this goal.

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Conflict of interest statement

AN serves on the PLOS Medicine Editorial Board.

Figures

Fig 1
Fig 1. Flow diagram of participant numbers.
Participant numbers and reasons for exclusion at each stage are shown.
Fig 2
Fig 2. Temporal trends of malaria positive fraction (MPF), age of slide positivity, and mortality among acute admissions.
Panel A shows the temporal trend of MPF. Panel B shows the trends of mean age over time for the slide-positive and slide-negative admissions. Panel C shows the temporal trends of absolute number of deaths, and Panel D shows case fatality rates. Shaded areas in panels A, B, and D represent 95% CIs.
Fig 3
Fig 3. Geographical distribution of MPF and age of slide-positive acute admissions over time.
Panels A, B, and C show spatial distributions of MPF for the predecline, decline, and postdecline periods respectively, and their associated pie charts show the proportion of the variability in MPF explained by the predictors: region (i.e., north versus south), location, time trend (i.e., as a continuous variable), an interaction between time and location, year to year variation (i.e., year as a stratified variable), and the unexplained variations. Panels D, E, and F show spatial distributions of age in months for the slide-positive admissions during the predecline, decline, and postdecline periods.
Fig 4
Fig 4. Temporal trends of MPF by age and parasite prevalence.
Panels A and B show the temporal trends of MPF in admitted children aged ≤ 1 y old and children aged > 1 y old, respectively; the red line represents the southern region, while the blue line represents the northern region of the creek. Panel C shows all age MPF for randomly selected locations from the southern and northern regions. Panel D shows the age-standardized parasite prevalence (i.e., Plasmodium falciparum parasite rate [PfPR2-10]) among the trauma cases. Shaded areas in panels A, B, and D represent 95% CIs.
Fig 5
Fig 5. Regression model prediction and variability explained by the predictors of the model.
Panel A shows the predicted probability of a positive slide result (y-axis) against the prevalence of ITN use in a 2 km radius around each admitted child’s residence (shaded area represents 95% CI). Panel B shows the pseudo R2 of the various variables assessed in the extended model (ITN use around a child’s residence, EVI, age, age–time interaction, time, region, and location). We repeated our analysis using a cut-off of >2,500 parasites per μl. The same patterns were seen—i.e., a pattern of a postdecline increase in MPF among older children (S6 Fig), and an inverse relationship between MPF and age of malaria (r = −0.63, p < 0.001) and a pattern of protection by community-level ITN use (S7 Fig). These patterns remained statistically significant.
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