Effect of a fall in malaria transmission on morbidity and mortality in Kilifi, Kenya

Abstract

Background: As efforts to control malaria are expanded across the world, understanding the role of transmission intensity in determining the burden of clinical malaria is crucial to the prediction and measurement of the effectiveness of interventions to reduce transmission. Furthermore, studies comparing several endemic sites led to speculation that as transmission decreases morbidity and mortality caused by severe malaria might increase. We aimed to assess the epidemiological characteristics of malaria in Kilifi, Kenya, during a period of decreasing transmission intensity.

Methods: We analyse 18 years (1990-2007) of surveillance data from a paediatric ward in a malaria-endemic region of Kenya. The hospital has a catchment area of 250 000 people. Clinical data and blood-film results for more than 61 000 admissions are reported.

Findings: Hospital admissions for malaria decreased from 18.43 per 1000 children in 2003 to 3.42 in 2007. Over 18 years of surveillance, the incidence of cerebral malaria initially increased; however, malaria mortality decreased overall because of a decrease in incidence of severe malarial anaemia since 1997 (4.75 to 0.37 per 1000 children) and improved survival among children admitted with non-severe malaria. Parasite prevalence, the mean age of children admitted with malaria, and the proportion of children with cerebral malaria began to change 10 years before hospitalisation for malaria started to fall.

Interpretation: Sustained reduction in exposure to infection leads to changes in mean age and presentation of disease similar to those described in multisite studies. Changes in transmission might not lead to immediate reductions in incidence of clinical disease. However, longitudinal data do not indicate that reductions in transmission intensity lead to transient increases in morbidity and mortality.

Figure 1

Hospital admissions per 1000 children resident in the study area

Figure 2

Annual parasite prevalence in patients admitted for trauma Bars are 95% CIs.

Figure 3

Mean age in years of slide-positive and slide-negative hospital admissions by year Bars are 95% CIs.

Figure 4

Annual incidence (A) and ratio (B) of cerebral malaria and severe malarial anaemia Data for cerebral malaria were unavailable in 1992 and 1998. SMA=severe malarial anaemia. CM=cerebral malaria.

Figure 5

Age distributions for children admitted with severe malarial anaemia (A) and cerebral malaria (B)

Figure 6

Mortality of slide-positive cases per 10 000 children resident in the study area