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Relationship Between Exposure, Clinical Malaria, and Age in an Area of Changing Transmission Intensity

The relationship between malaria transmission intensity and clinical disease is important for predicting the outcome of control measures that reduce transmission. Comparisons of hospital data between areas of differing transmission intensity suggest that the mean age of hospitalized clinical malaria is higher under relatively lower transmission, but the total number of episodes is similar until transmission drops below a threshold, where the risks of hospitalized malaria decline. These observations have rarely been examined longitudinally in a single community where transmission declines over time. We reconstructed 16 years (1991–2006) of pediatric hospital surveillance data and infection prevalence surveys from a circumscribed geographic area on the Kenyan coast. The incidence of clinical malaria remained high, despite sustained reductions in exposure to infection. However, the age group experiencing the clinical attacks of malaria increased steadily as exposure declined and may precede changes in the number of episodes in an area with declining transmission.

Received April 3, 2008. Accepted for publication May 25, 2008.

Acknowledgments: This work was published with the permission of the Director of KEMRI. The authors thank all the clinical, field, demography, and laboratory staff at the KEMRI/Wellcome Trust Programme who made this study possible. The KEMRI/Wellcome Trust Programme is a member of the INDEPTH Network of demographic surveillance sites.

Financial support: This investigation received financial support from The Wellcome Trust and the Kenya Medical Research Institute. WPO gratefully acknowledges the Fogarty International Center of the National Institutes for Health for funding and support. TNW is supported by The Wellcome Trust as a Senior Research Fellow (076934/Z/05/Z) and by funds awarded through the BioMalpar European Network 6 initiative. RWS is supported by the Wellcome Trust as Principal Research Fellow (079081).

Disclosure: There are no competing interests to declare.

^* Address correspondence to Wendy P. O’Meara, 16 Center Drive, Bethesda, MD 20892. E-mail: prudhomw{at}mail.nih.gov

Authors’ addresses: Wendy P. O’Meara, Tabitha W. Mwangi, Thom-as N. Williams, and Kevin Marsh, Centre for Geographic Medicine Research–Coast, Kenya Medical Research Institute, Wellcome Trust Laboratories, P.O. Box 230, Kilifi, Kenya, Tel: 254-125-22063, Fax: 254-125-22390, E-mail: prudhomw{at}mail.nih.gov. F. Ellis McKenzie, 16 Center Dr., Bethesda, MD 20892. Robert W. Snow, Centre for Geographic Medicine, KEMRI-University of Oxford-Wellcome Trust Collaborative Programme, Kenyatta National Hospital Grounds, P.O. Box 43640-00100, Nairobi, Kenya.

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