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RELATIONSHIP BETWEEN THE INTENSITY OF EXPOSURE TO MALARIA PARASITES AND INFECTION IN THE USAMBARA MOUNTAINS, TANZANIA

The relationship between exposure to Plasmodium falciparum malaria and parasite density and prevalence was studied in six communities along an altitude transect. Prevalence of parasitemia in children decreased by 5% for every 100 meter increase in altitude from 82% in the lowlands at 300 meters to 12% in the highlands at 1,700 meters. This decrease in prevalence corresponded to a 1,000-fold reduction in transmission intensity. The ability to suppress parasite density and prevalence with age increased proportionally with increasing transmission intensity when transmission rates were higher than 0.1 infective bites per year, but developed after 2–3 years of age, regardless of transmission intensity. However, at transmission rates less than 0.1 infective bites per year, prevalence remained similar in all age groups. We propose that both exposure-dependent acquired immunity and age-dependent acquired immunity regulate parasite prevalence and density and suggest that transmission control will not hinder the development of protective anti-parasite immunity.

Received February 2, 2005. Accepted for publication September 21, 2005.

Acknowledgments: We thank D. Shayo, L. N. Malle, Z. X. N. Savael, and W. Chambo (Amani Medical Research Centre), and G. Kilua and J. Raphael for invaluable help during the surveys. We are very grateful to L. N. Malle and Z. X. N. Savael for reading all the slides. We are also grateful for the collaboration of all villagers and for the excellent work done by the six village assistants. We also thank Tom Smith for the helpful comments on an earlier version of the manuscript.

Financial support: This study was supported by the Danish Council for Development Research and the Danish Bilharziasis Laboratory.

^* Address correspondence to René Bødker, Danish Institute for Food and Veterinary Research, Mørkhøj Bygade 19, 2860 Søborg, Denmark. E-mail: reb{at}dfvf.dk

Authors’ addresses: René Bødker, Danish Institute for Food and Veterinary Research, Mørkhøj Bygade 19, 2860 Søborg, Denmark, Telephone: 45-72-34-70-99, Fax: 45-72-34-70-28, E-mail: reb{at}dfvf.dk. Hamisi A. Msangeni and Willia, Kisinza, National Insitute of Medical Research, PO Box 4, Amani Tanga, Tanzania, E-mails: hmsangeni{at}hotmail.com and kisinza{at}hotmail.com. Steve W. Lindsay, School of Biological and Biomedical Sciences, Durham University, South Road, Durham DH1 3LE, United Kingdom, Telephone: 44-191-334-1349, Fax: 44-191-334-1289, E-mail: s.w.lindsay{at}durham.ac.uk.

Reprint requests: René Bødker, Danish Institute for Food and Veterinary Research, Mørkhøj Bygade 19, 2860 Søborg, Denmark.

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