1. Kintampo Health Research Centre, P.O. Box 200 Kintampo, Brong Ahafo Region, Ghana
2. Dodowa Health Research Centre, Ghana Health Service, P.O. Box 1 Dodowa, Dangme West District, Greater Accra Region, Ghana
3. Disease Control and Vector Biology Unit/Infectious and Tropical Disease Department, London School of Hygiene & Tropical Medicine, Keppel, WC1E 7HT St London, UK