1 Kintampo Health Research Centre, Research and Development Division, Ghana Health Service, Kintampo North Municipality, PO Box 200, Ghana; Department of Disease Control, London School of Hygiene & Tropical Medicine, London, UK. Electronic address: kwakupoku.asante@kintampo-hrc.org.
2 Department of Public Health, Environments and Society, London School of Hygiene & Tropical Medicine, London, UK.