1 Medical Research Council Unit in The Gambia at the London School of Hygiene and Tropical Medicine, Fajara, PO Box 273, The Gambia.
2 Ministry of Health, Banjul, The Gambia.
3 Medical Research Council Unit in The Gambia at the London School of Hygiene and Tropical Medicine, Fajara, PO Box 273, The Gambia. Electronic address: aroca@mrc.gm.