[1] Univ London London Sch Hyg & Trop Med, Dept Infect & Trop Dis, London WC1E 7HT, England
[2] Western Gen Hosp, Natl Creutzfeldt Jakob Dis Surveillance Unit, Edinburgh EH4 2XU, Midlothian, Scotland
[3] Open Univ, Dept Stat, Milton Keynes MK7 6AA, Bucks, England