[1] Western Gen Hosp, Natl CJD Surveillance Unit, Edinburgh EH4 2XU, Midlothian, Scotland
[2] Publ Hlth Lab Serv, Ctr Communicable Dis Surveillance, London NW9 5EQ, England
[3] Open Univ, Dept Stat, Milton Keynes MK7 6AA, Bucks, England
[4] London Sch Hyg & Trop Med, Dept Infect & Trop Dis, London WC1, England