1. Division of Epidemiology; Norwegian Institute of Public Health; Marcus Thranes Gate 6, P.O. Box 4404 0403 Oslo Norway
2. Epi-Centre; Department of Food-Safety and Infection Biology; Norwegian School of Veterinary Science; Ullevålsveien 72, P.O. Box 8146 Dep NO-0033 Oslo Norway
3. Norwegian Veterinary Institute; P.O. Box 750 Sentrum NO-0106 Oslo Norway