1. Department of Pathobiology and Epidemiology; DLO-Institute for Animal Science and Health (ID-DLO); PO Box 65 8200 AB Lelystad The Netherlands
2. Epidemiology Department; Central Veterinary Laboratory; Veterinary Laboratory Agency; New Haw, Addlestone Surrey KT15 3NB
3. Federal Research Centre for Virus Diseases of Animals; Tubingen Germany