1. Istituto Zooprofilattico Sperimentale del Piemonte; Liguria e Valle d'Aosta, Sezione di Aosta, Via Guido Rey 5 I-11100 Aosta Italy
2. Institute for Veterinary Bacteriology; University of Bern; Laenggassstrasse 122 CH-3012 Bern Switzerland
3. Centre for Fish and Wildlife Health; Institute of Animal Pathology; University of Bern; Laenggassstrasse 122 CH-3012 Bern Switzerland