1. Department of Arctic Veterinary Medicine, The Norwegian School of Veterinary Science, N-9292 Tromsø, Norway
2. Section of Immunoprophylaxis, National Veterinary Institute, P. O. Box 8156 Dep., N-0033 Oslo, Norway
3. Norwegian Polar Institute, N-9296 Tromsø, Norway
4. Zoological Museum, University of Oslo, Sars gt. 1, N-0562 Oslo, Norway
5. Department of Bacteriology and Immunology, Veterinary and Agrochemical Research Centre, 1180 Brussels, Belgium