1. Group of Animal Breeding; Institute of Animal Science; Swiss Federal Institute of Technology; Zurich Switzerland
2. Department of Parasitology; Institute of Zoology; University of Neuchâtel; Switzerland
3. Clinic of Large Animal Internal Medicine; University of Zurich; Switzerland
4. Institute of Comparative Tropical Medicine and Parasitology; Ludwigs Maximillian University Munich; Leopoldstrasse 5 D-80802 Munich Germany