1. Department of Laboratory Animal Science, Veterinary Faculty, State University, PO Box 80.166,3508 TD Utrecht, The Netherlands
2. Netherlands Organization for Scientific Research, Department of General and Industrial Microbiology, Zeist, The Netherlands
3. Department of Veterinary Pathology, Branch Laboratory Animal Pathology and Special Animal Diseases, Veterinary Faculty, State University, PO Box 80.158, 3508 TD Utrecht, The Netherlands