1. Department of Veterinary Pathology, State University, PO Box 80.166, 3508 TD Utrecht, The Netherlands; Department of Laboratory Animal Science, State University, !PO Box 80.166, 3508 TD Utrecht, The Netherlands
2. Department of Laboratory Animal Science, State University, PO Box 80.166, 3508 TD Utrecht, The Netherlands
3. Department of Veterinary Pathology, State University, PO Box 80.166, 3508 TD Utrecht, The Netherlands