1. Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, UK
2. Current address: Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
3. Current address: Animal Nutrition & Health Department, Animal Biology Division, Scottish Agricultural College, Bush Estate, Penicuik EH26 0PH, UK
4. Moredun Research Institute, International Research Centre, Pentlands Science Park, Bush Loan, Penicuik, Midlothian, EH26 0PZ, UK
5. Current address: Department of Microbiology and Immunology, Cornell University, Ithaca, New York 14853, USA