1. Animal Behaviour Group, Faculty of Veterinary Science, University of Liverpool, Leahurst, Chester High Road, Neston, Cheshire CH64 7TE, UK
2. Disease Ecology Group, Faculty of Veterinary Science, University of Liverpool, Leahurst, Chester High Road, Neston, Cheshire CH64 7TE, UK
3. Department of Medical Microbiology, University of Liverpool, Duncan Building, Daulby Street, Liverpool L69 3GA, UK