1. Centre for Comparative Infectious Diseases, The University of Liverpool, Liverpool L69 3BX, UK
2. Population Biology Research Group, School of Biological Sciences, Nicholson Building, The University of Liverpool, Liverpool L69 3BX, UK
3. Department of Veterinary Pathology, The University of Liverpool, Liverpool L69 3BX, UK
4. Department of Medical Microbiology, The University of Liverpool, Liverpool L69 3BX, UK