1. Istituto di Patologia Generale Veterinaria, Universitàdi Milano, Via Celoria 10, 20133 Milano, Italy
2. Wellcome Trust Centre for the Epidemiology of Infectious Diseases, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
3. Institute of Cell, Animal and Population Biology, King's Building, University of Edinburgh, Edinburgh EH9 3JT, UK