1. Department of Zoology, University of Guelph, Guelph, Ontario N1G 2W1, Canada
2. Centre for Tropical Veterinary Medicine, Easter Bush, Roslin, Midlothian EH25 9RG, UK
3. Eoscene Corporation, 700, 5th Avenue, Seattle, Washington 98104, USA
4. Veterinary Laboratories Agency, New Haw, Addlestone, Surrey KT15 3NB, UK
5. Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, University of London, Keppel Street, London WC1E 7HT, UK