1. Institute of Ecology, University of Georgia, Athens, GA 30602, USA.
2. Infectious Disease and Pathology Activity, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA.
3. Institute of Zoology, Zoological Society of London, Regent's Park, London NW1 4RY, UK.
4. Australian Animal Health Laboratory, CSIRO, Private Bag 24, Geelong, Victoria 3220, Australia.