David J Hampson1, Tom La1, Ben Adler2, Darren J Trott3. 1. School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, Western Australia 6150, Australia. 2. Department of Microbiology, Monash University, Victoria 3800, Australia. 3. School of Veterinary Science, University of Queensland, Queensland, St Lucia 4072, Australia.