1. The Pirbright Institute, Ash Road, Pirbright GU24 0NF, UK
2. MRC-University of Glasgow Centre for Virus Research, 464 Bearsden Road, Glasgow G61 1QH, UK
3. Department of Life Sciences, Faculty of Science, Claverton Down, University of Bath, Bath BA2 7AY, UK
4. Institute of Medical Sciences, School of Medicine, Medical Sciences and Nutrition, Foresterhill, University of Aberdeen, Aberdeen AB25 2ZD, UK