1. AgroParisTech/INRA UMR518, Department of Applied Mathematics and Informatics (MIA); MORSE team, F-75231 Paris cedex 5; France
2. Veterinary Laboratories Agency; New Haw; Addlestone; Surrey; KT15 3NB; UK
3. Department of Epidemiology and Public Health; Imperial College London; St Mary's Campus, Norfolk Place; London; W2 1PG; UK