[1] Royal Vet Coll, Dept Prod & Populat Hlth, Hatfield AL9 7TA, Herts, England
[2] Royal Vet Coll, Dept Pathol & Pathogen Biol, Hatfield AL9 7TA, Herts, England
[3] Univ Cambridge, Dept Vet Med, Cambridge CB3 0ES, England
[4] Anim Hlth Trust, Epidemiol Unit, Kentford CB8 7UU, England