[1] Univ London Royal Vet Coll, Surrey GU21 2LQ, England
[2] Food & Agr Org United Nations, Rome, Italy
[3] Univ London, Univ London Royal Vet Coll, Dept Pathol & Infect Dis, Hatfield AL9 TTA, Herts, England
[4] Minist Agr & Peche, F-75732 Paris 15, France