[1] Univ Bristol, Sch Biol Sci, Bristol BS8 1TQ, Avon, England
[2] Univ Bristol, Cabot Inst, Bristol BS8 1TS, Avon, England
[3] Univ Naples Federico II, CREMOPAR, Dept Vet Med & Anim Prod, Naples, Italy
[4] Univ Zurich, Sect Vet Epidemiol, CH-8057 Zurich, Switzerland
[5] Univ Coll Dublin, Sch Vet Med, Dublin 2, Ireland
[6] Moredun Res Inst, Penicuik EH26 0PZ, Midlothian, Scotland
[7] Univ Ghent, Fac Vet Med, Dept Virol Parasitol & Immunol, B-9820 Merelbeke, Belgium
[8] Univ Zurich, Inst Parasitol, CH-8057 Zurich, Switzerland
[9] Avia GIS BVBA, B-2980 Zoersel, Belgium
[10] Univ Bristol, Sch Vet Sci, Bristol BS40 5DU, Avon, England