[1] Vitagene & Hlth Res Ctr, Bristol BS4 2RS, Avon, England
[2] Moscow State Acad Vet Med & Biotechnol, Dept Hyg & Poultry Sci, Moscow 109472, Russia
[3] Trakia Univ, Fac Vet Med, Dept Microbiol & Biochem, Stara Zagora 6000, Bulgaria
[4] Szent Istvan Univ, Fac Agr & Environm Sci, Dept Anim Nutr, H-2103 Godollo, Hungary
[5] St Petersburg State Acad Vet Med, St Petersburg 196084, Russia