1. Faculty of Veterinary Medicine; University of Gent; B-9000 Gent Belgium
2. Regional Poultry Diagnostic Laboratory; B-2500 Lier Belgium
3. Regional Poultry Diagnostic Laboratory; B-8820 Torhout Belgium
4. Division of Veterinary Public Health and Food Safety; University of Bristol, Langford; Avon BS 18 7DU