1Barbon, Carnforth, Cumbria, UK
2Poultry Health Services Ltd, Poultry Health Centre, Thirsk, North Yorkshire, UK
3Department of Veterinary Pathology, University of Liverpool, Neston, UK
4VLA Weybridge, Department of Virology, Addlestone, Surrey, UK
*The authors wish to acknowledge the contribution of Clifford Wray and Robert Davies, coauthors of this chapter in the 5th edition.
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