1. Poultry Diagnostic and Research Center, College of Veterinary Medicine, University of Georgia, Athens, Georgia, USA
2. Present address: National Poultry Research Center, U.S. Department of Agriculture, Athens, Georgia, USA.
3. Present address: Boehringer Ingelheim Vetmedica, Inc., Ames, Iowa, USA.
4. Present address: Boehringer Ingelheim, Veterinary Research Center, GmbH Co. KG, Bemeroder Straße 31, 30559 Hannover, Germany.