1. Laboratory for Physiology and Immunology of Domestic Animals, Catholic University of Louvain, Kardinaal Mercierlaan 92, 3001 Heverlee, Belgium
2. Centre for Applied Poultry Research, Spelderholt 9, 7360 AA Beekbergen, The Netherlands
3. Department of Veterinary Sciences, R.U. Utrecht, Yalelaan 2, 3508 Utrecht, The Netherlands