1. National Veterinary Institute; Department of Animal Health and Antimicrobial Strategies; SE-751 89 Uppsala Sweden
2. Departamento de Sanidad Animal; Unidad de Microbiología e Immunología; Facultad de Veterinaria; Universidad de León; 24007 León Spain
3. Swedish University of Agricultural Sciences; Department of Biomedicine and Veterinary Public Health, Section of Immunology; PO Box 588, Biomedical Centre SE-751 23 Uppsala Sweden