1. Department of Veterinary and Microbiological Sciences, Great Plains Institute of Food Safety, 1523 Centennial Blvd., P.O. Box 6050-Dept. 7690; North Dakota State University; Fargo ND 58108
2. Great Plains Institute of Food Safety, 1523 Centennial Blvd., PO Box 6050-Dept. 7690; North Dakota State University; Fargo ND 58108
3. Department of Chemistry, Environment and Animal Feed; National Veterinary Institute; SE-75189 Uppsala Sweden
4. National Food Institute, Technical University of Denmark; Mørkhøj Bygade 19, Blok H DK-2860 Søborg Denmark