1. Nebraska Center for Virology, University of Nebraska-Lincoln, Lincoln, NE 68583, USA
2. School of Veterinary Medicine and Biomedical Sciences, University of Nebraska-Lincoln, Lincoln, NE 68583, USA
3. Veterinary & Biomedical Sciences Department, College of Veterinary Medicine, University of Minnesota, Twin Cities, MN 55108, USA
4. Veterinary Diagnostic Laboratory, College of Veterinary Medicine, Iowa State University, Ames, IA 50011, USA
5. Department of Animals Sciences, University of Nebraska-Lincoln, Lincoln, NE 68583, USA