[1] Iowa State Univ, Dept Vet Microbiol & Preventat Med, Ames, IA 50011 USA
[2] ARS, Infect Bacterial Dis Livestock Res Unit, Natl Anim Dis Ctr, USDA, Ames, IA 50010 USA
[3] ARS, Ruminant Dis & Immunol Res Unit, Natl Anim Dis Ctr, USDA, Ames, IA 50010 USA