作者单位:Texas A&M Univ, Coll Vet Med & Biomed Sci, Dept Vet Integrat Biosci, College Stn, TX USA[1]Univ Georgia, Coll Vet Med, Dept Populat Hlth, Athens, GA 30602 USA[2]Texas A&M Univ, Coll Vet Med & Biomed Sci, Dept Vet Pathobiol, College Stn, TX USA[3]Michigan State Univ, Coll Vet Med, Dept Large Anim Clin Sci, E Lansing, MI 48824 USA[4]Texas Tech Univ, Sch Vet Med, Amarillo, TX USA[5]