[1] Univ Maryland, Dept Anim & Avian Sci, College Pk, MD 20742 USA
[2] Univ Georgia, Dept Cellular Biol, Athens, GA 30602 USA
[3] China Agr Univ, Coll Vet Med, Dept Vet Microbiol & Immunol, Beijing 100083, Peoples R China
[4] Univ Minnesota, Ctr Immunol, Minneapolis, MN 55455 USA