[1] INIA, Ctr Invest Sanidad Anim, Madrid 28130, Spain
[2] Inst Virol & Immunoprophylaxis, CH-3147 Mittelhausern, Switzerland
[3] DLO, Cent Vet Inst, ID, NL-8200 AB Lelystad, Netherlands
[4] Finch Univ Hlth Sci Chicago Med Sch, Dept Microbiol & Immunol, N Chicago, IL 60064 USA
[5] Kansas State Univ, Dept Anat & Physiol, Manhattan, KS 66506 USA
[6] Univ Minnesota, Dept Vet Pathobiol, St Paul, MN 55108 USA