[1] Univ Milan, Dipartimento Patol Anim Igiene & Sanita Pubbl Vet, Sez Patol Gen & Parassitol, I-20133 Milan, Italy
[2] Univ Georgia, Coll Vet Med, Athens, GA USA
[3] Univ Parma, Dipartimento Prod Anim, I-43100 Parma, Italy
[4] Univ Pavia, Dipartimento Biol Anim, I-27100 Pavia, Italy
[5] Univ Rochester, Dept Biol, Rochester, NY 14627 USA